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A    MANUAL 


PHYSIOLOGY. 


MANUAL 


ri'  I  I  /    (fi  i\'i  r  (  I'l  \ 


PHYSIOLOGY. 


TEXT-BOOK 


STUDENTS   OF  MEDICINE. 


GERALD   F.   YEO,   M.D.,   F.R.C.S., 

PEOFESSOE  OF  PHYSIOLOGY  IN  KING'S  COLLEGE,  LONDON,  ETC. 


PHILADELPHIA: 
P.    B  E  A  K  I  8  T  O  N,    SON    &    C  0., 

1012    WALNUT    STREET. 
1884. 


Digitized  by  the  Internet  Arciiive 

in  2010  witii  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/manualofphysioloOOyeog 


PREFACE. 


The  present  volume  has  been  written  at  the  desire  on  the 
part  of  the  Publishers  that  a  new  elementary  treatise  on 
Physiology  should  be  added  to  the  series  of  admirable  students' 
manuals  which  they  had  previously  issued. 

In  carrying  this  desire  into  execution  I  have  endeavored 
to  avoid  theories  which  have  not  borne  the  test  of  time,  and 
such  details  of  methods  as  are  unnecessary  for  junior  students. 
I  do  not  give  any  history  of  how  our  knowledge  has  grown  to 
its  present  stand-point ;  nor  do  I  mention  the  names  of  the 
authorities  upon  whose  writings  my  statements  depend.  I 
have  also  omitted  the  mention  of  exceptional  points,  because 
I  find  that  exceptions  are  more  easily  remembered  than  the 
main  facts  from  which  they  differ ;  and,  since  we  must  often 
be  content  with  the  retention  of  the  one  or  the  other,  I  have 
tried  to  insure  that  it  shall  be  the  more  important. 

While  endeavoring  to  save  the  student  from  doubtful  and 
erroneous  doctrines,  1  have  taken  great  care  not  to  omit  any 
important  facts  that  are  necessary  to  his  acquirement  of  as 
clear  an  idea  as  possible  of  the  principles  of  Physiology. 

I  have  not  hesitated  to  lay  unwonted  stress  upon  those 
points  which  many  years'  practical  experience  as  a  teacher  and 


vm  PREFACE. 

an  oxainiiier  has  sliowii  inc  arc  difficult  to  grasp  and  are  com- 
monly misunderstood  ;  and  1  have  treated  such  subjects  as  are 
useful  in  ihe  practice  of  medicine  or  surgery,  more  fully  than 
those  which  are  essential  only  to  abstract  physiological  knowl- 
edge. 

As  medical  students  are  generally  obliged  to  commence  the 
study  of  Physiology  without  any  anatomical  knowledge,  I 
believe  it  to  be  absolutely  necessary  that  their  first  physiologi- 
cal book  should  contain  some  account  of  the  structure  and 
relationships  of  the  organs,  the  functions  of  which  they  are 
about  to  study.  I  have  therefore  added  a  short  account  of  the 
construction  of  the  various  })arts  discussed  in  each  chapter;  it 
has,  however,  been  found  necessary  to  curtail  this  anatomical 
portion  to  a  mere  introductory  sketch.  Numerous  illustrations, 
with  full  descriptions  attached  to  each,  are  introduced  to  sup- 
plement the  explanation  given  in  the  text. 

So  far  as  is  consistent  with  an  accurate  treatment  of  the  sub- 
ject, I  have  avoided  technical  terms  and  scientific  modes  of 
expression.  I  know  that  in  attempting  to  explain  physiologi- 
cal truths  in  every-day  language  and  in  a  plain  common-sense 
way,  I  run  the  risk  of  appearing  to  lack  the  precision  that 
such  a  subject  demands ;  but  after  mature  consideration  Lhave 
come  to  the  conclusion  that  great  scientific  nicety  and  a  scho- 
lastic style  of  expression  have  a  deterrent  effect  upon  the  be- 
ginner's industry ;  and  I  think  it  better  that  he  should  acquire 
the  first  principles  of  the  science  in  homely  language,  than  pick 
up  technical  odds  and  ends  in  learned  terms,  the  meaning  of 
which  he  does  not  comprehend. 

As  many  words,  strange  to  the  first  year's  student,  have  to 
be  used  and  must  be  learned,  it  has  been  thought  advisable  to 


PREFACE.  IX 

add  a  short  glossary,  containing  an  explanation  of  the  most 
ordinary  physiological  expressions. 

Great  difficulty  is  always  found  in  fixing  upon  a  starting 
point  at  which  to  begin  the  study  of  Physiology.  To  begin 
with  the  circulation  of  the  blood,  which  is  so  essential  for  the 
life  of  every  tissue,  one  should  have  some  knowledge  of  nerve 
and  muscle.  To  begin  with  nerves  and  muscles  the  mechan- 
isms and  the  uses  of  the  blood  current  should  be  understood  ; 
and  so  on  throughout  the  various  systems,  which  are  so  inter- 
dependent that,  for  the  thorough  comprehension  of  any  one,  a 
knowledge  of  all  is  required. 

I  have,  therefore,  adopted  the  time-honored  plan  of  com- 
mencing with  the  vegetative  systems  and  following  the  course 
of  the  aliments  to  their  destination  and  final  application,  as  I 
believe  that  this  arrangement  is  open  to  as  few  objections  as 
any  other  known  to  me. 

I  wish  here  to  express  my  most  cordial  thanks  to  many 
friends  who  have  aided  me  with  kind  assistance  and  advice. 
I  am  deeply  indebted  to  Mr.  W.  Tyrrell  Brooks  for  the  great 
help  he  afforded  me  by  compiling  the  chapters  on  Develop- 
ment; and  I  feel  I  cannot  sufficiently  thank  Mr.  E.  F.  Herroun 
for  his  untiring  and  valuable  assistance  in  the  revision  of  the 
proof-sheets. 

To  Mr.  G.  Hanlon  I  am  indebted  for  the  careful  and  skilful 
manner  in  which  he  has  executed  the  new  woodcuts,  most  of 
which  he  had  to  copy  from  my  rough  drawings. 

King's  College,  London,  January,  1884, 


CONTENTS. 


CHAPTER  I. 

The  Objects  of  Physiology. 

PAGE 

Introductory  Definitions, 17 

Structural  and  Physical  Properties  of  Organisms,  .         .         .         .20 

Chemical  Composition, 21 

Vital  Phenomena, 23 

CHAPTER  ir. 
General  View  of  the  Structube  of  Animal  Organisms. 


Cells 

.     25 

Protoplasm,  Nucleus.  Cell-wall, 

.     27 

Cell-contents 

.     29 

Varieties  of  Cells, 

.     30 

Modifications  of  Original  Cell-tissues,    . 

.    32 

I.  Epithelial  Tissues, 

.    35 

II.  Nerve  Tissues, 

.    39 

III.  Muscle  or  Contractile  Tissues, 

.    42 

IV.  Connective  Tissues, 

.    45 

CHAPTER  III. 

Chemical  Basis  op  the  Body. 

Elements  in  the  Body 54 

Classification  of  Ingredients  found  in  llie  Tissue.s,         .        .        .        .56 

Plasmata, .56 

Albuminous  Bodies,        ..........     59 

Classification  of  Albumins,    .........     60 

.\lbuniinoid8, 63 


CONTENTS. 


PACE 

Products  of  Tissue  Change, 05 

Carbohydrates, GO 

Fats '71 

Inorganic  Bodies,   .         .         .         .         .         .         .         .         .         .         .71 


CHAPTER  iV. 

The  Vital  Characters  of  Organisms. 

Protoplasmic  Movements 74 

Keproduction,         ...........  78 

Bacteria, 81 

Amoeba, 83 

Paramaecium, 80 

CHAPTER  V. 

Nutrition  and  Food  Stuffs. 

Classification  of  Foods, 91 

Composition  of  Special  Forms  of  Food, 93 

Milk, 95 

Cheese,  Meat,  Eggs,  etc., 97 

Vegetables, 99 

CHAPTER  VI. 

The  Mechanism  of  Digestion. 

Mastication, '  104 

Deglutition, 100 

Nervous  Mechanism  of  Deglutition, Ill 

Vomiting, .  115 

Movements  of  the  Intestines, 117 

Defecation, 119 

Nervous  Mechanism  of  the  Intestinal  Motion, 121 

CHAPTER  VII. 

Mouth  Digestion. 

Salivary  and  Mucous  Glands 125 

Characters  of  Mixed  Saliva, 129 


CONTEXTS.  XUl 

PAGE 

Nervous  Mechanism  of  Secretion  of  Saliva, 130 

Changes  in  the  Glaud-cells, 137 

Functions  of  the  Saliva,        .         .         .         .         .         .         .         ...  140 

CHAPTER  VIII. 

The  Stomach  Digestion. 

The  Gastric  Glands, .143 

The  Characters  of  Gastric  Juice, 145 

Mode  of  Secretion  of  Gastric  Juice,     .......  147 

Action  of  the  Gastric  Juice,          . 149 

CHAPTER  IX. 

Pancreatic  Juice. 

Structure  of  the  Pancreas, 155 

Characters  and  Mode  of  Secretion  of  Pancreatic  Juice,       .         .         .  156 

Changes  in  the  Gland-cells, 158 

Action  of  Pancreatic  Juice  on  Proteids,       .         .         .         .         .         .  160 

Action  on  Fats, 161 

Action  on  Starch, 162 

CHAPTER  X. 

Bile. 

Functions  of  the  Liver, 163 

Structure  of  the  Liver, 164 

Composition  and  Method  of  obtaining  Bile, 169 

Method  of  Secretion  of  Bile, 173 

Functions  of  the  Bile, 175 

CHAPTER  XL 

Functions  of  the  Intestinal  Mucous  Membrane. 


Structure  of  the  Small  Intestines, 
Method  of  obtaining  Intestinal  Secretion,    . 
Characters  and  Functions  of  the  Intestinal  Juice, 
Functions  of  the  Large  Intestine, 
Putrefactive  Fermentations  in  the  Intestine, 


178 
180 
181 
183 

•  184 


XIV 


CONTENTS. 


CHAPTER  Xri. 


Absorption 


Interstitial  Absorption, 
The  Lymphatic  System, 
Stnicture  of  Lymphatic  Glands, 
Intestinal  Absorption,  . 
Mechanism  of  Absorption, 
Materials  Absorbed, 
Lymph  and  Chyle, 
Movement  of  the  Lymph, 


PAGE 

186 
187 
190 
195 
198 
200 
203 
205 


CHAPTER  XIII. 

The  Constitution  of  the  Blood  and  the  Blood-plasma. 

General  Characters  of  the  Blood, 210 

Amount  of  Blood  in  the  Body, 211 

Physical  Construction  of  the  Blood,  Blood-plasma,      .         .         .         .213 

Chemical  Composition  of  Plasma,        .......     216 

Preparation  and  Properties  of  Fibrin,  .         .         .         .         .         .218 

Serum,. ,         .218 


CHAPTER  XIV. 

Blood-Corpuscles. 

Proportion  of  Red  to  White. 220 

White  Blood-cells, 221 

Origin  of  the  Colorless  Blood-cells, 223 

The  Red  Corpuscles,  Sizes  and  Shapes, 224 

Action  of  Reagents  on  Red  Corpuscles, 227 

Method  of  counting  Corpuscles, ;         .  230 

Chemistry  of  the  Coloring  Matter  of  the  Blood, 231 

Spectra  of  Haemoglobin, 234 

Haematin, 235 

Development  of  the  Red  Disks, 237 

The  Gases  of  the  Blood, 239 

CHAPTER  XV. 

Coagulation  of  the  Blood. 


Formation  of  the  Blood-clot, 
Circumstances  influencing  Coagulation, 


241 
244 


CONTENTS.  XV 

PAGE 

The  Cause  of  Coagulation, 245 

Coagulation  in  the  Vessels, 247 

Formation  of  Fibrin,    ...» 248 

CHAPTER  XVI. 

The  Heart. 

Pulmonary  and  Systemic  Circulations, 251 

Method  of  the  Circulation  of  the  Blood, 252 

The  Heart, 256 

Arrangement  of  Muscle  Fibres, 256 

Minute  Structure  of  the  Heart, 258 

Action  of  the  Valves, 260 

Movements  of  the  Heart,      .......                 .  262 

Cycle  of  the  Heart-beat, 263 

The  Heart's  Impulse, 266 

Heart-sounds, 269 

Innervation  of  the  Heart, 271 

Local  Centres, 272 

Inhibitory  Nerves, 276 

Acceleration  Nerves,    ........                 .  277 

Afferent  Cardiac  Nerves, 278 


CHAPTER  XVII. 

The  Bloodvessels. 

Structure  of  the  Vessels, 279 

The  Capillaries, 281 

Relative  Capacity  of  tlie  Vessels,          .......  283 

Physical  Forces  of  the  Circulation,       .......  285 

The  Blood-pressure,      ..........  287 

Measurement  of  Blood-pressure, 291 

Variations  in  the  Blood-pressure, 295 

Influence  of  Respiration  on  the  Blood-pressure,  .....  297 

The  Arterial  Pulse, 302 

Methods  of  obtaining  Pulse-tracings, 304 

Variations  in  the  Pulse, .         .  307 

Velocity  of  the  Blood-current 308 

Controlling  Mechanisms  of  the  Bloodvessels, 312 


XVI  CONTENTS. 

CHAPTER  XVIII. 

The  Mechanism  of  Respiration. 

PAGE 

Gas  Interchange,  .         .         . 318 

Structure  of  the  Lungs  and  Air-passages, 321 

The  Thorax 324 

Thoracic  Movements, .  326 

Inspiratory  Muscles,     ....;.....  329 

Expiration, 332 

Function  of  the  Pleura,         .........  334 

Pressure  Diflerences  in  tlie  Air,   ........  336 

The  Volume  of  Air, 337 

Nervous  Mechanism  of  Respiration, 339 

Modified  Respiratory  Movements,         . 345 


CHAPTER  XIX. 

The  Chemistry  of  Respiration. 

Composition  of  the  Atmosphere, 347 

Expired  Air, 348 

Changes  the  Blood  undergoes  in  the  Lungs,         .....  350 

Gases  in  the  Blood, 352 

Internal  Respiration, 355 

Respiration  of  Poisonous  Gases, 356 

Ventilation, 357 

Asphyxia,     ............  358 

CHAPTER  XX. 

Blood-Elaborating  Glands. 

Ductless  Glands, 361 

Supra-renal  Capsule, 362 

Thyroid  Body, 363 

Thymus, 364 

Spleen, 364 

Functions  of  the  Spleen, 365 

Glycogenic  Function  of  tlie  Liver,       .......  369 

Glycogen 372 


CONTENTS.  XVU 

CHAPTER  XXI. 

Secretions. 

PAGE 

Lachrymal  Glands, 374 

Mucous  Glands, 375 

Sebaceous  Glands,         ..........  377 

Mammary  Glands, 378 

Composition  of  Milk, 380 

Sudoriferous  Glands, 383 

Cutaneous  Desquamation,     .....         ....  385 

CHAPTER  XXII. 

Urinary  Excretion. 

Structure  of  the  Kidneys, 388 

Bloodvessels  of  the  Kidneys, 389 

Urine, 391 

Method  of  Secretion  of  tiie  Urine,         .......  393 

Chemical  Composition  of  the  Urine, 397 

Urea, 398 

Uric  Acid, 399 

Kreatinin,  Xanthin,  Hippuric  Acid,  etc.,     ......  400 

Coloring  Matters  and  Inorganic  Salts, 401 

Abnormal  Constituents, 403 

Urinary  Calculi, 404 

Source  of  Urea,  etc., 405 

Nervous  Mechanism  of  the  Urinary  Secretion, 407 

Outflow  of  Urine, 408 

Nervous  Mechanism  of  Micturition, 411 

CHAPTER  XXIII. 

Nutrition. 

Tissue  Changes  during  Starvation, 414 

Food  Requirements, 417 

Ultimate  Uses  of  Food-stuffs, 423 

CHAPTER  XXIV. 

Animal,  Heat. 

Warm-  and  Cold-blooded  Animals, 425 

Variations  in  the  Body  Temperature, *  426 


XV  111  CONTENTS. 

PAGE 

Mode  of  Production  of  Animal  Heat, 428 

Income  and  Expenditure  of  Heat, 429 

Maintenance  of  Uniform  Temperature, 432 

CHAPTER  XXV. 

Contractile  Tissues. 

Histology  of  Muscle, 439 

Properties  of  Muscle  in  the  Passive  State, 442 

Electric  Phenomena  of  Muscle,    ........  445 

Active  State  of  Muscle, 448 

Muscle  Stimuli, •         .        .        .        .  449 

Changes  occurring  in  Muscle  on  its  entering  the  Active  State,     .         .  452 

Muscle  Contraction, 457 

Graphic  Method  of  recording  Contraction, 458 

Tetanus,  Fatigue,  etc., 465 

Kigor  Mortis, '.         .  471 

Unstriated  Muscle, 473 

CHAPTER  XXVI. 

The  Application  of  Skeletal  Muscles. 

General  Arrangements, 474 

Joints, 475 

Standing, 478 

Walking  and  Running, 481 

CHAPTER  XXVII. 

Voice  and  Speech. 

Anatomical  Sketch, 483 

Mechanism  of  Vocalization,          . 486 

Properties  of  the  Human  Voice, 488 

Nervous  Mechanism  of  Voice, 491 

Speech, 491 

CHAPTER  XXVIII. 

General  Physiology  of  the  Neryous  System. 

Anatomical  Sketch,       ..........  494 

Functional  Classification, 496 

Chemistry  and  Electric  Properties  of  Nerves,      .....  498 


COXiENTS.  XIX 

PAGE 

The  Active  State  of  Nerve-fibres, 499 

Nerve  Stimuli, 499 

Velocity  of  Nerve-impulse, 502 

The  Electric  Changes  in  Nerves, 504 

Electrotonus,         ...........  504 

Irritability  of  Nerve-fibres, 506 

Law  of  Contraction, 509 

Nerve  Corpuscles  and  Terminals, 511 

Functions  of  the  Nerve  Cells, 513 


CHAPTER  XXIX. 

Special  Physiology  op  Nerves. 

Spinal  Nerves, 517 

The  Cranial  Nerves, 518 

The  Trochlear  Nerve, 521 

Portio  Dura,  etc., 522 

Eflferent  and  Afferent  Fibres, 524 

Ganglia  of  the  Fifth  Nerve, .526 

The  Glosso-pharyngeal  Nerve, 527 

The  Vagus  Nerve, 528 

The  Hypoglossal  Nerve 531 

CHAPTER  XXX. 

Special  Senses. 

Skin  Sensations, 535 

Nerve  Endings, 536 

Sense  of  Locality, 539 

Sense  of  Pressure,         .        .        '. 542 

Temperature  Sense, 543 

General  Sensations, 545 

CHAPTER  XXXI. 

Taste  and  Smell. 

Sense  of  Taste, 548 

Sense  of  Smell 551 


CONTENTS. 


CHAPTER  XXXII. 


Vision. 


The  Construction  of  the  Eyebiill, 

Dioptric  Media  of  tiie  Eyeball, 

Structure  of  the  Lens,  . 

The  Dioptrics  of  the  Eye,     . 

Accoinnioilatioii,  . 

Defects  of  Accommodation, . 

Defects  of  Dioptric  Apparatus, 

The  Iris,        .... 

The  Ophthalmoscope,  . 

Visual  Impressions, 

The  Fnnction  of  the  Ketina, 

Color  Perceptions, 

Mental  Operations  in  Vision, 

Movements  of  the  Eyeballs, 

IJinocular  vision,  . 


PAGE 

555 
558 
560 
502 
568 
571 
572 
574 
577 
578 
579 
586 
589 
590 
592 


CHAPTER  XXXIII. 

Hearing. 

Sound, 594 

Conduction  of  Sound  vibrations  through  the  Outer  Ear,      .         .         .  598 

Conduction  through  the  Tympanum, 601 

Conduction  through  the  Labyrinth, 603 

Stimulation  of  the  Auditory  Nerve,      . 607 

CHAPTER  XXXIV. 

Central  Nervous  Organs. 

Nerve  Cells, 611 

The  Spinal  Cord  as  a  Conductor, 614 

The  Spinal  Cord  as  a  Collection  of  Nerve  Centres,      ....  616 

Special  Reflex  Centres, 625 

Automatism, .  626 


CHAPTER  XXXV. 

The  Medulla  Oblongata. 


The  Medulla  Oblongata  as  a  Conductor, 
The  Respiratory  Centre, 


628 
630 


CONTENTS. 


XXI 


The  Yaso-motor  Centre, 
The  Cardiac  Centre, 


PAGE 

632 
634 


CHAPTER  XXXVI. 

The  Brain. 

The  Mesencephalon  and  Cerebellum, 637 

Crura  Cerebri, 640 

Basal  Ganglia, 641 

Cerebral  Hemispheres,          .........  644 

Localization  of  the  Cerebral  Functions, 646 

CHAPTER  XXXVII. 

Eeproduction. 

Origin  of  Male  and  Female  Generative  Elements,       ....  650 

Menstruation  and  Ovulation, 653 

Changes  in  the  Ovum  subsequent  to  Impregnation,      ....  656 

Formation  of  the  Membranes, 660 

The  Placenta, 666 


CHAPTER  XXXVIII. 


Development. 

Development  of  the  Vertebral  Axis, 671 

Development  of  the  Central  Nervous  System, 

677 

The  Alimentary  Canal  and  its  Appendages, 

683 

The  Genito-urinary  Apparatus,    . 

689 

The  Blood-vascular  System,          .         .         . 

696 

Development  of  the  Eye,     .... 

710 

Development  of  the  Ear,      .... 

715 

Development  of  the  Skull  and  Face,   . 

718 

Glossary .    723 

Index, 

737 

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CORRESPONDING  DEGREES   IN 

THE 

FAHRENHEIT  AND 

CENTIGRADE  SCALES. 

Fahr. 

Cent. 

Cent. 

Fahr. 

500°  

260°.0 

100°  

212°.0 

450°  

232°.2 

98°  

208°.4 

400°  

204°.4 

■      96°  

204°.8 

350°  

176°.7 

94°  

201°.2 

300°  

148°.9 

92°  

197°.6 

212°  

100°.0 

90°  

194°.0 

210°  

98°.9 

88°  

190°.4 

205°  

96°.l 

86°  

186°.8 

200°  

93°.3 

84°  

183°.2 

195°  

90°.5 

82°  

179°.6 

190°  

87°.8 

80°  

176°.0 

185°  

85°.0 

78°  

172°.4 

180°  

82°.2 

76°  

168°.8 

175°  

79°.4 

74°  

165°.2 

170°  

76°.7 

72°  

161°6 

165°  

73°.9 

70°  

158°.0 

160°  

71°.l 

68°  

1.54°.4 

155°  

68».3 

66°  

150°.8 

150°  

65°.5 

&4°  

147°.2 

145°  

62°.8 

62°  

143°.6 

140°  

60°.0 

60°  

140°  .0 

135°  

57«.2 

58°  

136°.4 

130°  

54°.4 

.56°  

132°.8 

125°  

51°.7 

54"  

129^.2 

120=  

48°.9 

.52°  

12.->°.6 

115°  

46°  1 

50°  

122°.0 

110°  

43°.3 

48°  

118°.4 

105°  

40°.5 

46°  

114°.8 

100°  

37°.8 

44°  

in°.2 

95°  

35°.0 

42°  

107°.6 

90°  

32°.2 

40°  

lfM°.0 

85°  

29°.4 

38°  

100°.4 

80°  

26°.7 

36°  

%°.8 

75°  

23°.9 

34°  

93°.2 

70°  

21°.l 

32°  

89°.6 

65°  

18°3 

30°  

86°.0 

60°  

15°.o 

28°  

82°.4 

55°  

12°.8 

26°  

78°.8 

50°  

10°.0 

240 

75°.2 

45°  

7°.2 

22°  

71°.6 

40°  

4°.4 

20«  

68".0 

35°  

1°.7 

18«  

M".4 

32°  

0°.0 

16"  

60".8 

30°  

—  l°.l 

14°  

57".2 

25°  

—  3°.9 

12"  

53".6 

20°  

—  6°.7 

10"  

50°.0 

15°  

—  9°.4 

80  

46°  .4 

10°  

— 12°.2 

6°  

42°.8 

5°  

-15°.0 

4°  

39».2 

0°  

— 17°.8 

2«  

35°.6 

—  5°  

— 20°.5 

0"  

32^.0 

—10°  

-23°.3 

—  2"  

28°  .4 

-15°  

— 26°.l 

—  4°  

24°.8 

-20°  

— 28°.9 

—  6°  

21".2 

—25°   

— 31°.7 

—  8°  

17°.6 

—30°  

— 34°.4 

—10"  

14°.0 

—35°  

— 37°.2 

—12"  

10°.4 

—10°  

-40°.0 

—14°  

6°.8 

—^°  

-42°.8 

—16°  

3".2 

—50°  

— 45°.6 

-18"  

-0".4 

—20°  

— 4°.0 

MANUAL  OF  PHYSIOLOGY, 


CHAPTER  I. 

THE  OBJECTS  OF  PHYSIOLOGY. 

Biology,  the  science  which  deals  with  living  beings  and  the 
phenomena  exhibited  by  them,  may  be  divided  into  two  great 
branches,  viz. : 

1.  Morphology,  which  treats  of  the  forms  and  structure  of  the 
bodies  of  living  creatures. 

2.  Physiology,  which  attempts  to  explain  the  modes  of  activity 
exhibited  by  them  during  their  lifetime,  and  may,  therefore,  be 
defined  asthescience  which  investigates  the  phenomena  presented 
by  the  textures  and  organs  of  healthy  living  beings;  or,  in  short, 
the  study  of  the  actions  of  organisms  in  contradistinction  to  that 
of  their  shape  and  structure. 

The  organic  or  living  world  is  naturally  divided  into  the  An- 
imal and  Vegetable  kingdoms.  We  have,  therefore,  both  animal 
and  vegetable  morphology  and  physiology.  In  studying  the  veg- 
etable kingdom,  the  form  and  the  structure,  as  well  as  the  activity 
of  plants,  are  associated  together  in  the  science  known  as  Botany. 
The  physiology  of  plants  may,  therefore,  here  be  omitted ;  though, 
indeed,  it  cannot  be  neglected  in  considering  the  processes  be- 
longing to  animal  life.  On  the  other  hand,  the  morphology  and 
the  physiology  of  animals  are  commonly  taught  separately,  and 
in  the  medical  curriculum  are  made  distinct  subjects. 

Morphology  properly  includes  the  external  form,  the  general 
construction  or  anatomy  of  organisms,  and  the  minute  structure 
of  their  textures  as  revealed    by   the  microscope.     This   latter 

2 


18  MANUAL   OF   PHYSIOLOGY. 

l)niiich  of  study,  under  tlie  name  Histology,  has  now  developed 
into  a  very  wide  subject,  which  is  inseparable  from  either  physi- 
(>l()<ry  or  auatoniy.  In  this  country  histology  is  commonly 
tau;,'ht  in  the  medical  schools  with  physiology,  for  while  the  time 
of  the  teachers  of  morphology  is  occupied  in  expounding  the  de- 
tails of  descriptive  anatomy,  the  microscope  is  in  everyday  use  in 
the  physiological  laboratory.  Moreover,  an  adequate  knowledge 
of  microscopic  methods,  and  of  the  various  form-elements  of  the 
diflerent  textures  of  the  body  is  one  of  the  first  essentials  for  phy- 
siological study. 

As  the  different  actions  of  the  body  are  performed  by  different 
tissues,  which  in  the  higher  animals  are  grouped  together  as  dis- 
tinct organs,  a  general  idea  of  the  position  and  construction  of 
these  different  parts  of  the  body  must  be  acquired  before  the 
study  of  physiology  can  be  commenced.  Anatomy  and  general 
morphology  are  the  frameworks  upon  which  physiological  knowl- 
edge is  built  up.  Some  knowledge  of  these  subjects  must  there- 
fore precede  the  study  of  physiology,  in  order  that  the  student 
may  be  in  a  position  to  grasp  even  the  simplest  facts  connected 
with  any  physiological  question. 

We  shall  soon  find  that  the  assistance  of  other  sciences  is  al.-o 
indispensable  to  physiology.  Thus  every  action  of  a  living  texture 
or  tissue  is  accompanied  by  some  chemical  change,  the  chemical 
process,  in  fact,  being  commonly  the  essential  part  of  the  phe- 
nomenon. The  student  of  physiology  must,  then,  know  something 
of  the  science  of  chemistry  ;  indeed,  the  mode  of  action  of  chemi- 
cal elements  forms  quite  as  important  a  groundwork  for  the 
study  of  the  activity  of  the  living  tissues  as  their  general  form 
or  minute  structure. 

Further,  the  laws  which  govern  the  motions  of  inanimate  bodies 
also  control  the  actions  of  living  tissues,  for  we  cannot  claim  to 
understand  or  recognize  the  existence  of  any  laws  aflfectiug  living 
organisms  other  than  those  known  to  be  applicable  to  dead  matter. 
There  are  a  great  number  of  activities  shown  by  living  textures 
which  we  cannot  explain  by  the  recognized  law's  of  nature.  We 
therefore  use,  for  convenience  sake,  the  term  "  vital  phenomena," 
to  indicate  processes  which  are  beyond  our  present  chemical  and 


THE   OBJECTS   OF    PHYSIOLOGY.  19 

physical  knowledge.  In  using  this  term  we  must  not  think  it 
implies  a  separate  set  of  laws  of  life.  We  cannot  discover  or  for- 
mulate any  special  laws  affecting  living  beings  only,  and  therefore 
we  must  not  assume  that  any  such  exist.  We  must  rather  en- 
deavor to  explain  all  the  so-called  "  vital  phenomena  "  by  means  of 
the  laws  known  to  chemists  and  physicists.  By  this  means  we 
shall  certainly  get  a  closer  insight  into  the  processes  of  life,  and 
if  there  be  laws  governing  the  living  beings  we  may  learn  to  know 
them.  This  method  of  working  has  already  given  good  results, 
for  within  comparatively  recent  times  many  of  the  processes  which 
were  regarded  as  specially  vital  in  character  have  been  shown  to 
be  within  the  power  of  the  experimenter  and  to  depend  on  purely 
physico-chemical  processes. 

It  is  therefore  necessary  for  the  physiologist,  before  he  attempts 
to  explain  the  activities  of  any  organism,  to  be  familiar  with  not 
only  the  structure  of  its  body  but  also  the  various  laws  which,  as 
chemists  and  physicists  teach  us,  control  the  operations  of  inani- 
mate matter. 

The  sciences  of  chemistry  and  physics  may,  in  fact,  be  regarded 
as  the  physiology  of  inorganic  matter,  just  as,  when  chemistry  and 
physics  are  applied  to  the  elucidations  of  the  functions  of  living 
creatures  by  the  biologist,  the  study  is  called  physiology.  When 
we  consider  bow  far  the  chemist  and  the  physicist  still  are  from 
thoroughly  grasping  and  interpreting  all  the  phenomena  presented 
by  the  various  kinds  and  conditions  of  matter,  we  cannot  be  sur- 
prised that  those  who  attempt  to  explain  the  actions  of  living  be- 
ings find  many  processes  that  they  are  unable  to  comprehend. 
But  while  physiologists  frequently  make  use  of  the  convenient 
term  "  vital  phenomena,"  they  do  not  thereby  imply  the  existence 
of  a  special  living  force  or  any  kind  of  energy  peculiar  to  living 
creatures. 

The  final  end  of  physiology  is,  then,  not  yet  within  the  reach 
of  our  modern  methods  of  research.  To  explain  the  mode  of  ac- 
tivity of  living  beings,  and  grasp  the  exact  relation  borne  by  their 
living  phenomena  to  the  laws  which  govern  them,  is  a  task  of 
enormous  difficulty.  Indeed  the  manifestations  of  certain  ener- 
gies in  living  organisms  are  so  complicated  that  it  is  often,  if  not 


20  MANUAL   OF    PHYSIOLOGY. 

generally,  impossible  to  say  exactly  how  they  are  brought  about, 
and  we  are  therefore  obliged,  for  the  present  at  least,  to  be  satis- 
fied with  the  mere  recognition  and  description  of  the  phenomena. 
Since  the  human  organism  is  the  special  study  of  students  of 
medicine,  the  contents  of  this  volume-should  properly  be  restricted 
to  the  physiology  of  man.  But  human  physiology  cannot  be 
studied  alone :  because  in  man  we  cannot  watch  sufficiently  closely, 
or  question  fully,  by  experiment,  the  phenomena  of  life.  Further, 
no  sharp  line  of  separation  can  be  drawn  between  the  actions  of  the 
various  organs  of  man  and  those  of  the  lower  animals.  The  consid- 
eration of  the  physiology  of  those  animals  which  are  akin  to  man 
must  therefore  go  hand  in  hand  with  the  study  of  the  physiology 
of  man  himself.  Much  light  has  been  thrown  on  the  actions  of 
the  most  complex  textures  of  the  highest  animals,  by  the  obser- 
vation of  the  activities  of  the  lowest  organisms,  where  the  mani- 
festations of  life  may  be  carefully  watched  with  the  microscope  in 
the  living  animal  under  perfectly  normal  conditions. 

GENERAL  CHARACTERS  OF  ORGANISMS. 

The  term  organism,  which  is  commonly  used  as  having  the 
same  meaning  as  living  being,  owes  its  derivation  to  the  complex- 
ity of  structure  common  among  the  higher  forms  of  life,  which 
are  made  up  of  several  distinct  organs.  This  organic  construction 
no  longer  holds  good  as  a  distinguishing  mark  between  living 
beings  and  inanimate  matter,  because  we  are  acquainted  with  a 
vast  number  of  living  organisms,  both  plants  aud  animals,  which 
are  not  made  up  of  organs,  but  are  composed  of  a  minute  piece 
of  a  soft  jelly-like  material,  which  is  simply  granular  throughout, 
and  devoid  of  structural  differentiation  during  the  life  of  the 
creature. 

We  may  classify  the  general  characters  of  living  beings  as  fol- 
lows : 

1.  Structural  and  physical  properties. 

2.  Chemical  composition. 

3.  Activities  during  life  (vital  phenomena). 

1.  Structural  Characters  of  Organisms. — The  minute 
structure  of  living  beings  as  shown  by  the  microscope  no  doubt 


CHARACTERS    OF    ORGANISMS.  21 

helps  to  distinguish  the  textures  of  organisms  from  inorganic 
structures.  Although  organic  textures  are  found  to  differ  very 
widely  in  their  characters,  they  are  all  related  in  one  respect, 
namely,  that  at  some  period  of  their  existence  they  consisted  of 
cells,  i.e.,  minute  masses  of  a  substance  called  Protoplasm.  In 
plants  this  cellular  structure  remains  obvious  in  nearly  all  parts 
of  the  adult  organism,  no  matter  how  much  the  texture  may  be 
modified  by  adaptation  to  the  requirements  of  any  given  duty  or 
function.  If  we  examine  with  the  microscope  the  leaves,  bark, 
wood,  or  pith  of  a  plant,  in  all  of  them  a  cellular  structure  can  be 
recognized..  In  the  less  developed  members  of  the  animal  king- 
dom, and  during  the  initial  stages  in  the  existence  of  the  highest 
animals,  the  textures  are  composed  exclusively  of  aggregations  of 
living  cell-elements.  We  shall  shortly  see  that  in  the  more  fully 
developed  condition  of  the  higher  animals,  the  cells  become  vari- 
ously modified  in  form  and  function,  and  the  protoplasm  manufac- 
tures various  structures  adapted  to  the  performance  of  the  diverse 
functions  of  the  different  parts.  In  all  organic  textures  which  can 
be  said  to  be  living,  cells  are  dispersed  in  greater  or  less  number 
throughout  them,  and  regulate  their  nutrition  and  repair. 

2.  Chemical  Composition. — There  are  no  characters  in 
the  chemical  composition  of  the  textures  of  organic  beings  which 
can  be  said  to  be  absolutely  distinctive  or  to  separate  them  from  in- 
organic matter.  No  doubt  their  chemical  construction  commonly 
exhibits  certain  peculiarities,  not  seen  in  dead  matter,  which  may 
be  taken  as  characteristic,  but  living  textures  only  differ  in  the 
general  plan  of  arrangement  and  composition  from  that  most  com- 
monly met  with  in  the  construction  of  inorganic  materials. 

In  the  first  place,  the  great  majority  of  the  chemical  elements 
which  we  know  of,  take  no  share  in  the  formation  of  living 
creatures,  and  are  never  found  to  enter  their  composition.  Prac- 
tically only  fifteen  of  the  sixty-three  elements  known  to  chemists 
take  part  in  making  up  the  tissues  of  animals.  The  majority  of 
these  are  only  present  in  very  small  quantity  and  with  no  great 
constancy.  On  the  other  hand,  there  are  four  elements,  namely, 
carbon,  oxygen,  hydrogen,  and  nitrogen,  which  are  found  with 


22  MANUAL    OF    PHYSIOLOGY. 

such  great  regularity,  and  in  so  great  quantity,  that  they  may  be 
said  to  make  up  the  great  bulk  (97  per  cent.)  of  the  animal  frame. 
The  great  constancy  with  which  the  first  three  of  these  elements 
occurs  must  be  regarded  as  the  most  important  character  of  or- 
ganic tissues. 

Secondly,  in  organic  textures  the  chemical  elements  are  a.sso- 
ciated  iu  a  much  more  complex  and  irregular  proportion.  Gener- 
ally a  large  number  of  atoms,  of  each  element,  are  grouped 
together  to  form  the  molecule,  and  often  the  compound  is  so  com- 
plex that  its  chemical  formula  remains  a  matter  of  doubt.  As 
an  example,  a  remarkable  body,  called  lecithin,  which  appears 
iu  the  analysis  of  protoplasm  and  many  tissues,  may  be  mentioned  ; 
it  is  a  peculiar  compound  containing  nitrogen  and  phosphorus, 
and  in  construction  said  to  be  like  a  fat.  It  may  be  expressed 
thus: 

C\,H^NPO,. 

In  inorganic  substances,  on  the  other  hand,  the  elements  are 
found  to  be  combined,  as  a  general  rule,  in  simple  and  regular 
proportions.  The  molecules  are  made  up  of  but  few  elements 
arranged  in  a  definite  manner  and  firmly  bound  together,  so  that 
they  are  not  prone  to  undergo  spontaneous  decomposition.  As 
an  example,  we  may  take  water,   which   has   the   well-known 

formula, 

H,0. 

Though  these  bodies  may  be  taken  as  types  of  organic  and  in- 
organic substances  respectively,  it  must  not  be  imagined  that  all 
organic  bodies  are  as  complex,  irregular,  and  unstable  as  lecithin, 
or  that  inorganic  compounds,  as  a  rule,  are  invariably  simple  and 
stable  like  water. 

It  is  further  remarkable  that  Carbon — an  element  which  is 
exceptional  in  forming  but  few  associations  iu  the  mineral  world, 
where  it  chiefly  combines  with  oxygen  to  form  CO.^ — is  almost 
invariably  present  in  living  textures,  in  which  it  is  combined 
with  hydrogen  and  nitrogen  as  well  as  oxygen  in  various  propor- 
tions. The  constancy  of  carbon  as  an  ingredient  of  organic  bodies 
is  so  great  that  what  formerly  was  called  organic  chemistry  is 
now  often  called  the  chemistry  of  the  carbon  compounds. 


CHARACTERS   OF   ORGANISMS.  23 

These  complex  associatious  of  many  atoms  of  carbon  with  mauy 
atoms  of  other  elements,  are  readily  dissociated  when  exposed  to 
the  air  under  even  slightly  disturbing  influences.  When  heated 
to  a  certain  degree  they  burn,  i.e.,  unite  rapidly  with  the  oxygen 
of  the  air,  and  in  the  presence  of  minute  organisms  they  putrefy. 
Thus  instability  is  a  general  feature  comtnonly  met  with  in  most 
substances  of  organic  origin. 

Chemical  instability  reaches  the  highest  pitch  in  tissues  which 
are  actually  alive  and  engaged  in  vital  processes.  So  long  as  any 
texture  lives  it  must  constantly  undergo  certain  chemical  changes, 
one  of  which  is  regarded  as  a  kind  of  decomposition,  tending  to 
produce  disintegration,  and  the  other,  a  re-integration  by  means 
of  new  chemical  associations  with  fresh  materials.  A  tissue  may 
then  be  said  to  deserve  the  term  living,  only  as  long  as  it  under- 
goes these  antagonistic  chemical  changes.  The  tendency  to  de- 
structive oxidation,  or  disintegration  is  intimately  connected  with 
the  functional  activity  of  the  living  texture  and  increases  with 
this  activity.  The  re-integration  or  constructive  process  requires 
the  presence  of  suitable  materials  with  which  the  texture  may 
combine,  in  order  to  make  up  for  the  loss.  Thus  living  tissues 
are  ever  on  the  point  of  destruction,  which  can  only  be  warded 
off  by  the  timely  reconstruction  of  their  chemical  ingredients  by 
suitable  fresh  materials.  This  reconstruction  by  means  of  fresh 
matter  from  without  is  called  assimilation,  and  forms  the  most,  if 
not  the  only,  satisfactory  critlerion  by  which  adequately  to  dis- 
tinguish living  beings  from  inorganic  matters. 

The  whole  object  of  all  these  chemical  processes  is  to  supply 
suitable  fresh  materials  to  the  various  textures  for  their  assimi- 
lation. This  will  be  found  to  form  a  great  part  of  physiological 
study.  Further,  the  energy  manifested  in  the  living  activity  of 
the  textures  depends  upon  the  various  oxidizing  processes,  and 
the  exact  laws  which  govern  these  combustions,  and  the  results 
they  give  rise  to  in  the  various  tissues,  practically  make  up  the 
other  part  of  physiology. 

3.  Vital  Phenomena. — The  so-called  vital  phenomena 
which  take  place  in  the  textures  of  organisms  are,  for  the  most  part, 


24  MANUAL   OF   PHYSIOLOGY. 

performed  by  the  agency  of  the  living  cell-elements,  in  which  we 
can  recognize  independent  manifestations  of  life,  such  as  the  re- 
sponse to  stimuli,  motion,  nutrition,  growth,  etc.  The  living  ac- 
tivity of  organisms  requires  for  its  perfect  development  certain 
external  conditions,  namely,  a  certain  degree  of  warmth  and 
vioisture.  Without  heat  and  moisture  the  chemical  interchanges 
just  mentioned  cannot  go  on,  and  the  organism  is  either  destroyed 
or  remains  in  a  state  of  inactivity. 

The  nutrition  of  the  animal  body  which  is  accomplished  by 
means  of  the  processes  of  assimilation  already  mentioned  enables 
it  to  grow,  and,  up  to  a  certain  point,  increase  in  size,  and  further 
to  undergo  many  changes  in  form  and  texture.  There  is,  how- 
ever, a  limit  to  this  assimilative  power:  nutrition  loses  in  activity, 
growth  gradually  stops,  and  after  a  time  decay  appears  and  is  fol- 
lowed by  death. 

Thus  organisms  exist  only  for  a  limited  period  of  time,  during 
which  their  size,  form,  and  functional  activity  are  constantly 
undergoing  some  alteration  dependent  on  the  incessant  changes 
in  their  molecular  construction. 

This  cycle  of  changes  through  which  organisms  pass  we  speak 
of  as  their  lifetime.  During  this  lifetime,  at  the  period  when 
their  functional  activity  is  at  its  height,  they  possess  the  remark- 
able faculty  of  producing  individuals  like  themselves. 

This  is  accomplished  by  setting  apart  a  cell  which,  under  favor- 
able circumstances,  assumes  special  powers  of  growth,  increases  in 
size  by  the  rapid  formation  of  new  cells,  and  develops  into  an 
independent  living  unit.  In  time  it  arrives  at  maturity,  and  be- 
comes like  its  parent,  and  then  passes  through  the  same  cycle — 
by  its  power  of  assimilation  it  grows  to  maturity,  reproduces  its 
like,  decays,  and  dies. 


CHAPTER  ir. 


GENEKAL  VIEW  OF  THE  STRUCTURAL  CPIARACTERS  OF 
ANIMAL  ORGANISMS. 

The  parts  played  in  Physiology  by  cells  are  so  many  and  so 
important  that  it  is  necessary  at  the  very  outset  to  consider  their 
properties  somewhat  in  detail. 

Fig.  1. 


Cells  from  the  root  of  a  plant  (X550). — 1.  Showing  youngest  cells  witli 
thin  walls  (w),  filled  with  protoplasm  and  containing  nucleus  (n)  and  nu- 
cleolus (n^).  2.  Older  cells  with  thicker  walls  with  vacuoles  and  cell-sap 
(s).  3.  Shows  further  diminution  of  protoplasm  and  increase  in  cavity  (s) 
in  proportion  to  tiie  growth  of  the  cell  wall  (w). 

The  demonstration  of  the  cellular  structure  of  plants  was  first 
made  in  1832  by  a  distinguished  German  botanist  named  Schlie- 


26 


MANUAL   OF    PHYSIOLOGY. 


(Icn,  who  considered  the  cells  to  be  characteristic  of  plant  tissue. 
A  few  years  later  Schwann  showed  that  the  animal  tissues  were 
also  made  up  of  cells,  though  not  so  completely,  and  that  they 
owed  their  origin  and  development  to  cell  elements.  Thus  origi- 
nated the  cellular  theory,  which,  with  some  modification,  is  now 
the  basis  of  all  physiological  inquiry. 

The  first  idea  which  was  conveyed  by  the  term  cell  varied  much 
from  that  which  we  now^  accept  as  a  proper  definitiou  of  such  an 
organic  unit. 

Vegetable  cells  being  the  first  discovered  were  taken  as  the 
type  of  all.  The  main  characteristics  of  these  may  be  briefly 
summed  up.     Firstly,  a  membranous  sac  called  the  cell  wall,  geu- 


FiG.  2. 


Fig.  3. 


Fig.  2. — Diagram  of  animal  ceil  (ovum).  (Gegenbauer.) — a.  (jranular 
protoplasm,     b.  Nucleus,     c.  Nucleolus. 

Fig.  3. — Liver  cell  of  man,  containing  fat  globules  [b)  and  biliary  mat- 
ters.    (Cadiat.) 


erally  very  well  defined,  and  secondly,  within  the  cell  wall  various 
cell  contents.  Among  the  more  conspicuous  portion  of  the  latter 
may  be  mentioned  the  (1)  nucleus,  which  lies  in  a  soft,  clear, 
jelly-like  substance  called  protoplasm,  and  (2)  certain  cavities 
called  vacuoles,  which  are  filled  with  a  clear  fluid  or  "cell  sap." 
Further  investigation  of  the  life  history  of  cells,  particularly  in 
the  early  stages  of  their  development,  showed  that  the  cell  wall, 
which  played  so  important  a  part  in  the  original  conception  of  a 
cell,  was  not  always  present,  but  was  formed  by  the  protoplasm  in 
the  later  stage  of  growth.  The  cell  sap  and  other  matters  were 
found  less  commonly  present,  and  appeared  still  later  in  the  life- 
time of  the  vegetable  cell ;  hence  it  was  concluded  that  they  were 


STRUCTURAL   CHARACTERS    OF    ANIMAL    ORGANISMS.       27 

the  outcome  of  changes  due  to  the  activity  of  the  protoplasm,  and 
that  this  latter  was  the  only  real  essential  and  vital  part  of  the 
cell. 

Subsequently,  from  the  fact  that  some  vegetable  cells  in  the 
youngest  and  most  active  stage  of  their  growth  have  no  limiting 
wall,  and  that  most  animal  cells  have  none  during  any  part  of 
their  life,  it  was  proposed  to  define  a  cell  as  a  mass  of  protoplasm 
containing  a  nucleus.  But  further  research  showed  that  the  nu- 
cleus was  not  always  present.  In  many  cryptogamic  plants  no 
nucleus  can  be  found,  and  in  some  animal  cells,  which  must  be 
regarded  as  independent  individuals  (Protamoeba),  there  is  no 
nucleus  at  any  part  of  its  lifetime.  This  would  lead  us  to  suppose 
that  a  mass  of  protoplasm  capable  of  manifesting  all  the  phenomena 
of  life  would  be  a  sufficient  definition.  Though  this  is  probably 
correct  in  a  few  cases,  the  vast  majority  of  cells  do  contain  nuclei. 
As  it  is  difficult  to  divest  our  minds  of  the  connection  between 
the  two,  it  has  been  proposed  to  give  the  name  cytode  to  the  non- 
nucleated  forms,  which  certainly  are  very  exceptional,  reserving 
the  term  cell  for  the  common  nucleated  unit.  Each  part  of  the 
cell  may  now  be  considered  in  the  order  of  its  importance,  viz., 
protoplasm,  nucleus,  cell  wall,  and  cell  contents. 

I.  Protoplasm  is  a  colorless,  pale,  milky,  semi-translucent 
substance,  more  or  less  altered  in  appearance  by  various  foreign 
matters  which  lie  in  it.  These  latter  also  give  it  a  granular  ap- 
pearance, and  when  dead  it  commonly  exhibits  a  linear  marking 
or  fine  network.  During  life  its  consistence  is  nearly  fluid,  vary- 
ing with  the  circumstances  in  which  it  is  placed,  from  that  of  a 
gum  solution  to  a  soft  jelly.  When  living  unmolested  in  its  nor- 
mal medium  it  seems  to  flow  into  various  shapes,  but  this  is  a 
living  action  which  does  not  prove  it  to  be  diffluent,  for  any  at- 
tempt to  investigate  it  by  experiment  causes  a  change  in  its  con- 
sistence approaching  to  rigidity. 

As  the  full  comprehension  of  the  function  of  this  substance  lies 
at  the  root  of  the  greater  part  of  Physiology,  the  reader  is  referred 
for  a  detailed  account  of  its  properties  to  Chapter  III.  on  Vital 
Phenomena,  where  it  will  be  discussed  at  greater  length. 


28  MANUAL   OF   PHYSIOLOGY. 

II.  The  Nucleus. — ]\[ost  independent  masses  of  protoplasm, 
and  all  highly  organized  cells,  contain  one  or  more  nuclei  in  their 
substance.  The  nucleus  is  in  a  greater  or  less  degree  sharply  di- 
vided off  from  the  surrounding  protoplasm.  Its  presence  can 
generally  be  made  much  more  conspicuous  by  treating  the  cell 
with  various  chemical  reagents,  notably  dilute  acids  and  certain 
dyes.  The  nucleus  is,  in  the  former  case,  able  to  resist  the  action 
of  dilute  acetic  acid  for  a  much  greater  length  of  time  than  the 
remainder  of  the  cell,  so  that  it  stands  out  clearly,  while  the  rest 
becomes  quite  transparent.  In  the  latter  case,  magenta  (one  of 
the  aniline  dyes)  stains  the  nucleus  sooner  and  deeper  than  the 
protoplasm.  Although  it  has  been  accredited  with  special  inde- 
pendent movements,  it  may  safely  be  said  that  in  comparison  with 
the  protoplasm  it  is  not  very  contractile.  Yet  it  appears  to  be 
intimately  associated  with  the  vital  phenomena  of  the  cell,  and 
may  be  said  to  control  or  initiate  the  most  important  activities  of 
the  cell,  namely,  its  division.  The  small  size  of  the  nucleus  adds 
greatly  to  the  difficulty  of  investigating  its  functions,  and  much 
remains  to  be  made  out  concerning  both  its  structure  and  proper- 
ties, although  recently  considerable  progress  has  been  made  in 
this  direction. 

III.  The  Cell  Wall. — It  has  already  been  stated  that  the 
most  active  form-elements,  such  as  the  cells  in  the  earliest  stages 
in  the  life  of  an  organism  (embryonic  cells),  have  no  inclosing 
membrane  or  cell  wall.  But  in  the  more  advanced  stages  of  cell 
life  we  find  this  second  form  of  protoplasmic  differentiation  to  be 
common  enough.  In  animal  cells  the  limiting  membrane  has 
never  the  same  importance  as  the  cell  wall  in  vegetable  tissues, 
where  some  of  the  principal  textures  may  be  traced  to  a  direct 
modification  of  the  cell  wall,  still  recognizable  as  such.  When- 
ever such  a  limiting  membrane  does  exist,  it  is  always  formed  by 
the  outer  layers  of  protoplasm  undergoing  changes  so  as  to  become 
of  greater  consistence.  In  the  animal  tissues  the  protoplasmic 
units  form  various  structures,  which,  however,  do  not  hold  the 
relation  to  them  of  limiting  membranes,  but  rather  give  the  idea 
of  lying  between  the  cells.     Hence,  in  one  large  group  of  tissues, 


STRUCTURAL   CHARACTERS    OF    ANIMAL    ORGANISMS.       29 


Fig.  4. 


they  have  been  called  intercellular  substance,  while  in  others  they 
appear  as  materials  specially  modi6ed  for  the  furtherance  of  the 
functions  of  the  special  tissues. 

IV.  Cell  Contents. — Regarding  protoplasm  as  the  essential 
living  part  of  the  cell,  under  this  heading  will  come  only  those 
extraneous  matters  which  are  the  outcome  of  protoplasmic  ac- 
tivity. 

The  cell  contents  which  are  present  with  such  constancy  and 
in  such  variety  in  vegetable  cells,  form  in  them  an  all-important 
part ;  but  in  most  animal  cells  the  contents  do  not  occupy  such  a 
striking  position. 

No  doubt  animal  protoplasm  is  quite  as  capable  as  that  of  vege- 
tables of  making  out  of  its  own  substance,  or  the  nutriment  sup- 
plied to  it,  a  great  variety  of  mate- 
rials, but  these  are  seldom  stored 
in  such  large  quantities  in  animal 
cells  as  in  those  of  plants. 

In  the  cells  of  some  kinds  of 
animal  textures,  particularly  that 
called  Connective  Tissue,  we  com- 
monly find  large  quantities  of  fat 
formed  and  accumulated  to  such  a 
degree  in  the  cell  that  the  proto- 
plasm can  be  no  longer  recognized 
as  such.  Its  remnant  is  devoted 
to  forming  a  limiting  membrane  for 
the  fatty  contents,  so  that  the  cell 
is  converted  into  an  oil  vesicle,  and 
here  certainly  what  may  be  termed  the  contents  become  the  most 
important  part  of  the  cell.  In  various  gland  cells  also,  as  will 
be  seen  hereafter,  different  substances  are  made  and  stored  up 
temporarily  in  the  protoplasm,  and  these  can  be  seen  as  bright 
refracting  granules,  and  are  subsequently  discharged  in  the  secre- 
tion of  the  gland. 

In  other  cells  again  (liver)  nutrient  material  allied  to  starch 
may  be  deposited  in  considerable  quantity,  just  as  starch  is  stored 
in  certain  cells  of  a  plant,  but  owing  to  the  greater  and  more  con- 


Cell  from  connective  tissue  con- 
taining large  fat  globule  (a),  and 
showing  protoplasm  (p),  and  nu- 
cleus (n),  (m)  membrane.  (Ran- 
vier.) 


30 


MANUAL    OF    PHYSIOLOGY. 


slant  activity  of  animals,  the  amount  laid  by  never  attains  any- 
thing like  that  found  in  the  store  textures  of  vegetables,  where 
the  result  of  an  entire  summer's  active  work  is  put  by  as  a  pro- 
vision for  the  next  winter,  and  the  fresh  burst  of  energy  which 
follows  it  in  the  spring. 

But  while  the  above  are  all  more  or  less  temporary  contents  of 
cells,  we  have  an  example  of  a  permanent  deposit  in  them,  viz., 
Pigment.  This  substance  is  formed  by  the  protoplasm  in  various 
parts  and  has  a  special  physiological  use.  Thus  in  the  cells  of 
the  tissue  behind  the  retina — or  nerve  layer  of  the  eye-ball — the 
cells  are  filled  with  a  number  of  granules  composed  of  a  substance 
very  rich  in  carbon,  which  when  in  a  sufficiently  thick  layer  is 
able  completely  to  absorb  any  light  that  may  fall  upon  it,  thus 
j)reveutiug  the  reflections  that  would  otherwise  occur,  and  which 
would  interfere  with  the  clearness  of  sight. 

It  also  occurs  in  the  skin  of  the  negro  and  other  races,  and  in 
that  of  the  frog  and  other  animals,  but  in  these  its  function  is  not 
fully  known. 

Varieties  of  Cells. — Great  varieties  of  cells  are  found  in 
the  various  mature  tissues  of  the  higher  animals,  all  of  which  have 
passed  through  the  stage  of  being  a  simple  nucleated  mass  of  pro- 


FlG. 


Transverse  section  of  Ijlastoderm,  showing  the  elements  in  the  earlier  stage 
of  the  development. — A,  epiblast ;  B,  mesoblast;  C,  hypoblast. 

toplasra  in  the  earlier  periods  of  their  development  and  differentia- 
tion. All  cells  may  then  be  divided  into  two  chief  types,  the  indif- 
ferent and  the  differentiated. 

Under  the  category  of  indifferent  cells  may  be  placed  all  such 
as  retain  the  characters  of  the  first  embryonic  cell  and  have  not 
acquired  any  special  structure  or  property  by  which  they  can  be 


STEUCTURAL    CHARACTERS    OF    ANIMAL    ORGANISMS.       31 

distiuguished  from  the  simplest  form.  Such  cells  are  the  only 
ones  in  the  early  stages  of  the  embryo.  In  the  adult  tissues  they 
also  occur,  having  various  duties  to  perform.  They  are  mainly 
found  in  the  adult  in  the  blood  and  lymph,  and  scattered  through- 
out the  tissues,  and  are  without  a  cell  wall,  and  have  no  special 
contents  to  mark  their  function. 

Among  the  differentiated  cells  we  find  many  special  characters, 
adapting  them  to  certain  special  duties,  for  all  these  cells  are  mod- 
ified from  the  original  type  and  applied  to  the  performance  of  some 
special  function. 

Space  prevents  even  a  short  enumeration  of  the  varieties  of  cells 
met  with  in  the  tissues  of  plants,  where  they  carry  on  all  the  active 
function  of  the  organism  as  well  as  form  the  firm  supporting 
structures. 

The  differentiation  of  a  cell  is  accomplished  by  its  own  proto- 
plasm, which  forms  new  structural  parts  and  sometimes  seems  to 
diminish  in  quantity  until  finally  an  element  is  produced  in  which 
there  is  no  longer  any  protoplasm  recognizable. 

"We  find  then  matured  and  differentiated  cells  which  vary  — 

1.  In  shape,  being  spherical,  flattened,  fusiform,  stellate,  etc. 

2.  In  size,  becoming  smaller  or  larger. 

3.  In  their  mode  of  connection,  becoming  attached  in  one 

way  or  another  to  neighboring  cells  or  structures. 

Cells  may  also  be  classified  according  to  their  function,  e.g., 
Glandular,  Nervous,  etc.,  and  the  greater  portion  of  the  following 
pages  will  be  devoted  to  the  functions  of  these  various  forms  of 
cells. 

So  long  as  a  cell  remains  in  its  indifferent  stage  it  possesses  the 
properties  of  ordinary  protoplasm  only.  But  by  its  further  devel- 
opment it  acquires  special  properties  not  common  to  all  protoplasm. 
These  properties  may  or  may  not  be  accompanied  by  structural 
change.  Thus  the  protoplasm  of  a  gland  cell  differs  in  little  from 
that  of  any  other  cell  except  in  the  capabilities  of  its  nutritive 
changes  and  its  chemical  products  ;  while  on  the  other  hand,  those 
epithelial  cells  which  form  the  outer  layer  of  the  skin  lose  com- 
pletely their  protoplasmic  characters  and  are  profoundly  modified 
in  .structure. 


32 


MANUAL    OF    PHYSIOLOGY. 


P^iG.  G. 


Modifications  of  Original  Cell — Tissues. — In  the  pre- 
ceding pages  the  special  characters  of  a  single  ceil  liave  been  dwelt 
on,  and  it  has  also  been  seen  that  a  cell  may  become  changed  from 

its  original  form  in  order  to  fit  it 
more  perfectly  for  some  special  pur- 
pose. But  it  is  necessary  to  consider 
this  latter  fact  more  fully  in  order  to 
understand  the  relation  of  the  va- 
rious tissues  of  the  adult  body  to 
each  other. 

The  first  stage  in  the  existence  of 

any    organism,   from    the    simplest 

form  of  plant  to  man,  is  composed 

of  a  single  cell  (in  animals  called  the  ovum  or  egg),  which  differs 

in  no  essential  points  of  structure  from  an  ordinary  cell. 


Unicellular  organism.   Small 
amoeba.     (Cadlat.) 


Stages  in  the  division  of  the  egg  cell  (ovimi),  showing  the  production  of 
a  multiple  mass  by  division.     (Gegenbauer.) 

There  is  moreover  a  class  of  organisms  which  never  goes  beyond 
this  one-celled  stage,  and  the  individuals  pass  their  entire  lifetime 
in  the  state  of  a  simple  unicellular  organism.  This  group,  called 
Protista,  though  insignificant  in  point  of  its  size,  may  vie  with  the 
higher  plants  and  animals  in  number,  species,  and  variety  of  form, 


STRUCTURAL   CHARACTERS   OF    ANIMAL,   ORGANISMS.      33 


SO  that  they  might  well  be  placed  in  a  kingdom  by  themselves  (as 
has  been  proposed)  apart  from  the  vegetable  and  animal  kingdoms. 

The  group  of  these  organisms  which  most  resemble  animals, 
is  called  Protozoa,  and  is  divided  from  other  animal  forms  by 
the  manner  of  development  of  the  ovum  of  the  latter,  which 
differentiates  by  division  into  cells.  This  group  is  called  the 
Metazoa.  In  the  Protozoa  the  ovum  never  divides,  the  animal 
always  remaining  a  single  cell. 

On  the  contrary,  the  ovum  of  the  Metazoa  changes  its  charac- 
ters during  its  development.  At  first  possessing  a  stage  common 
to  both  divisions,  viz.,  a  single  cell,  it  soon  passes  through  rapid 
stages  of  cell  proliferation,  and  is  converted  into  a  multiple  mass, 
the  mulberry  stage  or  Morula. 

The  cells  forming  this  Morula  stage  work  towards  the  periphery 
of  the  mass,  where  they  tend  to  arrange  themselves  in  two  layers, 
at  the  same  time  forming  a  cavity  in  the 
centre.  This  is  known  as  the  Gastrula  stage. 
Following  then  this  cell  multiplication  or 
quantitative  differentiation,  we  find  a  quali- 
tative differentiation  of  the  cells,  by  which 
certain  groups  of  cells  assume  special  peculi- 
arities, fitting  them  for  some  specific  duty. 

Thus  we  arrive  at  the  production  of 
special  textures  and  organs  such  as  are  met 
with  in  the  higher  animals,  and  which  are 
necessary  for  the  efiicient  discharge  of  the 
various  functions  carried  on  during  their 
live.s.  The  division  of  the  original  mass  of 
indifferent  cells  into  two  layers  of  special 
cells  is  the  first  step  towards  tissue  differen- 
tiation, and  in  some  animals  is  the  only  one 
arrived  atin  their  entirelife  history  through- 


FiG.  8. 


Diagram  showing  the 
first  differentiation  of 
the  organism  into  an 
external  and  internal 
layer. — (a)  Mouth,  (b) 


enteric  cavity,  (d)  ecto- 
out  which  they  remain  a  simple  sac  made^derm,    (<;)    endoderm. 
up  of  an  external  layer,  Ectoderm,  and  an  .£  (Gegenbauer.) 
internal  layer,  Endoderm. 

The  groups  or  layers  of  cells  forming  the  outer  and  inner  layers 
of  this  stage  of  development,  not  only  form  the  primitive  tissues, 

3 


34 


MANUAL   OF   PHYSIOLOGY. 


but  they  also  represent  the  first  appearance  of  organs  or  parts 
with  a  specific  function.  The  external  or  ectodermic  layer  is  the 
supporting,  protecting,  motor,  and  respiratory  organ,  while  the 
inner  or  endodermic  layer  is  devoted  to  a  primitive  form  of  di- 
gestion, preparing  the  food  for  assimilation,  and  generally  pre- 
siding over  tlie  nutrition  of  the  body. 

Although  this  sac-like  (Gastrula)  stage  is  supposed  to  have 
formed  a  step  in  the  life  history  of  nearly  all  animals,  yet  it  forms 
a  less  striking  part  in  the  development  of  the  individuals  as  we 
ascend  the  scale,  and  in  the  higher  animals  no  such  stage  has  been 
recognized.     In  the  Vertebrates,  the  germ  cells  derived  from  the 

Fig.  9. 


Transverse  section  of  blastoderm  of  chick. — A.  Epiblast.     B.  Mesoblast. 
C.  Hypoblast.    j)r.  Primitive  groove. 

ovum  are  from  an  early  period  divided  into  three  distinct  layers, 
owing  to  the  layers  which  correspond  to  the  Ectoderm  and  Endo- 
derm  of  the  lower  organisms,  forming  between  them  a  third  layer 
or  Mesoblast. 

Fi'om  these  germinal  layers  all  the  organs  and  tissues  of  the 
body  are  subsequently  evolved.  In  embryological  language  the 
three  primitive  layers  are  called  epi-,  meso-,  and  hypoblast. 

Thus  it  can  be  seen  that,  as  we  can  compare  the  primitive  uni- 
cellular state  of  the  lowest  animals  with  the  first  egg-cell  stage  of 
existence  of  the  highest  animals,  so  we  can  compare  all  the  steps 
of  tissue  and  organ  differentiation  as  we  trace  them  in  the  embyro 
of  a  mammal,  with  the  steps  of  elaboration  in  organic  and  textural 
parts  that  we  find  in  ascending  the  scale  of  animal  life. 


STRUCTUEAL    CHARACTERS    OF    ANIMAL    ORGANISMS.       35 

The  history  then  of  the  development  of  any  mammal  from  a 
single  cell  or  egg  to  the  complex  adult  individual,  is  strictly 
analogous  with  the  more  protracted  history  of  the  evolution  of  the 
animal  kingdom  from  the  Protista  upwards. 

It  is  impossible  to  separate  the  differentiation  of  tissues  and 
organs,  or  to  say  which  is  of  older  date  in  the  history  of  animal 
evolution.  Even  in  unicellular  animals,  where  we  have  no  trace 
of  tissue  difference  (Paramgeciura,  Vorticella,  there  being  only  one 
cell),  we  have  a  distinct  foreshadowing  of  organ  and  functional 
differentiation  (vide  Chapter  III.).  And  in  creatures  made  of 
many  parts,  the  same  cells  have  several  duties  to  perform.  But 
when  an  aggregation  of  cell  units  exists,  it  may  be  said  that  a 
tissue  is  formed.  If  these  cells  be  indifferent,  that  is,  have  no 
special  characteristic,  then  the  tissue  may  be  called  primitive  or 
embryonic.  But,  as  has  just  been  stated,  the  aggregation  of 
embryonic  cells — in  the  higher  forms  of  life — have  special  char- 
acters from  the  very  first,  which  mark  them  off  from  one  another 
as  destined  for  different  functions. 

The  middle  germ  layer  (mesoblast)  is  derived  from  the  upper 
(epiblast)  and  lower  (hypoblast),  the  part  contributed  by  each 
being  doubtful.  From  the  first  the  middle  layer  has  distinctive 
characteristics,  and  ultimately  gives  rise  to  a  set  of  tissues  which 
can  always  be  distinguished  from  those  which  originate  from  the 
upper  and  lower  layers. 

From  the  inner  and  outer  germ  layers  are  formed  several  layers 
of  tissues,  which,  in  a  more  or  less  perfect  degree,  retain  the  activ- 
ity of  the  original  protoplasm,  and  hence  may  be  called  active 
tissues.  From  the  middle  germinal  layer  is  developed  a  set  of 
textures,  in  the  majority  of  which  the  protoplasmic  elements  are 
reduced  to  a  minimum,  and  are  therefore  grouped  together  as  sup- 
porting ti.«sues. 

The  tissues  formed  in  the  adult  may  be  classified  into  four 
groups : 

1.  Epithelial  Tissues.   The  primitive  surface  tissues  of  the 

outer  and  inner  germ  layers,  which  are  variously  modified 
for  several  distinct  duties. 

2.  Nerve  Tissues.   Springing  from  the  former,  are  modified 


36  MANUAL   OF   PHYSIOLOGY. 

for  receiviug,  conducting,  controlling,  and  distributing 
impressions. 

3.  Muscles  or  Contractile  Tissues.  In  close  relation  to  both,  the 
previous  and  the  next  groups. 

4.  Connective  tissues  formed  ouly  from  the  middle  germ  layer. 
They  are  much  modified  in  different  parts,  so  as  to  give 
shape  to  the  body,  and  to  support  and  hold  the  various 
organs  and  parts  firmly  together.  They  are  in  fact  the 
materials  used  in  the  general  body  architecture. 

Epithelial  Tissue,  although  the  oldest  kind  of  tissue  both  in 
the  animal  series  and  in  the  germinal  layers,  retains  the  embry- 
onic character  of  being  entirely  composed  of  cells  placed  in  close 
relationship  to  each  other  on  the  internal  and  external  surfaces 
of  the  body.  The  individual  cells,  moreover,  retain  the  embry- 
onic character  in  form  and  function,  being  soft  rounded  masses  of 
protoplasm,  only  altered  in  shape  by  the  pressure  of  their  neigh- 
bors. The  cells  which  lie  next  the  nutrient  vessels  of  the  meso- 
blast,  are  endowed  with  energetic  powers  of  growth  and  reproduc- 
tion. As  the  young  cells  are  produced  they  take  the  place  of  the 
parent  cell,  whose  future  life-history  determines  the  special  char- 
acters of  the  diflferent  kinds  of  tissues. 

Sometimes  the  cells  are  retained,  as  in  the  skin,  and  are  arranged 
in  several  layers,  one  over  the  other.  As  the  cells  are  conveyed 
from  the  deeper  layer,  where  they  take  their  origin,  towards  the 
surface,  the  eflTorts  of  the  waning  nutritive  power  of  the  protoplasm 
are  devoted  to  the  manufacture  of  a  tough  insoluble  substance. 
The  cells  thus  gradually  lose  their  vital  activities,  and  are  con- 
verted into  horny  scales,  which  form  the  external  protecting  skin, 
and  its  many  modifications  that  give  rise  to  the  diflferent  dermal 
appendages,  such  as  hair,  feathers,  etc.  Instead  of  a  horny  sub- 
stance, the  protoplasm  may  manufacture  fat  in  the  bodies  of  the 
cells,  and  the  adult  cells,  being  moved  on  by  the  young  cells  aris- 
ing beneath  them,  are  heaped  together  as  an  indefinite  mass  which 
passes  off  as  a  fatty  mixture.  In  other  cases  the  reproductive 
activity  of  the  cell  is  in  abeyance,  and  its  remaining  nutritive  en- 
ergy is  devoted  to  the  manufacture  of  a  material  which  is  poured 
out  of  the  cell  at  certain  periods.     Thus  we  have  another  great 


STRUCTURAL    CHARACTERS    OF    ANIMAL   ORGANISMS.       37 

function  performed  by  the  epithelial  tissues,  namely,  that  of  man- 
ufacturing certain  materials  which,  being  collected  by  suitable 
channels,  appear  as  secretions. 

The  active   elements  of  glandular  tissue  are  epithelial   cells 
whose  nutrition  seems  to  lead  to  the  formation  of  specific  chemical 

Fig.  10. 


Section  of  the  epiderm  of  the  prepuce,  showing  the  superimposed  layers 
of  cells  of  a  stratified  epithelium.  (Cadiat.) — a.  Young  proliferating  cells. 
b-d.  Ceils  advancing  towards  surface,  c.  Flattened  cell  of  horny  layer. 
/.  Basement  membrane,     g.  Connective  tissue. 


products  within  their  protoplasm.  These  products  pass  out  com- 
monly as  fluids,  and  form  various  substances  of  great  importance 
in  the  economy.  A  gland  then  is  simply  a  special  arrangement 
of  epithelial  cells,  generally  lining  the  sacs  or  tubes  into  which  the 
secretion  is  poured. 

The  covering  of  epithelium  is  in  various  places  found  to  be 


38 


MANUAL   OF   THYSIOLOGY. 


modified  in  different  ways,  so  as  to  suit  it  for  the  special  part  in 
which  it  is  placed.     Sonic  tracts  are  covered  with  fine  moving 


Fi(i.  11. 


Fig.  12. 


Fig.  11. — Two  cells  of  scaly  epithelium  from  the  inside  of  the  cheek. 
(Ranvier.) 

Fig.  12. — Section  of  milk  gland  of  cat,  showing  secreting  cells  containing 
fat  globules,  and  some  secretion  in  alveoli. 

hair-like  processes,  called  cilia,  which  give  rise  to  a  slight  motion 
of  the  fluids  in  contact  with  them. 


Fig.  13. 


Fig.  14. 


Fig.  13. — Ciliated  epithelial  cells  from  the  gills  of  mussel.     (Cadiat.) 
Fig.  14. — Stratified  ciliated   epithelial  cells  from  the  trachea  of  man, 
(Cadiat.) — a.  Large  surface  cells,  with  cilia  on  surface,     b.  Lower  cells  in 
earlier  stage  of  development,     e.  Cell  charged  with  mucus. 

Other  differences  will  be  given  in  detail  with  the  description  of 
the  duties  of  the  many  mucous  surfaces.     The  most  striking,  and 


STRUCTURAL    CHARACTERS   OF    ANIMAL    ORGANISMS.       39 

at  the  same  time  the  most  iuterestiug  modificatioos  are  those  in 
the  special  seuse  organs,  where  the  cells  are  in  immediate  connec- 
tion with  nerves,  and  aid  in  forming  the  special  nerve  terminals.* 


Fig.  l"). 


P^io.  IG. 


Fig.  15.— Epithelial  cells,  some  of  which  are  filled  with  mucus  (d),  form- 
ing goblet-like  cells.     (Cadiat.) 

Fig.  16. — Nenro-muscular  cells  of  hydra,  m.  Contractile  fibres.  (Klein- 
enberg.) 

Nerve  Tissue. — The  great  nervous  centres  are  formed  from 
certain  cells  of  the  outer  germinal  layer,  which,  in  the  earliest 
days  of  the  embryo,  dip  in  as  a  furrow,  and  are  gradually  cut  off 
from  the  parent  tissue  by  the  rapid  growth  of  the  middle  germ-layer. 
In  looking  for  special  conducting  tissue  in  animals  possessing  the 
irost  simple  structure,  we  find  cells  which  would  seem  to  possess 
certainly  a  two-fold,  and  possibly  a  three-fold  function,— one  of 
which  is  conduction.  In  the  so-called  "  neuro-muscular  "  cells  of 
the  hydra,  processes  are  described  as  found  to  pass  off  from  them, 
and  to  unite  beneath  the  ectoderm  with  other  fibre-like  processes, 
which  are  eminently  contractile.  Here  we  find  for  the  first  time 
a  portion  of  protoplasm  specially  devoted  to  acting  as  a  conductor 
of  impulses,  and  attached  by  the  one  end  to  a  contractile  fibre, 
and  by  the  other  to  a  surface  (sensory)  cell.  The  intimate  rela- 
tion between  the  development  of  nerve  and  muscle  fibres  is  thus 
established,  and  we  have  the  first  attempt  at  a  nerve  mechanism, 
viz.,  a  cell  capable  of  receiving  impressions,  and  a  fibre  capable 
of  transmitting  the  results  of  these  stimuli.  As  further  differen- 
tiation proceeds,  each  of  these  parts  becomes  more  distinct  from 


*  A  fuller  account  of  the  Histology  of  these  tissues  will  be  found  in  the 
chapters  .specially  devoted  to  tlicse  subjects. 


40 


MANUAL   OF   PHYSIOLOGY. 


the  other,  and  ultimately  the  adult  nerve  tissue  is  found  to  be 
made  up  (»f  nerve  or  ganglion  cells,  nerve  fibres,  and  special  nerve 
endings.  The  fibres  commonly  act  as  lines  of  communication  be- 
tween two  cells;  they  connect  together  the  numerous  cells  in  the 
various  parts  of  the  brain  and  spinaJ  cord,  or  pass  between  the 
cells  of  these  central  nerve  organs  and  special  cells  situated  through- 
out the  body,  which  might  be  called  the  peripheral  nerve  organs. 
The  simplest  idea  then  of  a  special  nerve  apparatus  is  a  fibre 
connecting  two  cells.  The  peripheral  cell  may  be  a  receiving  organ 
(fig.  17,  s),  from  which,  when  stimulated,  impulses  are  transmitted 


Fig.  17. 


Fig.  18. 


Fig.  17. — S.  Sensory  receiving  organ  with  attached  afferent  nerve  fibre. 
G.  Central  organs — ganglion  cells.  M.  Peripheral  organ  and  efferent 
nerve. 

Fig.  18. — Three  medullated  nerve  fibres,  the  medullary  sheath  of  which 
is  stained  dark  with  osmic  acid.  X.  Nodes  of  Ranvier. — Two  non-iuedullated 
nerve  fibres,  with  nuclei  in  the  primitive  sheath. 


along  the  fibre  to  the  central  nerve  cell,  where  they  give  rise  to 
certain  impressions,  and  so  we  have  a  sensory  nerve  apparatus. 
Or  the  central  nerve  cell  may  be  the  receiving  agent,  getting 
stimuli  from  its  central  neighbors,  and  transmitting  impulses  to 
a  peripheral  nerve  terminal,  by  which  the  energy  is,  as  it  were, 
handed  over  to  a  muscle  (m)  or  gland,  and  so  we  have  a  simple 


STRUCTUEAL   CHARACl^ERS    OF    ANIMAL    ORGANISMS.      41 


motor  or  secretory  apparatus.  Where  the  effect  of  a  stimulus  can 
be  definitely  traced  from  one  nerve  cell  to  another,  and  from  thence 
by  a  second  fibre  to  a  third  cell,  the  impulse  is  said  to  be  reflected 
by  the  second  cell  to  the  third.  And  there  we  have  what  is  called 
a  reflex  act. 

The  essential  part  of  a  nerve  fibre  is  a  kind  of  protoplasmic 
band,  in  which  the  finest  fibrilla  or  thread-like  marking  can  be 
made  out  with  the  aid  of  reagents  and  a  powerful  microscope. 

Fig.  19. 


Multipolar  cells  from  the  anterior  gray  column  of  the  spinal  cord  of  the 
dog-fish — (a)  lying  in  a  texture  of  fibrils;  (6)  prolongation  from  cells;  (c) 
nerve  fibres  cut  across.     (Cadiat.) 

This  is  called  the  axis-cylinder.  In  some  nerve  fibres  (mostly  in 
the  brain  and  spinal  cord)  the  axis-cylinder  is  naked,  and  even 
a  single  fibrilla  may  so  pass  from  one  cell  to  another  in  the  brain 
matter.  In  other  parts  the  axis-cylinder  is  generally  covered  by 
a  thin  membrane,  called  the  primitive  sheath,  or  with  a  soft  oil-like 
substance,  called  the  medullary  sheath,  or,  as  is  commonly  the  case 
in  all  peripheral  nerves,  by  both.  The  primitive  sheath  incloses 
the  medullary  sheath,  which  surrounds  the  axis-cylinder. 

These  fibres  are  made  of  peculiarly  modified  cells,  which  are, 
however,  so  elongated  as  not  to  be  very  easily  recognized  as  such 
Id  adult  tissue. 

4 


42 


MANUAL   OF    PHYSIOLOGY. 


The  nerve  or  ganglion  cells  vary  extremely  in  general  form  and 
size.  The  commonest  in  the  nerve  centres  are  large  bodies  with 
a  clear  well-defined  single  nucleus,  and  distinct  nucleolus;  they 
commonly  have  two  or  more  processes,  which  are  connected  by 


Fig.  20. 


Fig.  21. 


Fig.  20. — Ganglion  cells  of  frog,  showing  straight  and  spiral  fibres.  (After 
Beale  and  Arnold.) 

Fig.  21. — Cells  from  tlie  sympathetic  ganglion  of  a  cat.  The  delicate 
protoplasm  has  shrivelled  here  and  there  from  the  cell  wall. 

nerve  fibres  to  other  cells,  and  to  the  axis-cylinder  of  nerve?. 
This  latter  does  not  branch  as  the  other  processes  sometimes  do, 
and  when  it  can  be  traced,  appears  to  enter  the  protoplasm,  run- 
ning towards  the  nucleus. 

The  peripheral  nerve  cells  are  generally  much  modified,  and 
often  small  compared  with  those  in  the  centres.  Besides  the  cells 
in  the  sporadic  ganglia,  which  are  large  rounded  corpuscles  with 
but  few  processes — there  are  many  other  bodies  connected  with 
the  peripheral  nerves  which  cannot  be  called  ganglion  corpuscles. 
They  are,  however,  nevertheless  nerve  cells. 


Muscles  or  Contractile  Tissues. — When  changes  take 
place  in  protoplasm  adapting  it  specially  for  contraction,  it  is 
termed  muscle  tissue.  The  large  masses  of  this  tissue  attached  to 
the  skeleton  so  as  to  move  its  various  parts,  form  the  flesh  of  the 


STRUCTURAL   CHARACTERS    OF    ANIMAL   ORGANISMS.      43 

higher  animals.     Muscle  tissue  is,  almost  invariably,  connected 
Fig.  22.  Fig.  23. 


Fig.  22. — Cells  of  smooth  muscle  tissue  from  the  intestinal  tract  of  rabbit. 
(Ranvier.) — A  and  B.  Muscle  cells  in  which  differentiation  of  the  proto- 
plasm can  be  well  seen.     (Schafer.) 

Fig.  23. — Two  fibres  of  striated  muscle,  in  which  the  contractile  substance 
(m)  has  been  ruptured  and  separated  from  the  sarcolemma  (a)  and  (s) ;  (p) 
space  under  sarcolemma.     (Ranvier.) 


with  nerve  tissue,  and  acts  in  response  to  stimuli  communicated 


44  MANUAL    OF    PHY8I0LOGY. 

from  the  nerves.  In  some  of  tlie  lower  animals  the  two  tissues 
are  so  intimately  related  that  it  is  not  easy  to  separate  them,  and 
the  development  of  both  progresses  equally  as  we  ascend  the  scale 
of  animal  life.  They  are  nearly  related  in  their  origin,  or  even 
spring  from  the  same  primitive  tissue.  In  fact,  as  has  already 
been  mentioned  (vide  p.  39),  they  form  but  one  structure  in  some 
of  the  more  simple  and  less  differentiated  animals.  The  neuro- 
muscular tissue  which  is  formed  from  the  outer  layer  of  the  embryo, 
is  the  forerunner  of  the  muscles  as  well  as  of  the  nerves  of  the 
embryo  of  the  higher  animals. 

In  the  higher  animals  and  man  muscle  tissue  consists  of  two 
distinct  kinds  of  textures,  commonly  spoken  of  as — 

a.  Smooth  or  non-striated  muscle. 

b.  Striated  muscle. 

In  the  smooth  muscle  the  individual  elements  present  all  the 
characters  of  a  cell,  but  very  much  elongated  and  flattened,  and 
contain  a  single  long  nucleus.  They  contract  very  slowly  and 
persistently,  and  require  a  comparatively  long  time  for  the  nerve 
influence  to  effect  them,  so  that  an  obvious  interval  exists  between 
the  moment  of  their  stimulation  and  their  contraction.  They  are 
therefore  commonly  found  in  the  internal  organs  and  in  situations 
where  gradual  and  lasting  contractions  are  required.  They  re- 
ceive their  nervous  supply  generally  from  the  sympathetic  system, 
and  perform  their  duty  without  our  being  conscious  of  their  ac- 
tivity or  being  able  to  control  it  by  our  will. 

Striated  muscle  tissue  is  made  up  of  cylindrical  fibres  of  such 
length  that  both  extremities  cannot  be  brought  into  the  field  of 
the  microscope  at  the  same  time.  Their  exact  relation  to  cells  is 
not  so  easily  made  out  as  in  smooth  muscle,  and  doubtless  varies 
in  different  muscles.  Sometimes  the  fibres  are  made  up  of  single 
cells,  and  in  other  cases  they  are  formed  by  the  permanent  fusion 
of  several  cell  elements  which  never  differentiate  into  separate  ele- 
ments, owing  to  the  imperfect  divisit)n  of  the  cells,  but  make  up 
one  mass,  the  multiple  nuclei  of  which  alone  make  its  mode  of  ori- 
gin apparent.  The  contractile  substance  is  made  up  of  two  kinds 
of  material,  one  of  which  refracts  light  singly,  while  the  other  is 


STRUCTURAT.    CHARACTERS    OF    ANIMAL    ORGANISMS.       45 

doubly  refracting.  These  are  ranged  alternately  across  the  fibre, 
making  the  transverse  markings  or  strise  from  which  it  gets  its 
name.  This  striated  material  is  quite  soft,  and  is  incased  in  a  thin 
homogeneous  elastic  sheath  called  sarcolemma,  which  keeps  the 
fibre  in  its  normal  shape. 

This  form  of  muscle  may  be  considered  the  widest  departure 
from  the  primitive  protoplasmic  type  imparting  to  it  contractility. 
It  moves  with  wonderful  rapidity,  contracting  almost  the  instant 
its  nerve  is  stimulated.  It  forms  the  great  mass  of  the  quick-acting 
skeletal  muscles,  being  attached  to  the  bones  by  bands  composed 
of  a  form  of  fibrous  connective  tissue,  which  form  the  tendons  and 
fasciae.  Muscles  made  of  striated  tissue  are  commonly  under  the 
control  of  the  will,  and  hence  are  frequently  spoken  of  as  voluntary 
muscles,  but  this  term  is  misleading,  for  many  striated  muscles 
are  not  governed  by  voluntary  control. 

The  Connective  Tissue  group,  coming  exclusively  from 
the  middle  germinal  layer,  exhibits  very  great  varieties  of  form. 
Its  cells  diflTer  much  from  the  epithelial  cells  both  in  their  char- 
acter and  their  relations,  and  particularly  in  the  adult  tissues. 

Under  the  heading  Connective  Tissues  are  generally  classed  all 
those  which  support  the  frame  and  hold  together  the  various  other 
tissues  and  organs.     They  are — 

1.  Mucous  and  retiform  connective  tissue. 

2.  White  and  yellow  fibrous  tissue. 

3.  Cartilage. 

4.  Bone — as  well  as  certain  modifications  of  these  types. 

The  cells  of  all  these  tissues  have  the  property  of  manufacturing 
some  material,  which  however  does  not  generally  inclose  thera  as 
a  cell  wall,  but  remains  between  the  cells  and  forms  the  intercel- 
lular substance.  The  younger  the  tissue  the  greater  is  the  pro- 
portion of  its  cell  constituents,  and  the  older  the  issue  the  greater 
will  be  found  the  preponderance  of  the  intercellular  substance. 

Mucous  Tissue. — In  certain  parts  of  the  embryo  and  in  some 
of  the  lower  animals  a  kind  of  conDective  tissue  is  found  in  which 


46 


MANUAL   OF   PHYSIOLOGY. 


there  is  but  little  intercellular  substance,  the  mass  of  the  tissue 
being  thus  made  up  of  cells.  This  cellular  connective  tissue  never 
forms  an  important  texture  in  the  adult  man,  but  is  interesting 
as  the  probable  tissue  from  which  all  the  connective  tissues  are 
formed  in  the  embryo,  and  as  occurring  in  abnormal  growths  or 
tumors. 

The  first  step  in  differentiation  is  the  secretion  of  a  large  quan- 
tity of  soft  homogeneous,  semi-gelatinous  or  fluid  material  like  the 
mucus  secreted  by  epithelium.     In  this  the  cells  lie,  either  free 

Fig.  24. 


Transverse  section  of  the  chorda  dorsalis  and  neigliboring  substance,  a, 
cartilage  cells;  b,  cell  of  the  middle  layer  of  embryo;  c,  mucous  tissue;  d, 
boundary  of  chorda.     (Cadiat.) 


or  united  by  long  protoplasmic  processes.  This  is  the  mucous  tis- 
sue common  in  the  lower  animals,  in  textures  of  the  embryo  in  the 
adult  and  in  pathological  growths  of  the  connective  tissue  type. 
The  processes  uniting  the  cells  may  not  be  present,  and  the  cells 
may  be  reduced  to  a  minimum,  as  occurs  in  the  vitreous  humor 
of  the  eye.  But  more  commonly  the  soft  gelatinous  substance  is 
reduced  in  amount,  and  the  processes  connecting  the  cells  are  con- 
verted into  a  dense  network  of  delicate  threads  to  form  the  retiform 
tissue  of  lymphoid  structures. 


STRUCTURAL    CHARACTERS    OF    ANIMAL   ORGANIS]\LS.       47 

Fibrous  Tissue. — The  cells  may  become  further  difFeren- 
FiG.  25. 


Cells  of  mucous  tissue  with  branching  processes  (Bj  and  a  couple  of  elastic 
fibres  (Fj.     (Eanvier). 

tiated  and  fibriilated.    When  the  thickness  of  the  cell  processes  is 

Fig.  26. 


Portion  of  tendon  from  the  tail  of  a  young  rat,  stained  with  gold  chloride, 
showing  arrangement  of  flattened  cells  on  bundles  of  fibrils.     (After  Klein.) 

great,  and  their  fibrillation  well  marked,  the  cells  appear  to  devote 


4« 


MANUAL   OF    PHYSIOLOGY. 


all  their  reproductive  energy  to  the  formation  of  this  fibrillated 
substance  whicli  ultimately  forms  the  great  bulk  of  the  tissue, 
while  the  cells  become  gradually  and  proportionately  fewer  in 
number.  In  this  case  only  sufficient  of  the  mucous  substances 
generally  remains  to  cement  the  fibrils  together  into  bundles.    A 


Fig.  27. 


Fig.  28. 


Fig.  27. — Coarse  (a)  and  fine  (b)  yellow  elastic  fibres  after  treatment  with 
strong  acetic  acid.     (Cadiat.) 

Fig.  28. — Elastic  membrane  from  inner  coat  of  aorta,  and,  below,  mesh- 
work  of  elastic  fibres  from  a  yellow  ligament.     (Cadiat.) 


few  of  the  cells,  however,  remain  between  the  bundles  of  fibrils 
to  preside  over  the  nutrition  of  the  tissue.  Thus  is  formed  the 
non-elastic  or  white  fibrous  tissue  of  tendon. 

As  a  general  rule  these  fibrils  are  easily  affected  by  chemical 
re-ageuts.  Weak  acids  cause  them  to  swell  up  and  become  indis- 
tinct. Baryta  water  aflTects  the  cement  and  renders  them  easily 
separable.  They  swell  and  dissolve  in  boiling  water,  yielding  gel- 
atine, which  forms  a  jelly  on  cooling. 


STRUCTURAL   CHARACTERS    OF    ANIMAL   ORGANISMS.      49 

In  some  parts  of  the  body,  however,  a  different  kind  of  inter- 
cellular substance  is  formed,  which  is  highly  elastic,  does  not  give 
gelatine  on  boiling,  and  is  not  affected  by  weak  acids  or  alkalies. 
This  is  spoken  of  as  Yellow  elastic  tissue.  It  is  sometimes  found 
alone  forming  an  elastic  band  or  ligament,  but  more  commonly 
mingled  wiih  fibrillar  tissue  to  form  the  common  connecting 
medium  which  lies  under  the  skin  and  between  the  various  other 
textures. 

In  Cartilage  the  intercellular  substance  secreted  by  the  cells 
is  hard,  and  forms  in  the  earlier  stages  of  its  development  cases  or 
cell  walls  for  the  cells.     These  cases  subsequently  increase  in  thick- 


FlG 


A  teased  preparation  of  connective  tissue,  showing  fine  and  coarse  elastic 
fibres  mingled  with  bundles  of  fibrillar  tissue  and  connective  tissue  corpus- 
cles. 

ness,  and  become  fused  together  into  a  homogeneous  intercellular 
substance,  where  ultimately  the  capsules  belonging  to  the  different 
cells  can  no  longer  be  distinguished  from  one  another,  so  that  in 
the  adult  tissue  there  is  seen  to  be  a  tough  matrix  of  intercellular 
substance,  in  which  the  cells  are  scattered,  apparently  occupying 
small  cavities.  These  cells,  which  are  the  remote  offspring  of 
those  which  formed  the  tissue  permanently,  preside  over  its  nu- 
trition. 


50 


MANUAL   OF  PHYSIOLOGY. 


The  intercellular  substance,  which  is  quite  homogeneous  in  com- 
mou  hyaline  cartilage,  is  sometimes  modified  so  as  to  resemble 


FiC4.  30, 


Fig.  31. 


Fig.  30. — Section  of  liyaline  cartilage  froiu  the  end  of  a  growing  bone, 
showing  a  decrense  in  the  intercelhilar  substance  comiiared  witli  the  number 
of  cell  elements,  which  are  arranged  in  rows. 

Fig.  31. — Elastic  fibro-cartihige,  showing  cells  in  capsules  and  elastic 
fibres  in  matrix.     (Cadiat.) 

fibrous  tissue,  Fometiraes  the  fibrillar,  and  sometimes  the  elastic 
form  being  produced.     (Figs.  31  and  32,) 

Fig.  32. 


White  fibro-cartilage,  showing  cells   (a)  in  capsules  and  fibrillar  matrix 
{h).     (Cadiat.) 

Bone  is  probably  the  most  advanced  differentiation  of  the  con- 
nective tissue  group.     The  intercellular  substance  is  characterized 


STRUCTURAL    CHARACTERS    OF    ANIMAL    ORGANISMS.       51 

by  containing  a  great  quantity  of  earthy  or  inorganic  matter, 
which  gives  the  tissue  its  enormous  strength.  It  is,  moreover, 
everywhere  traversed  by  the  processes  of  the  cells  lying  in  little 

Fig.  33. 


Transverse  section  of  a  system  of  Havers,  showing  Haversian  canal  in 
centre,  with  bone  cells  arranged  aronnd  it  in  lacunas,  which  are  connected 
by  the  delicate  canaliculi.     (Cadiat.) 

canals  (canaliculi),  which  connect  the  spaces  (lacunae)  in  which 
the  protoplasmic  bone  cells  sojourn. 

In  the  formation  of  bone  from  fibrous  or  cartilaginous  tissue 
the  original  intercellular  substance  disappears,  and  a  set  of  cells 
with  new  formative  powers  come  upon  the  field  (Fig.  34).  These 
new  cells  (osteoblasts)  cover  the  growing  surface  of  the  bone  and 
secrete  and  lay  down  in  layers  a  new  kind  of  intercellular  sub- 
stance, which  is  the  bone  matrix.  Here  and  there  at  wonderfully 
regular  intervals  an  osteoblast  ceases  to  secrete  the  calcareous  inter- 
cellular substance,  while  its  neighbors  continue  formative  activity. 
Conse<]uently,  this  osteoblast,  or  as  it  may  now  be  called  young 
bone  cell,  becomes  surrounded  by  calcareous  intercellular  sub- 
stance, and  is  thus  permanently  lodged  in  the  bone  tissue. 


52 


MAJS'UAL   OF   PHYSIOLOGY. 


Fig.  S4. 


Section  through  ossifying  cartilage  and  newly-formed  bone.  (Cadiat.) — 
a.  Developing  cartilage  cells,  b.  Degenerating  cartilage  cells,  c.  Cell  space, 
empty.  (/.  Spiculae  of  calcareous  deposit,  e.  Blood  corpuscles.  /.Osteoblasts. 
g.  Ditto  of  periosteum,     h.  Bone  cells. 


STRUCTURAL,   CHARACTERS    OF    ANIMAL    ORGANISMS.       53 

The  Vascular  System  is  developed  in  the  middle  germinal 
layer  with  the  earliest  stages  of  the  connective  tissue.  The  blood- 
vessels, which  are  chiefly  made  up  of  connective  tissues,  soon 
traverse  all  the  parts  of  the  body,  and  distribute  the  nutrient  fluid 
or  blood.  And  even  the  blood  may  be  considered  as  an  outcome 
of  the  connective  tissues,  since  the  cells  of  the  blood  are  at  first 
formed  from  the  mesoblast,  and  later  from  the  connective  tissue 
corpuscles. 

An  arrangement  of  special  cells,  such  as  epithelial  or  muscle 
cells,  with  a  special  function,  constitutes  an  organ.  However,  in 
the  higher  animals  and  man,  an  organ  is  almost  invariably  a 
complex  structure,  having  various  tissues  entering  into  its  con- 
struction. Thus  a  skeletal  muscle  is  made  up  of  a  quantity  of 
muscle  fibres  held  together  by  sheets  of  connective  tissue,  and 
attached  to  bones  by  connecting  bands.  It  is  further  traversed 
by  many  bloodvessels,  and  the  fibres  are  in  immediate  relation 
to  certain  nerves  which  terminate  in  them.  The  various  secreting 
organs  are  made  up  of  epithelial  cells,  held  together  by  connective 
tissue  and  in  close  relation  to  bloodvessels  and  nerves,  and  are  so 
arranged  that  they  pour  their  secretion  into  a  duct.  The  bones^ 
which  are  the  organs  which  give  the  body  support,  contain,  in 
addition  to  the  bone  tissue  of  which  they  are  composed,  a  great 
quantity  of  indifferent  cells,  fat  cells,  nerves,  and  bloodvessels. 
They  are  covered  on  the  outside  with  a  tough  vascular  coat,  which 
gives  them  strength,  assists  their  nutritive  repair  and  reproduction, 
and  acts  as  a  point  of  attachment  for  the  muscles  and  ligaments. 
Where  the  bones  are  in  immediate  relation  at  the  joints,  they  are 
commonly  tipped  with  cartilage. 

If,  then,  we  analyze  anatomically  the  architecture  of  the  human 
body,  we  shall  find  that  it  is  made  up  of  a  number  of  complex 
parts,  each  adapted  to  some  special  function,  and  composed  of  such 
an  association  of  the  simple  tissues  as  the  special  part  demands. 

The  general  arrangement  of  these  organs  and  their  modes  of 
action  will  be  discussed  in  a  future  chapter. 


CHAPTER  III. 

CHEMICAL  BASIS  OF  THE  BODY. 

It  seems  natural  to  commence  the  description  of  the  molecular 
changes  that  take  place  in  the  various  tissues  and  organs  of  the 
body,  with  a  brief  account  of  the  chemical  composition  of  the 
most  characteristic  substances  found  in  animal  textures,  because 
none  of  the  mysterious  processes  of  cell  life,  or  tissue  activity,  can 
be  satisfactorily  studied  without  familiarity  with  the  more  common 
terms  occurring  in  physiological  chemistry. 

The  chapter  on  this  subject  here  introduced,  is  intended  rather 
to  give  the  medical  student  a  general  view  of  the  chemical  com- 
position and  characters  of  those  substances  most  frequently  met 
with  in  the  chemical  changes  specially  connected  with  animal  life, 
than  to  supply  a  complete  or  systematic  account  of  the  relation- 
ships of  the  chemical  bases  of  the  body,  for  which  reference  must 
be  made  to  more  advanced  text  books,  or  treatises  on  the  special 
subject  of  physiological  chemistry.  This  review  must  moreover 
be  inadequate  in  the  case  of  many  bodies,  but  they  will  be  again 
referred  to  when  speaking  of  the  function  with  which  they  are 
associated. 

It  has  already  been  stated  that  of  the  sixty-three  elements  known 
to  chemists,  a  comparatively  small  number  form  the  great  bulk  of 
the  animal  body,  although  traces  of  many  are  constantly  present. 
Thus,  we  shall  see  that  four  elements,  namely,  (1)  oxygen,  (2)  car- 
bon, (3)  hydrogen,  (4)  nitrogen,  are  present  in  large  proportions  in 
every  tissue,  and  together  make  up  about  97  per  cent,  of  the  body  ; 
and  sulphur,  phosphorus,  chlorine,  fluorine,  silicon,  potassium,  so- 
dium, magnesium,  calcium  and  iron,  are  indispensable  to  the  econ- 
omy, and  are  widely  distributed,  but  are  found  in  comparatively 
minute  quantities.  Occasionally  traces  of  zinc,  lead,  copper,  lith- 
ium, and  other  minerals  may  be  detected,  but  these  must  be  re- 
garded rather  as  accidental  than  indispensable  ingredients. 


CHEMICAL   BASIS   OF   THE   BODY.  55 

The  attempt  to  investigate  the  composition  of  a  living  tissue  by 
chemical  analysis,  must  cause  its  death,  and  thus  alter  the  arrange- 
ments of  its  constituents,  so  that  its  true  molecular  constitution 
when  alive,  cannot  be  determined. 

We  know  that  the  composition  of  all  living  textures  is  extremely 
complicated,  every  one  being  made  up  of  a  great  number  of  com- 
ponents, most  of  which  contain  many  chemical  elements  associated 
together  in  very  complex  proportions. 

But  as  has  already  been  pointed  out,  the  complexity  of  their 
chemical  constitution  is  not  so  wonderful  as  another  fact  which 
sounds  paradoxical,  that,  in  order  to  preserve  their  elaborate  com- 
position, they  must  constantly  undergo  a  change  or  renewal,  which 
is  necessary  for,  and  forms  the  one  essential  characteristic  of,  their 
life.  In  fact,  their  complexity  and  instability  is  such,  that  they 
require  constant  reconstruction  to  make  up  for  the  changes  insep- 
arable from  their  functional  activity. 

Their  chemical  constituents  are  easily  and  permanently  disso- 
ciated, and  the  various  components  are  themselves  readily  decom- 
posed, generally  uniting  with  oxygen  to  form  more  stable  com- 
pounds. 

The  investigation  of  the  chemical  changes,  known  as  assimila- 
tion, forms  a  great  part  of  physiological  study,  and  therefore  will 
occupy  many  chapters  of  this  book.  Here  we  can  only  call  atten- 
tion to  the  chief  characteristic  substances  to  be  found  in  the  animal 
body,  as  the  result  of  the  primary  dissociation  or  death  of  the 
textures,  and  briefly  enumerate  the  products  of  their  further  de- 
composition as  obtained  by  the  analysis  of  the  different  substances. 

The  tissues  of  the  higher  animals  present  a  greater  variety  of 
substances,  materially  differing  in  chemical  composition ;  they 
have,  however,  all  been  made  from  protoplasm,  and  contain  a  pro- 
portion of  some  substance  forming  a  leading  chemical  constituent 
of  protoplasm.  Every  living  tissue  contains  either  protoplasm  or 
a  derivative  of  it,  and  the  special  characters  of  each  tissue  depend 
upon  the  greater  development  of  some  one  of  these  substances. 

It  is  of  little  use  to  classify  the  numerous  chemical  constituents 
found  in  the  animal  body,  in  such  a  systematic  manner  as  to  sat- 
isfy the  rules  of  modern  chemistry,  because  their  classification  from 


56  MANUAL    OF    PII VSIOI.OGY. 

a  strictly  chemical  point  of  view,  does  not  set  forth  their  physio- 
logical importance  or  express  in  any  way  the  relation  they  bear  to 
the  vital  phenomena  of  organisms. 

The  following  enumeration  of  the  chief  chemical  ingredients 
found  in  the  tissues  has  regard  to  their  physiological  dignity  as 
well  as  to  their  chemical  construction,  and  will  thus  it  is  hoped 
assist  the  student  to  distinguish  the  different  groups,  and  give  him 
a  better  idea  of  their  vital  relationships,  than  a  more  strictly  sys- 
tematic classification. 

A.  NITROGENOUS. 

I.  Complex  bodies  forming  the  active  portions  of  all  tis- 
sues— Plasmata.     E.g.,  protoplasm,  blood-plasma. 
II.  Bodies  entering  into  the  formation  of,  and  which  can 
easily  be  obtained  by  analysis  of  Group  I.^Albu- 
mins.     E.g.,  serum-albumin. 

III.  Bodies  the  outcome  of  differentiation,  manufactured 

in  the  tissues  by  Group  I. — Albuminoids.     E.  g., 
gelatin,  etc. 

IV.  Bodies  containing  nitrogen,  being  intermediate,  bye, 

or  effete  products  of  tissue  manufacture.     E.g.,  leci- 
thin, urea,  etc. 

B.  NON-NITROGENOUS. 

V.  Carbohydrates  in  which  the  hydrogen  and  oxygen  ex- 
ist in  the  proportion  found  in  water.     E.g.,  starch 
and  sugar. 
VI.  Hydrocarbons  containing  oxygen  in  less  proportions 

than  the  above.     E.g.,  fats. 
VII.  Salts. 
VIII.  Water. 

Class  A.  NITROGENOUS. 

Group  I.     Plasmata. 

Under  this  group  may  be  placed  a  great  variety  of  materials 
which  must  be  acknowledged  to  exist  in  the  living  tissues  as  ex- 
alted chemical  compounds,  of  whose  chemical  constitution,  how- 


PLASMATA.  57 

ever,  we  are  ignoraut,  siuce  it  is  altered  by  the  death  of  the 
tissue. 

There  are  some  exceedingly  unstable  associations  of  albuminous 
bodies  with  other  substances,  and  they  at  once  break  up  into  their 
more  stable  constituents,  dead  albumins,  fats,  salts,  etc.,  as  soon 
as  they  are  deprived  of  the  opportunities  of  chemical  interchange 
and  assimilation  which  are  necessary  for  their  life. 

Although  we  can  only  theorize  as  to  the  real  chemical  consti- 
tution of  such  substances,  we  must  believe  that  they  really  exist 
in  the  living  tissues  as  chemical  compounds,  and,  moreover,  as 
chemical  compounds  endowed  with  special  properties  which  impart 
the  specific  activity  of  their  textures,  whose  molecular  motions  in 
fact  are  the  essence  of  the  life  of  the  tissues. 

Protoplasm:  By  far  the  most  widely  spread  and  important  of 
these  is  the  soft  jelly-like  substance,  Protoplasm.  This  is  the 
really  active  part  of  growing  textures  of  all  organisms,  whether 
animal  or  vegetable,  and  forms  the  entire  mass  of  those  interme- 
diate forms  of  life,  the  protista,  which  are  now  generally  regarded 
as  the  original  fountain  head  of  life  on  the  globe. 

This  material  commonly  exists  in  small  independent  masses 
(cells),  in  which  we  can  watch  all  the  manifestations  of  life,  assim- 
ilation, growth,  motion,  etc.,  taking  place.  We  must  assume  that 
this  substance  is  a  definite  chemical  compound  ;  and,  further,  since 
the  living  phenomena  are  exhibited  only  so  long  as  it  pre.serves 
its  chemical  integrity,  we  may  conclude  that  its  manifestations  of 
life  depend  upon  thesustenation  of  a  special  chemical  equilibrium. 
Not  only  is  this  equilibrium  destroyed  by  any  attempt  to  ascertain 
the  chemical  composition  of  protoplasm  by  analysis,  but  even  for 
its  preservation  the  protoplasm  must  be  surrounded  by  those  cir- 
cumstances which  are  known  to  be  necessary  for  life,  viz.,  moisture, 
warmth,  and  suitable  nutritive  material,  or  its  chemical  destruc- 
tion must  be  warded  off  by  a  degree  of  cold  that  checks  its  chem- 
ical activity. 

If  the  chemical  integrity  of  protoplasm  be  destroyed  and  its 
death  produced,  many  new  substances  appear,  amongst  which  are 
repre.sentatives  of  each  of  the  great  chemical  groups  found  in  the 
animal  tissues.     Thus,  besides  water  and  inorganic  salts  we  get 

5 


58  MANUAL   OP   I'HYSIOLOGV. 

from  protoplasm  carbohydrates  represented  by  glycogen,  lecithin, 
and  other  fats,  and  several  alhiuninous  bodies,  which  will  be  de- 
scribed in  the  groups  to  which  they  belong.  In  addition  to  these, 
protoplasm  often  contains  some  foreign  bodies  which  have  come 
from  without,  and  sjiccial  ingredients  of  its  own  manufacture,  such 
as  oil,  pigment,  starch,  and  chlorophyll. 

Blood-plasma:  There  is  another  body  in  living  blood  which 
must  be  included  in  this  group,  as  it  undoubtedly  has  a  much 
more  complex  constitution  than  any  of  the  individual  albuminous 
bodies,  presently  to  be  described,  which  can  be  obtained  from  it. 
This  is  proved  by  the  following  facts :  first,  its  death  is  accompa- 
nied by  a  series  of  chemical  changes,  viz.,  disappearance  of  free 
oxygen,  diminution  of  alkalinity,  and  a  rise  in  temperature,  and 
secondly,  that  several  albuminous  bodies  appear  which  were  not 
present  as  such,  in  the  living  plasma. 

The  spontaneous  decomposition  of  separated  blood- plasma  may 
be  delayed  by  cold  ;  at  freezing  point  the  chemical  processes  are 
thus  held  in  check.  During  life  the  exalted  constitution  of  the 
plasma  is  sustained  by  certain  chemical  interchanges  which  go  on 
between  it  and  its  surroundings.  This  question  will  be  more  fully 
discussed  when  the  coagulation  of  the  blood  is  described. 

Muscle-plasma:  Likewise,  as  will  be  found  in  the  chapter  on 
Muscles,  there  exists  in  the  soft,  contractile  part  of  striated  mus- 
cles a  plasma  which  at  its  death  spontaneously  breaks  up  into 
several  distinct  albuminous  bodies  and  forms  a  coagulum.  These 
changes  are  accompanied  by  acidity  of  reaction,  the  disappearance 
of  oxygen  and  an  elevation  of  temperature,  showing  that  distinct 
chemical  change  is  taking  place. 

Oxy-hcemoglobin,  the  coloring  matter  of  the  blood,  should  be 
included  here  amongst  the  important  chemical  bodies  more  com- 
plex than  the  albumins.  This  singular  body  can  be  broken  up 
into  a  globulin  and  a  coloring  matter  hcemai'm  containing  iron. 
It  differs  from  all  other  bodies  of  a  similarly  complex  nature  from 
the  fact  that  it  readily  crystallizes,  and  also  in  the  very  remark- 
able manner  in  which  it  combines  with  oxygen,  and  again  yields 
it  up. 


ALBUMINOUS    BODIES.  59 


Group  II.     Albuminous  Bodies. 

It  is  difficult  to  say  how  far  these  bodies  exist  as  such  in  the 
living  organism,  but  they  can  be  obtained  from  nearly  all  parts, 
particularly  those  which  contain  active  protoplasm,  and  after  its 
death  they  can  be  detected  in  abundance.  As  may  be  seen,  by 
testing  for  their  presence  in  living  protoplasm,  the  addition  of  any 
chemical  reagent  or  treatment  causes  its  death,  so  that  although 
albumins  appear  in  the  test-tube,  this  cannot  be  accepted  as  proof 
that  they  would  have  answered  to  the  tests  before  the  protoplasm 
was  changed  by  its  death. 

They  do  not  occur  normally  in  any  secretion  except  those  sub- 
stances which  tend  to  nourish  the  adult  body,  and  to  form  and 
nourish  the  offspring,  viz.,  the  ovum,  semen,  and  milk.  No  satis- 
factory formula  has  been  suggested  to  express  their  chemical  com- 
position, but  the  average  percentage  of  the  elements  they  contain 
is  remarkably  alike  in  all  members  of  the  group.  This  may  be 
said  to  be  in  round  numbers  as  follows : 

Oxygen, 21  per   cent. 

Hj^drogen, 7         " 

Nitrogen, 16         " 

Carbon, .  53        " 

Sulphur, 1         " 

They  are  amorphous,  of  varying  solubility,  and  with  one  excep- 
tion indiffusible  in  water. 

As  far  as  we  know  at  present,  albumins  cannot  be  constructed 
de  novo  in  the  animal  body,  but  must  be  supplied  in  one  form  or 
another  as  part  of  the  food.  Albumins  are  therefore  always  the 
outcome  of  the  activity  of  vegetable  life. 

They  can  be  recognized  by  the  following  tests  : 

1.  Strong  nitric  acid  gives  a  pale  yellow  color  to  strong  solu- 

tions or  solid  albumin,  which  turns  to  deep  orange  when 
ammonia  is  added  {Xanthoproteic  test). 

2.  Millon'sReagent(acidsolutionofproto-nitrate  of  mercury) 

gives   a   white    precipitate   which   soon   turns    yellow, 


GO  MANUAL   OF    I'lIYSIOLOGY. 

changing  to  rosy-red  on  prolonged  boiling,  or  after  stand- 
ing for  some  days. 

3.  Solution  of  caustic  soda  and  a  drop  of  cupric  sulphate 

solution  give  a  violet  color  to  the  liquid. 

4.  Acetic  acid  and  boiling  give  a  white  precipitate. 

5.  Acetic  acid  and  potassium  ferro-cyanide  give  a  flocculent 

white  precipitate. 

6.  Acetic  acid  and  equal  volumes  of  sodium  sulphate  solution 

give  a  precipitate  on  boiling. 

7.  With  sugar  and  sulphuric  acid  they  become  violet. 

8.  Crystals  of  picric  acid  added  to  their  solutions  dissolve 

and  at  the  same  time  cause  bead-like  local  coagulations. 

Classification  op  Albumins. 

Under  the  head  of  the  albuminous  bodies  we  find  several  classes 
which  differ  from  each  other  in  slight  but  very  important  points. 
The  first  class  may  be  called 

A.  Albumins  Proper,  or  Native  Albumins. 
They  consist  of: 

1.  Egg-Albumin,  which  does  not  occur  in  the  ordinary  tissues 
of  the  animal,  can  be  procured  by  filtration  from  the  white  of  an 
egg.  It  makes  a  clear  or  slightly  opalescent  solution  in  water, 
from  which  it  is  precipitated  by  mercuric  chloride,  silver  nitrate, 
lead  acetate,  and  alcohol.  It  is  coagulated  by  heat,  strong  nitric 
and  hydrochloric  acids,  or  prolonged  exposure  to  alcohol  and  ether. 

2.  Serum-Albumin,  on  the  other  hand,  is  one  of  the  chief  forms 
of  albumin  found  in  the  nutrient  fluids. 

It  diflfers  from  egg-albumin  in — 

(a)  Not  coagulating  with  ether. 

(b)  The  precipitate  obtained   by  strong  hydrochloric  acid 
being  readily  redissolved  by  excess  of  the  acid. 

(c)  Coagulum  being  more  readily  soluble  in  nitric  acid. 

(d)  Its  specific  rotatory  power  being  56°,  while  that  of  egg- 
albumin  is  35.5°. 

(e)  If  introduced  into  the  circulation,  it  is  not  eliminated 
with  the  urine  as  is  effff-albumin. 

DO 


GLOBULINS    AND    ALBUMINATES.  ol 

B.   Globulins. 

Associated  with  the  last  during  the  life  of  the  tissues  we  find 
another  class  of  albumins,  namely,  the  globulins,  which  do  not 
dissolve  in  pure  water,  but  are  more  or  less  soluble  in  a  solution 
of  common  salt.     These  may  be  divided  as  follows : 

1.  Globulin  (crystalline)  occurs  in  many  tissues,  but  is  usually 
obtained  from  an  extract  of  the  crystalline  lens  made  by  tritura- 
ting it  with  fine  sand  in  a  weak  solution  of  common  salt,  and  then 
passing  a  current  of  carbon  dioxide  through  the  solution.  The 
globulin  falls,  being  easily  precipitable  from  its  saline  solution  by 
very  weak  acid.  This  form  of  globulin  does  not  cause  coagulation 
on  its  being  added  to  a  serous  fluid,  and  in  this  respect  differs 
from  the  next  members  of  this  division. 

2'.  Paraglobulin  (fibrinoplastin)  can  be  obtained  by  passing 
through  diluted  serum  a  brisk  stream  of  carbon  dioxide.  When 
a  fluid  containing  paraglobulin  is  added  to  a  serous  transudation, 
it  causes  coagulation  of  the  fluid  giving  rise  to  fibrin. 

3.  Fibrinogen,  a  viscous  precipitate  got  from  serous  fluids  or 
blood  in  the  same  way  as  the  last,  but  with  greater  dilution  and 
more  prolonged  use  of  carbon  dioxide.  It  is  similar  in  its  charac- 
ters to  the  last,  but  coagulates  at  a  lower  temperature  (55°  C.) 
(paraglobulin  coagulating  at  60°-70°  C).  On  its  addition  to 
defibrinated  blood,  or  a  fluid  containing  paraglobulin,  it  forms  a 
coagulum. 

4.  Myosin,  obtained  from  dead  muscle,  beiug  the  soft  jelly-like 
clot  formed  during  rigor  mortis  from  the  dying  muscle  plasma. 
It  is  not  so  soluble  as  globulin,  for  it  requires  a  stronger  solution 
of  salt  to  dissolve  it,  and  is  precipitated  from  its  saline  solution  by 
solid  salt  or  by  dilution.     It  is  coagulated  at  60°  C. 

5.  Vitellin,  a  white  granular  proteid  obtained  from  the  yolk  of 
egg.  It  is  very  soluble  in  10  per  cent,  saline  solution,  from  which 
it  can  be  precipitated  by  extreme  dilution,  but  not  by  saturation 
with  salt.     It  coagulates  between  70°  and  80°  C. 

C.  Derived  Albumins  {Albuminates). 

1.  Acid  Albumin  (Syntonin)  can  be  made  from  any  of  the  pre- 
ceding by  the  slow  action  of  a  weak  acid  ;  or  by  the  addition  of 


62  MANUA].   OF   PHYSIOLOGY. 

strong  acetic  or  hydrochloric  acids  to  native  albumin,  such  as 
exists  in  white  of  egg,  and  dissolving  the  jelly,  thus  formed,  in 
water.  It  is  only  soluble  in  weak  acids — exact  neutralization 
precipitating  it.  With  the  least  excess  of  alkali  the  precipitate 
re-dissolves,  becoming  changed  into  alkali-albumin. 

So  long  as  it  is  dissolved  in  weak  acid  it  will  not  coagulate  on 
boiling,  but  it  coagulates  and  becomes  incapable  of  re-solution  if 
heated  while  precipitated  by  neutralization. 

2.  Alkali- Albumin. — Similar  to  the  last,  but  produced  by  the, 
action  of  either  weak  alkalies  and  dilute  solutions,  or  strong  solution 
of  potash  on  white  of  egg.  Its  general  behavior  is  the  same  as  the 
above — but  as  it  differs  in  composition,  containing  no  sulphur — 
it  can  therefore  be  distinguished  by  the  aKsence  of  the  brown 
coloration  which  appears  on  heating  acid-albumin  with  caustic 
potash  and  lead  acetate. 

3.  Casein  is  the  proteid  existing  in  milk,  and  resembles  alkali- 
albumin  in  its  reactions.  It  can  be  precipitated  from  milk  by 
rennet,  or  acetic  acid  in  excess,  but  not  by  exact  neutralization, 
owing  to  the  presence  of  potassium  phosphate. 

D.  Fibrin. 

A  solid  filamentous  body,  the  result  of  chemical  changes  accom- 
panying the  death  of  the  blood-plasma,  during  which  the  so-called 
fibrin  generators  are  .*et  free.  It  swells  in  weak  hydrochloric  acid, 
but  does  not  dissolve  while  cold.  If  heated  to  60°  C.  in  acid,  it 
changes  to  acid-albumin  and  dissolves.  By  10  per  cent,  neutral 
saline  solutions,  a  substance  like  a  globulin  may  be  extracted  from 
it.     If  heated,  it  assumes  the  characters  of  a  coagulated  proteid. 

E.   Coagulated  Albumin. 

If  any  of  the  above  be  heated  to  70°  C.  (except  acid  and  alkali- 
albumin,  which  must  first  be  precipitated  by  neutralization)  they 
coagulate  and  become  extremely  insoluble  and  lose  their  former 
characters.  They  are  but  very  slightly  acted  on  by  weak  acids, 
even  when  warmed.  Strong  acids  dissolve  them,  but  this  solution 
is  associated  with  a  destructive  change.      They  are,  however, 


ALBUMINOIDS.  63 

readily  converted  by  the  digestive  ferments  and  juices  into  pep- 
tones, and  thus  dissolved. 

F.  Peptone. 

This  substance  is  formed  by  the  action  of  the  digestive  ferments 
from  any  of  the  above  albumins,  in  the  stomach  by  pepsin  in  the 
presence  of  dilute  acid,  and  in  the  small  intestines  by  trypsin  in 
the  presence  of  dilute  alkali.  This  change  renders  them  more 
soluble  and  diffusible,  and  thus  enables  them  to  pass  out  of  the 
alimentary  canal  into  the  system,  and  makes  them  more  suited  to 
take  part  in  the  nourishment  of  the  body. 

The  leading  characteristics  of  peptones  may  be  thus  enumerated  : 

1.  Very  ready  solubility  in  hot  or  cold  water,  acids  or  alka- 

lies. 

2.  Not  coagulable  by  heat. 

3.  They  are  precipitated  by  alcohol  but  not  changed  to  the 

coagulated  form. 

4.  They  diffuse  readily  through  animal  membrane. 

5.  They  are  not  precipitated  by  sulphate  of  copper,  chloride 

of  iron,  or  ferro-cyanide  of  potassium  and  acetic  acid. 

6.  They  are  precipitated  by  iodine,  chlorine,  tannin,  chloride 

of  mercury,  and  the  nitrate  of  silver  and  mercury. 

7.  Caustic  potash  and  a  trace  of  sulphate  of  copper  added  to 

their  solutions  give  a  i-ed  color  which  deepens  to  violet 
if  too  much  of  the  copper  salt  be  used. 

The  formation  of  peptones  is  a  gradual  process  having  many  in- 
termediate steps,  in  the  earlier  stages  of  which,  precipitates  are 
formed  by  ferro-cyanide  of  potassium  and  acetic  acid.  {Vide 
Chaps.  VIII.  and  IX.  and  on  Chemistry  of  Digestion,  pp.  131  and 
141.) 

Group  HI.     Albuminoids. 

These  are  the  outcome  of  nutritive  modification  of  protoplasm, 
and  may  be  said  to  be  directly  manufactured  by  that  substance, 
and  to  be  specially  adapted  to  meet  the  requirements  of  certain 
textures  differing  widely  in  function.  They  are  allied  to  one 
another  and  to  the  last  group  by — (a)  their  percentage  composi- 


64 


MANUAL   OF    PHYSIOLOGY. 


tion*  ;  (b)  containiug  nitrogen  ;  (c)  being  amorphous  colloids. 
They  differ  from  albuminous  bodies  in — (a)  their  solubility;  (b) 
their  behavior  to  heat,  acids,  alkalies  and  the  digestive  fluids;  and 
(c)  their  value  as  food-stuffs. 

1.  Mucin  is  the  characteristic  ingredient  of  the  mucus  manufac- 
tured by  epithelial  cells,  and  is  also  found  in  connective  tissue 
(abundantly  in  that  of  the  foetus)  and  in  some  pathological  growths. 
It  gives  a  peculiar  thick  ropy  consistence  to  the  fluid  containing 
it,  enabling  it  to  be  drawn  into  threads.  It  is  p)ecipitated  by 
mineral  acids,  alum,  and  alcohol,  and  the  precipitate  swells  in 
water  and  is  re-di.ssolved  in  excess  of  the  acid.  With  acetic  acid 
a  precipitate  is  formed  which  does  not  re-dissolve  in  an  excess  of 
the  acid.  When  boiled  with  sulphuric  acid  it  yields  leucin  and 
ty  rosin. 

2.  Chondrin  is  obtained  by  the  prolonged  boiling  in  water  of 
slices  of  cartilage  cleared  of  the  perichondrium.  On  cooling,  this 
solution  forms  a  jelly.  The  jelly  dissolves  easily  in  hot  water  or 
alkalies,  and  can  be  precipitated  by  acetic  or  weak  mineral  acids, 
alum  or  acetate  of  lead.  It  gives  only  leuciu  on  boiling  with  sul- 
phuric acid. 

3.  Gelatin  is  produced  by  boiling  fibrous  connective  tissues,  such 
as  ligaments,  tendons,  the  true  skin  and  bones  in  water.  On  cool- 
ing, the  fluid  forms  a  jelly,  which  can  be  dried  to  a  colorless  brit- 
tle body  which  swells  in  cold  water  and  dissolves  on  being  heated. 
It  is  not  precipitated  by  acetic  acid,  but  yields  precipitates  with 
chloride  of  mercury  or  tannin,  which  latter  is  seen  iu  making 
leather.  On  boiling  with  sulphuric  acid  it  yields  glycin  and  leu- 
cin but  no  ty  rosin. 


*  The  following  Table  gives  the  composition  of  the  principal  albumi- 
noids and  albuiuin  : 


Gelatin. 

Elastin. 

Chondrin. 

Mucin. 

Keratin. 

Albumin. 

c 

H 

N 
0 

s 

50  p.  c. 

7 
18 
23 

0.5 

55  p.  c. 

7 
17 
20 

47  p.  c. 

6 
14 
31 

O.G 

50  p.  c. 

7 
10 
33 

51  p.  c. 
6 

17 

21 

3 

51-54  p.  C. 

6-  7 
15-17 
20-23 

2-  2.3 

INTERMEDIATE    PRODUCTS.  65 

4.  Elastin  is  obtained  from  yellow  elastic  tissue  by  boiling  with 
caustic  alkalies.  It  is  little  affected  by  boiling  water,  strong  acetic 
acid,  or  weak  alkalies,  but  dissolves  in  concentrated  sulphuric 
acid.  It  is  precipitated  by  tannin,  and  yields  leuciu  when  boiled 
with  sulphuric  acid. 

b.  Keratin  exists  in  the  epidermic  appendages  (hair,  horn,  nails, 
etc.).  It  is  like  the  albuminous  bodies  in  containing  a  coiisid*  ra- 
ble  quantity  of  sulphur,  but  differs  from  thrni  and  the  other  albu- 
minoids in  general  properties.  It  is  soluble  in  alkalies,  swells  in 
strong  acetic  acid,  gives  the  xanthoproteic  reaction,  and  is  ex- 
tremely insoluble  in  the  digestive  juices. 

Group  IV.     Products  of  Tii<suE  Change. 

Intermediate  or  bye  products. 

These  are,  no  doubt,  protoplasmic  manufactures  destined  for 
some  useful  purpose,  but  they  do  not  long  exist  in  their  original 
form,  being  often  broken  up  into  other  compounds  they  are  re- 
absorbed, or  pass  away  with  the  faeces.  These  bodies  are  found 
in  the  various  secretions.  Most  of  fhem,  however,  can  be  better 
described  with  the  function  of  the  gland  which  forms  the  secre- 
tion in  which  they  occur. 

Attention  must  here  be  drawn  however  to  certain  complex 
bodies  existing  in  the  bile.  Some  complex  nitrogenous  substances 
and  the  monatomic  alcohol,  cholesterin,  will  also  be  now  men- 
tioned. But  the  reader  must  remember  that  chemically  they  are 
not  connected  with  the  other  bodies  the  description  of  which  im- 
mediately follows  theirs,  namely,  the  effete  products. 

Bile  Snltfi. — Two  acids  exist  in  the  bile  united  with  soda  to  form 
soluble  soap-like  salts.  They  may  be  recognized  by  the  purple- 
violet  color  produced  by  cane-sugar  and  sulphuric  acid  at  a  tem- 
perature of  about  70°  C.  (Petteukofer's  reaction). 

Taurocholic  Acid,  C^jH^jNSO,,  is  most  plentiful  in  the  bile  of 
carnivora  and  man,  where  it  occurs  con)bined  with  soda.  It  is 
decomposd  by  prolonged  boiling  with  water  into  taurin  and  cholic 
acid,  thus : 

Taurocholic  acid.  Taurin.  Cholic  acid. 

C,.H,,NSO,  -f  H,0  =  C,H,NS03  -f  C,H,„0,- 
6 


(i6  MANUAL  OF    PHYSIOLOGY. 

Glycocholic  ^cirf,  C^gH^^NOg,  found  in  the  bile  of  lierbivora  and 
man.  It  crystallizes  in  fine  white  glistening  needles.  It  exists 
as  the  glycocholate  of  soda  in  the  bile.  By  boiling  with  weak 
acid  it  yields  glyciu  and  cholic  acid. 

Glvfocholic  acid.  Olvcin.  Cholic  acid. 

C,„H,3N0«  +  U,0  =  C.,NH,0,  +  a,H,„0,. 

In  the  bile  certain  matters  also  exist  to  which  the  color  is  due, 
the  principal  being  bilirubin  m  man  and  carnivora,  and  biliverdin 
in  herbivora.  They  are  probably  derived  from  the  coloring  mat- 
ter of  the  blood.  They  can  be  recognized  by  treating  the  solution 
with  nitric  acid  which  is  colored  with  red  fumes,  when  a  play  of 
colors  ending  in  a  dull  purple  is  seen. 

Lecithin,  Cj^HynNPO,,,  is  a  complex  nitrogenous  fat  found  in 
most  tis-ues  and  fluids  of  the  body,  particularly  in  the  nerve  tissues 
and  yolk  of  egg.  It  is  an  interesting  product  of  decomposition  of 
the  constituents  of  the  brain,  which  is  related  in  constitution  to 
the  neutral  fats,  and  it  may  be  regarded  as  an  acid-glycerin  ether. 
It  is  easily  decomposed  when  heated  with  baryta-water,  splitting 
into  glycerin,  phosphoric  acid,  neurin,  and  barium  stearate. 

Another  body  called  Cerebrin,  not  containing  any  phosphorus 
and  of  doubtful  composition,  can  be  obtained  from  brain  sub- 
stance, and  is  also  found  in  nerve  fibres  and  pus  corpuscles.  It 
is  a  light  colorless  powder  which  swells  in  water. 

Protagon,  C^^H^^J^J^O^^,  is  by  some  supposed  to  be  the  chief 
constituent  of  brain  substance,  and  by  others  said  be  a  mixture 
of  the  last  two  bodies. 

Neurin  (C'holin),  C^HjjNO.^,  is  an  oily  liquid  only  found  in  the 
body  as  a  product  of  the  decomposition  of  lecithin,  but  it  has 
been  obtained  synthetically. 

Cholesterin,  C.,gH.^^O,  exists  throughout  the  body  where  active 
tissue  change  is  going  on,  particularly  the  nervous  centres.  It  is 
a  monatomic  alcohol,  and  is  the  only  one  existing  free  in  the  body. 
It  may  be  obtained  from  gall-stones,  some  of  which  consist  entirely 
of  cholesterin.  It  may  occasionally  be  found  in  a  crystallized  foroi 
in  any  of  the  fluids  of  the  body  normal  or  pathological,  but  it 
only  seems  to  be  an  effete  product,  nearly  all  that  produced  in  the 


EFFETE    PRODUCTS.  67 

body  being  discharged  along  with  the  effete  portions  of  the  bile. 
It  may  be  recognized  by  the  shape  of  the  crystals,  which  are  rhom- 
bic plates,  in  which  one  corner  is  generally  deficient. 

Effete  Products. 

These,  as  has  been  stated  before,  are  generally  the  outcome  of 
the  active  chemical  changes  necessary  for  the  growth  and  vitality 
of  the  living  protoplasm,  and  are  for  the  most  part  soon  eliminated 
by  the  excretory  glands,  so  that  but  small  quantities  of  them  can 
be  found  in  the  active  tissues  where  they  are  produced. 

Urea,  CO(NH2)2,  is  the  most  important  constituent  of  the  urine 
of  mammalia,  but  not  of  that  of  birds  or  reptiles.  Traces  of  it 
may  be  found  in  the  fluids  and  tissues  of  the  body.  It  is  readily 
soluble  in  water  and  alcohol,  and  forms  crystals  when  its  solution 
is  concentrated.  It  decompt)ses  when  treated  with  some  strong 
acids  or  alkalies,  taking  up  water  and  yielding  COj  and  NHj, 
and  with  nitrous  acid  gives  CO^+N  +  H.^O.  It  was  the  first  of 
the  so-called  "  organic"  compounds  to  be  made  artificially,  being 
obtained  in  1828  by  mixing  watery  solutions  of  cyanate  of  potas- 
sium and  sulphate  of  ammonium,  evaporating  to  diyness  and  ex- 
tracting with  alcohol,  or  in  short  by  heating  cyanate  of  ammonia 
with  which  it  is  isomeric. 

Ammonium  Cyanate  =  N  H^  ^  0  =  H^      ,'  N.^  =  Urea. 

)  H,     ) 

It  can  now  be  produced  artificially  in  other  ways. 

It  has  also  been  considered  to  be  a  diamide  of  carbonic  acid 
(C0(0H)2),  the  two  atoms  of  hydroxy!  being  replaced  by  two 
atoms  of  amidogen,  NH^,  thus — (COCNH,)^).  In  the  presence 
of  septic  agencies,  in  a  watery  solution,  urea  takes  up  two  atoms 
of  water  and  is  converted  into  carbonate  of  ammonium — 

CO(NH,,),  +  2H,0  =  COCONHJ^. 
The  so-called  alkaline  fermentation  of  urine  depends  upon  this 
change.     The  reader  is  referred  to  the  Chapter  on  Excretions, 
where  more  complete  information  is  given. 

Kreatin,  (^^\l^f).^,  occurs  in  muscle  and  many  other  textures. 


68  MANUAL    OF    PHYSIOLOGY. 

It  may  be  converted  into  kreatiniu  by  the  action  of  acids  by  sim- 
ple dehydration.    It  can  also  be  split  up  into  sarcosin  and  urea. 

Kreatinin,  C^H^NsO,  is  a  dehydrated  form  of  kreatiu,  which  is 
a  normal  constituent  of  urine.  In  watery  solutions  it  is  slowly 
converted  into  kreatin. 

Allanioin,  C^HgN^Oj,  found  in  the  allantoic  fluid  and  the  urine 
of  the  foetus  and  pregnant  women.  It  is  crystallizable,  and  is 
converted  into  urea  and  allantoic  acid  by  oxidation. 

Glycin  ( Glycocoll  or  Glycocine),  C,H,(NH,)O.OH,  or    H      )■  N, 


s  ; 


is  regarded  as  amido-acetic  acid.  It  does  not  occur  free  in  the 
body,  but  enters  into  the  composition  of  the  bile  acids  and  hip- 
puric  acid.     It  is  soluble  in  water. 

Leucin,  C-H.oCNHj^O.OH,  or     H        ^  N,  amido-caproic  acid, 

H        ) 

is  found  in  the  secretion  of  the  pancreas  and  some  other  glands. 
It  is  one  of  the  principal  products  of  the  decomposition  of  albu- 
minous bodies,  from  which  it  can  be  obtained  by  boiling  with  sul- 
phuric acid. 

Tyrosin,  CgHnNOg,  though  belonging  to  a  distinct  chemical 
series  (aromatic),  is  only  found  in  company  with  leucin  in  the 
decomposition  of  albuminous  bodies,  and  normally  in  the  pan- 
creatic secretion. 

Taurin,  C^H^NOgS,  or   ^^tt^  >  SO3,  is  a  constituent  of  one  of 

the  bile  acids,  and  is  also  found  in  muscle-juice.  It  may  be  re- 
garded as  amido-ethyl-sulj)honic  acid. 

Uric  Add,  CjH^N^Og  (dibasic),  is  found  in  large  quantities  in 
the  excrement  of  birds  and  reptiles,  but  in  a  small  and  variable 
quantity  in  the  urine  of  man.  Traces  have  been  found  in  many 
tissues,  in  some  of  which  large  quantities  accumulate  as  the  result 
of  pathological  processes  (gout).  It  forms  salts  which  are  much 
less  soluble  in  cold  than  in  hot  water,  and  make  the  common 
sediment  in  urine.  The  acid  salts  are  less  soluble  than  the  neutral. 
The  common  test  for  uric  acid  consists  of  slowly  evaporating  the 


CARBOHYDRATES.  69 

substance  to  dryness  with  a  little  nitric  acid,  and  to  the  residue 
adding  amraonia.when  a  bright  purple  color  is  produced  (tnurexide 
test).  Uric  acid  is  supposed  to  be  a  step  in  the  production  of  urea, 
which  is  one  of  the  results  of  its  oxidation  iu  the  presence  of  acids, 
thus : 

Uric  acid.  Alloxan.  Urea. 

C,H,N,0,  +  H.p  +  O  =  C,H,N,0,  +  CO(NH,),. 
Hippuric  Acid,  CgHgNO.,,  or  C.Hp   V  N,  occurs  in  considerable 

quantities  iu  the  urine  of  the  horse  and  herbivora  generally.  It 
is  found  but  very  sparingly  in  man's  urine,  but  it  appears  in  large 
quantities  after  benzoic  acid  and  some  other  medicaments  have 
been  taken.  In  constitution  it  is  an  araido-acetic  acid  in  which 
one  atom  of  the  hydrogen  is  replaced  by  the  radical  benzoyl 
(C.H.O).  In  the  body  it  is  combined  with  bases,  and  is  formed 
out  of  benzoic  acid  and  glyciu  (amido-acetic  acid),  thus : 

Glvcin.  Benzoic  acid.  Hippuric  acid.  Water. 

C,II/NH,)O.OH  +  C,HA  =  C,,H(C,H50)(NH,)0.0H  +  H,0. 

By  heating  or  putrefaction  it  is  resolved  into  these  constituents. 

Indol,  CgH^N,  is  produced  in  the  intestinal  canal  by  the  putre- 
factive changes  brought  about  by  septic  agencies  during  pancreatic 
digestion.  It  gives  an  odor  to  the  faeces  and  a  red  color  with 
nitrous  acid. 

Indican,  a  peculiar  substance  sometimes  found  in  the  urine  and 
sweat.  With  oxidizing  agents  it  yields  indigo  blue.  By  this  fact 
it  is  easily  recognized.  An  equal  volume  of  hydrochloric  acid  and 
a  very  small  quantity  of  calcium  hypochlorite  (bleaching-lime)  is 
added,  and  the  indigo  which  is  formed  can  then  be  dissolved  and 
separated  by  agitation  with  chloroform. 

Class  B.  NON-NITKOGENOUS. 
Group  V.     Carbohydrates, 

Carbohydrates  (general  formula,  CmH^nOn)  are  bodies  in  which 
the  hydrogen  and  oxygen  exist  in  the  same  proportion  as  in  water, 


70  MANUAL   OF    PHYSIOLOGY. 

the  carbon  being  variable.     The  following  examples  of  this  group 
are  met  with  in  the  textures  of  the  body  : 

Grape  Sur/ar  (Dextrose),  C„H,,^Og,  occurs  in  minute  quantities  in 
the  blood,  chyle,  and  lymph.  It  forms  crystals  which  readily  dis- 
solve in  their  own  weight  of  water.  The  watery  solution  has  a 
dextro-rotatory  power  on  the  ray  of  polarized  light.  When  mixed 
with  yeast,  the  fungus  (Saccharomycea  cervisice)  of  the  yeast  causes 
alcoholic  fermentalion  of  the  sugar,  whereby  alcohol  and  carbon- 
dioxide  are  formed. 

Dextrose.  Alcohol. 

C«H,A=2C,H„0  4-2CO, 

Moderate  heat  (25°  C.)  aids  the  process,  and  cold  below  5°  C. 
checks  it ;  an  excess  of  either  sugar  or  alcohol  stops  it. 

The  presence  of  casein  or  other  proteid  material,  when  de- 
composing, gives  rise  to  lactic  fermentalion,  producing  first  lactic 
acid,  then  butyric  and  carbon-dioxide  and  hydrogen. 

Dextrose.         Lactic  acid.       Butyric  acid. 

C«H,,0,  =  2C3H,03  =  C.HP,  -f  2C0,  +  H, 

Milk  Hugar  {Lactose),  QyJi^^O-^^-\-  HgO,  isomeric  with  cane  sugar 
(sucrose).  It  is  the  characteristic  sugar  found  in  milk.  It  is  not 
so  soluble  as  dextrose,  and  does  not  undergo  direct  alcoholic 
fermentation,but  under  the  influence  of  certain  organisms  it  readily 
gives  rise  to  lactic  acid  by  lactic  fermentation  in  the  same  way 
as  dextrose.     (See  Milk.) 

Inosit,  CgHj.^Og  +  2H.^0,  is  an  isomer  of  grape  sugar,  which  is 
incapableof  undergoing  alcoholic  fermentation.  It  is  crystal!  izable, 
and  easily  soluble  iu  water.  It  has  no  effect  on  the  polarized  ray. 
It  is  found  in  the  muscles,  and  also  iu  the  lungs,  spleen,  liver,  and 
brain. 

Glycogem,  CgHjuO^,  a  body  like  dextrin,  first  found  in  the  liver. 
It  gives  an  opalescent  solution  in  water,  and  is  readily  converted 
into  dextrose  by  an  amylolytic  ferment,  or  weak  acids.  It  has  a 
strong  dextro-rotatory  power.  It  can  be  found  in  most  rapidly 
growing  tissues.     (See  Glycogenic  Function  of  the  Liver.) 


INORGANIC  BODIES.  71 

Group    VI.    Fat8. 

The^e  bodies  have  the  same  elements  in  their  compositicii,  but 
the  hydrogen  and  oxygen  have  variable  proportions — not  that 
of  water.  Fats  are  found  in  large  masses  in  some  tissue?,  and 
also  as  fine  particles  suspended  in  many  of  the  fluids.  The  fat  of 
adipose  tissue  in  man  is  a  mixture  of  olein,  palmitin,  and  stearin, 
which  are  commonly  spoken  of  as  the  neutral  fats. 

The  first  is  liquid,  and  the  last  two  solid  at  normal  temperatures, 
and  the  varying  consistence  of  the  fat  of  different  animals  depends 
upon  the  relative  proportions  of  the  more  solid  or  liquid  fats. 

Fats  are  soluble  in  ether  and  chloroform,  but  quite  insoluble 
in  water.  When  agitated  in  water  containing  an  albuminous 
body,  or  an  alkaline  carbonate  in  solution,  fluid  fat  is  broken  up 
into  small  particles,  which  remain  suspended  in  the  liquid  so  as 
to  form  an  opaque  milky  emulsion. 

Chemically,  they  are  regarded  as  ethers  derived  from  the  tri- 
atomic  alcohol  glycerin,  €3115(011)3,  by  replacing  the  hydrogen 
atoms  of  the  OH  group  by  the  oxidized  radicals  of  the  fatty  acids  ; 
thus — 

Glycerin  +    Palmitic  acid    =  Tripalmitin  -r    Water. 

C3H,(  OH  )3  +  3(C,eH3P,)  =  C.HA  iC,Ji,fi\  +  3H,0. 

Under  the  influence  of  certain  ferments  they  separate  into  glyce- 
rin and  the  fatty  acid,  taking  up  the  necessary  elements  of  water. 

When  the  neutral  fats  are  boiled  with  alkaline  solutions  they 
are  similarly  decomposed,  and  uniting  with  the  elements  of  water, 
form  glycerin  and  fatty  acids.  The  glycerin  is  thus  set  free,  but 
the  fatty  acid  combines  with  the  alkaline  metal  to  form  a  kind  of 
soluble  soap.  An  insoluble  soap  may  be  obtained  by  substituting 
lead  or  lime,  etc.,  for  the  alkali. 

This  splitting  up  of  the  neutral  fats,  stearin,  palmitin,  and  olein 
into  sodium  stearate,  palmitate,  or  oleate  goes  on  during  digestion, 
and  is  said  to  be  useful  in  aiding  the  absorption  of  fatty  matters. 

INORGANIC  BODIES. 

Water  '^  H._,0 j  is  present  in  nearly  all  tissues  in  larger  proportion 
than  any  other  compound,  making  up  about  70  per  cent,  of  the 


72  MANUAL   OF    PHVSIOLOGy. 

entire  body-weight.     The  amount  in  each  texture  varies,  and  thus 
the  different  tissues  have  widely  different  consistence. 

Water  is  introduced  into  the  body  in  all  kinds  of  drink,  and  a 
large  quantity  is  also  taken  with  our  solid  food.  It  is  highly 
probable,  that  in  the  chemical  changes  which  take  place  in  the 
tissues,  some  water  is  formed  by  the  oxidization  of  the  hydrogen 
of  the  more  complex  substances. 

In  the  economy  it  acts  as  the  universal  solvent  in  the  fluids  of 
the  body,  and  as  the  agent  by  means  of  which  the  chemical  changes 
of  the  various  organs  are  enabled  to  be  accomplished. 

The  water  leaves  the  body  by  the  lungs  as  vapor,  and  by  the 
skin,  kidney,  and  many  other  glands,  as  the  fluid  in  which  their 
secretions  are  dissolved. 

Inorganic  acids  occur  either  combined  to  form  salts,  in  which 
condition  we  find  several  in  the  body  (sulphuric,  phosphoric,  sili- 
cic), or  uncombined.     In  the  latter  state  we  have  only  two,  viz.: 

Hydrochloric  Acid,  HCl,  which  is  manufactured  by  the  mucous 
membrane  of  the  stomach,  and  takes  an  important  part  in  gastric 
digestion. 

Carbonic  Acid  Gas,  QO^,  exists  in  most  of  the  fluids  of  the  body, 
having  been  absorbed  by  them  from  the  tissues  during  their  com- 
bustion. The  venous  blood  contains  a  considerable  quantity,  which 
is  got  rid  of  during  the  passage  of  the  blood  through  the  lungs. 
It  is  distinctly  a  waste  product,  that  must  be  constantly  eliminated 
from  the  body  (see  Respiration). 

A  large  number  of  salts  occur  in  the  tissues,  generally  in  small 
quantity,  in  solution.  In  the  teeth  and  in  bone  tissue  salts  exist 
in  the  solid  form,  and  in  much  greater  proportion  than  in  any  of 
the  soft  parts.  Most  of  the  salts  are  introduced  into  the  economy 
with  the  food,  but  some,  doubtless,  are  formed  in  the  body  itself. 
Our  knowledge  of  the  exact  position  occupied  by  the  salts  in  the 
textures  is  very  incomplete,  as  their  amount  is  only  estimated  from 
the  ash  of  the  tissue  which  remains  after  ignition,  by  which  pro- 
cess they  become  altered,  so  that  it  is  impossible  to  say  what  are 
the  exact  salts  that  are  present  in  the  body.  They  doubtless  form 
chemical  combinations  with  the  complex  organic  compounds, 
which  we  do  not  understand,  and  probably  have  important  func- 
tions to  perform,  such  as  rendering  certain  materials  (globulins) 


INORGANIC   BODIES.  73 

si)luble,  or  otherwise  facilitating  tissue  change.  The  salts  pass 
out  of  the  body  in  many  secretious,  notably  in  the  urine,  where 
they  have  great  influence  on  the  elimination  of  urea,  and  therefore 
form  a  most  important  constituent  of  that  secretion. 

Common  Salt  (Sodium  Chloride),  NaCl,  is  the  most  widely 
spread,  and  is  present  in  greater  quantity  than  any  other  salt  in 
all  fluids  and  tissues,  except  in  bones,  teeth,  red  blood  corpuscles, 
and  red  muscle. 

Potassium  Chloride  commonly  accompanies  sodium  chloride  in 
small  quantity.  In  the  red  blood  corpuscles  and  in  muscle  it 
occurs  in  greater  amount  than  the  sodium  salt,  while  in  the  blood 
plasma  but  little  is  found  in  comparison  with  the  soda  salts,  and 
any  excess  seems  to  act  as  a  poison  to  the  heart. 

Carbonates  and  phosphates  of  calcium,  sodium,  potassium  and 
magnesium  occur  in  small  quantities  in  most  tissues.  The  earthy 
part  of  bone  is  chiefly  composed  of  calcium  and  magnesium  phos- 
phate and  calcium  carbonate,  together  with  some  calcium  fluoride. 

Sulphates  of  sodium  and  potassium,  probably  formed  in  the  body 
from  the  oxidization  of  the  sulphur  in  the  complex  proteid  mate- 
rials, occur  in  most  tissues,  and  arc  removed  from  the  body  by 
the  kidneys. 

Finally  we  find  two  of  the  elements  free  in  the  textures.  Of 
these  Oxygen  plays  by  far  the  most  important  part.  It  is  widely 
distributed  among  the  fluids  of  the  body,  from  which  it  can  be 
removed  by  reducing  the  pressure  of  the  oxygen  of  the  atmosphere 
by  means  of  an  air  pump.  Oxygen  is  introduced  into  the  body 
by  the  lungs,  where  the  blood  takes  it  from  the  air.  In  the  blood 
only  a  small  (juantity  of  that  which  can  be  removed  by  the  air- 
pump  is  really  free,  the  remainder  is  chemically  combined  with 
the  coloring  matter  of  the  blood.  It  is  absolutely  necessary  for 
life,  as  it  alone  can  enable  the  chemical  changes  of  the  tissues, 
which  are  mostly  oxidizations,  to  go  on.  It  is,  in  fact,  the  element 
necessary  for  the  slow  combustion  which  takes  place  in  the  nutri- 
ent material  after  its  assimilation. 

Nitrogen,  also  occurs  in  the  blood,  but  in  insignificant  quantity. 
It  is  absorbed  from  the  atmosphere  as  the  blood  passes  through 
the  lungs.  So  far  as  we  know,  it  has  no  physiological  importance 
in  the  body. 


chapt?:r  IV. 

THE  VITAL  CriARACTKRS  OF  ORGANISMS. 

The  inanifcstation  of  so-called  vital  j)henoincna  in  man  forms 
the  subject-matter  of  the  following  chapters,  and  some  kind  of 
explanatory  definition  of  the  vital  characters  of  the  simpler  organ- 
isms will  he  useful  in  preparing  the  beginner's  mind  for  the  more 
intricate  questions  in  human  physiology.  This,  with  the  foregoing 
short  account  of  the  chemical  and  structural  peculiarities  of  ani- 
mals, will  complete  a  rough  outline  of  the  general  character  of 
organisms. 

Protoplasm  has  already  been  referred  to  as  the  material  capable 
of  showing  "  vital  phenomena,"  the  most  obvious  and  striking  of 
which  are  its  movements. 

Besides  the  common  molecular  or  Brownian  movement  of  the 
granules  of  protoplasm — which  may  be  seen  in  most  cases  where 
fine  granules  are  suspended  in  a  less  dense  medium — protoplasm 
can  perform  motions  of  different  kinds  which  must  be  regarded 
as  distinctly  vital  in  character.  This  movement  may  be  said  to 
be  of  three  different  kinds,  according  to  the  results  produced,  viz.: 
(1.)  The  production  of  internal  currents.  (2.)  Changes  in  form. 
(3.)  Locomotion.  In  reality  the  two  latter  are  dependent  on  the 
first. 

The  occurence  of  currents  from  one  part  of  a  portion  of  proto- 
plasm to  another  can  be  well  seen  in  vegetable  cells,  wheu  the 
cell  wall  restricts  the  more  obvious  change  in  form  or  place. 
Thus  in  the  cells  forming  the  hair  on  the  stamens  of  Tradescantla 
Virginica  the  various  currents  can  be  seen  in  the  layers  of  proto- 
plasm which  line  the  cell  wall. 

The  granular  particles  course  along  in  varying  but  definite 
directions,  passing  one  another  like  foot-passengers  in  a  crowded 
street.  The  first  and  most  obvious  result  of  this  is,  that  the  vari- 
ous parts  of  the  substance  are  constantly  brought  into  contact  with 


PROTOPLASMIC    MOVEMENTS. 


75 


Fig.  35. 


one  another,  and  thus  the  products  of  any  chemical  changes  taking 
place  at  any  given  part  of  the  cell  body  are  rapidly  distributed 
over  the  entire  mass  of  the  protoplasm. 

If  there  be  no  definite  cell  wall — as  in  naked  vegetable  spores, 
and  amoeboid  forms  of  animal  life — to  restrict  or  direct  the  cur- 
rent of  protoplasm,  it  flows  out  in  various  directions  in  budlike 
processes,  which  appear  at  various  parts  of  the  protoplasmic  mass, 
60  as  to  cause  a  constant  change  in  the  form  of  the  cell.  These 
outstretched  processes  sometimes  flow  together  and  unite  com- 
pletely, often  inclosing  some  of  the  medium  in  which  the  creature 
is  suspended,  or  catching  some  foreign  particle  floating  near  them. 

The  flowing  out  of  these  pseudopodia  commonly  takes  place  for 
some  little  time  persistently  from  one  side  of  the  cell ;  and  the 
body  of  the  cell,  as  it  were,  has  to 
follow  the  protrusion  of  the  pro- 
cesses in  such  a  manner  that  in  a 
little  time  definite  change  in  posi- 
tion or  movement  in  a  certain  di- 
rection occurs;  thus  the  unit  of 
protoplasm  may  be  said  to  perform 
definite  progression  of  locomotion. 
All  these  movements  may  be  seen 
in  the  white  blood  corpuscles  of  a 
cold-blooded  animal,  such  as  afrog, 
and  still  more  easily  in  the  unicel- 
lular being  known  as  amoeba. 

Various  influences  may  be  seen 
to  affect  the  rate  of  movements, 
and  probably  influence  at  the  same 
time  the  other  activities  of  the  protoplasm.  Foremost  among 
these  must  be  named  :  (1.)  Temperature.  If  a  protoplasmic  unit 
which  is  observed  to  be  motile  be  gently  warmed,  the  movemen-ts 
become  more  and  more  active  as  the  temperature  is  raised,  until 
at  a  certain  point,  about  35°-42°  C,  a  spasm  occurs,  resulting  in 
the  withdrawal  of  the  pseudopodia  ;  soon  after  which  the  cell  as- 
sumes a  spherical  shape.  If  the  heat  be  carefully  abstracted  by 
the  gentle  and  short  application  of  cold,  the  protoplasm  may  be 


An  amoeba  figured  at  two  dif- 
ferent moments  during  movement, 
showing  a  clear  outer  layer  and  a 
more  granular  central  portion. — 
(«)  Nucleus;  (i)  Ingested  food. 
( Gegenbauer.  j 


76  MANUAL   OF    PHYSIOLOGY. 

made  to  recover  and  again  commence  its  movements.  If,  on  the 
other  hand,  cold  be  applied  to  moving  protoplasm,  the  motions 
become  less  and  less  active,  and  commonly  cease  at  a  temperature 
about  or  a  little  over  0°  C  (2.)  Mechmdeal  irritation  also  pro- 
duces a  marked  effect  on  the  movements  of  protoplasm.  This 
may  be  well  seen  in  the  behavior  of  a  protoplasmic  cell  of  frog's 
blood  under  the  microscope.  It  is  spherical  when  first  mounted, 
owing  to  the  rough  treatment  it  goes  through  while  being  placed 
on  the  glass  slide  and  covered  ;  shortly  its  movements  become 
obvious  by  its  change  in  form,  which  may  again  be  checked  by  a 
sudden  motion  of  the  cover-glass.  (3.)  Electric,  shocks  given  by 
means  of  a  rapidly- broken  induced  current  cause  spasm  of  the 
protoplasm,  the  cell  becoming  spherical.  (4.)  Chemical  stimuli 
also  have  a  marked  effect ;  carbonic  acid  causing  the  movements 
to  cease,  and  a  supply  of  oxygeu  making  it  active.  The  move- 
ments and  other  activities  of  protoplasm  are,  during  life,  frequently 
modified  and  controlled  by  nerve  influence,  as  will  appear  in  the 
following  pages:  this  may  readily  be  seen  in  the  stellate  pigment 
cells  of  the  frog's  skin,  which  can  be  made  to  contract  into  spheres 
by  the  stimulation  of  the  nerves  leading  to  the  part. 

The  motions  of  protoplasm  are  thus  seen  to  be  greatly  affected 
by  external  influences,  but  the  most  careful  observer  cannot  find 
physical  explanations  of  the  various  movements  which  have  been 
described.  It  is  necessary,  therefore,  to  ascribe  this  power  of 
motion  to  some  property  inherent  in  the  protoplasm,  and  hence 
the  movements  are  called  automatic.  We  are  unable  to  follow 
the  chemical  processes  upon  which  the  activities  of  the  protoplasm 
depend,  and  therefore  we  call  them  vital  actions;  but  we  must 
assume  that  these  so-called  vital  properties  depend  on  certain  de- 
compositions in  the  chemical  constitution  of  the  protoplasm.  We 
know  that  some  chemical  changes  do  take  place,  as  we  can  esti- 
mate the  products  which  indicate  a  kind  of  combustion  ;  but  we 
know  little  or  nothing  of  the  details  of  the  chemical  process. 

From  the  foregoing  description  of  the  manner  in  which  proto- 
plasm responds  to  external  stimuli,  it  may  be  gathered  that  it  is 
capable  of  appreciating  impressions  from  without;  indeed  we  may 
say,  it  can  feel.     We  can  only  judge  of  the  sensitiveness  of  any 


REPRODUCTION. 


77 


creature  by  the  manner  in  which  it  responds  to  stimuli,  and  we 
may  tiierefore  conclude  that  the  smallest  particle  of  living  proto- 
plasm is  endowed  with  definite  sensitiveness;  this  must  be  noted 
as  one  of  the  most  striking  properties  of  protoplasm. 

Every  particle  of  living  protoplasm  has  the  power  of  assimila- 
tion. Taking  into  its  structure  any  nutrient  matters  it  meets  with, 
by  flowing  around  them  in  the  way  mentioned,  it  brings  them 
into  direct  contact  with  different  parts  of  the  protoplasmic  sub- 
stance. This  nutrition  of  the  form-units  gives  rise  to  growth,  and 
finally  leads  to  their  reproduction,  and  these  facts  will  be  more 
closely  examined  when  speaking  of  their  relation  to  cell  life. 


Fig.  36. 


Fig.  37. 


Fig.  36. — Cells  of  the  yeast-plant  in  process  of  budding,  between  which 
are  some  bacteria. 

Fig.  37. — Cartilage  from  young  animal,  showing  the  division  of  the  cells 
(a,  6,  c,  d). 

When  a  certain  size  has  been  attained,  the  cell  does  not  in- 
crease any  more,  but  tends  to  bring  forth  a  cell  unit  similar  to 
itself.     Tliis  is  spoken  of  as  the  reproduction  of  cells. 

Different  kinds  of  cell  reproduction  have  been  observed,  which 
are  all,  however,  modifications  of  the  same  general  plan.  The 
first  is  that  by  the  formation  of  a  bud  from  the  side  of  the  parent 
cell ;  this  bud  then  increases  in  size,  and  finally  separates  from 
the  parent  and  becomes  a  separate  individual.  This  process, 
which  is  called  gemmation,  can  readily  be  seen  in  all  its  stages  in 
growing  yeast,  where  the  torula  cells  have  various  sized"  buds 


78  MANUAL   OF    PHYSIOLOGY. 

growing  from  theTii.  If  the  biuUike  protrusion  be  large,  nearly 
equal  in  size  to  the  cell  itself,  the  process  receives  the  name  of 
fis^sion,  or  division.  In  well  marked  typical  fission  the  parent 
cell  divides  into  two  parts  of  equal  size,  each  of  which  becomes  a 
perfect  individual.  Various  gradations  may  be  traced  between 
the  two  processes,  so  that  it  is  difficult  to  draw  any  very  distinct 
line  between  budding  and  fission.  The  budding  and  fission  may 
be  multiple;  many  buds  and  several  units,  products  of  division 
may  remain  together,  and  form  what  is  called  a  colony.     When 


Cells  t)f  a  fungus  ( Glwocapsa)  showing  different  stages  (1-4)  of  endogenous 
division.     (After  Sachs.) 

this  multiple  budding  or  divi.«ion  takes  place,  so  that  the  new  units 
are  included  within  the  body  of  the  parent  cell,  then  the  pro- 
cess is  called  endogenous  reproduction  or  spore  formation.  As  in 
the  gradations  between  budding  and  fission,  so  it  is  difficult  to 
draw  a  hard  and  fast  line  between  what  may  be  called  multiple 
fission  and  spore  formation. 

In  tracing  the  stages  of  development  of  the  highly  differentiated 
cells  of  some  tissues,  we  have  to  pass  through  a  series  of  changes 
which  form  a  cycle  that  may  well  be  called  the  lifetime  of  the 
cell.  The  duration  of  this  cycle  varies  greatly  in  different  indi- 
vidual cells.  Some  cells  are  very  short-lived,  being  destroyed  in 
the  act  of  secretion  ;  others  probably  endure  for  the  entire  life  of 
the  animal.  The  life-history  of  every  cell  begins  with  the  stage 
when  it  is  composed  entirely  of  indifferent  protoplasm,  in  which 
various  modifications  are  subsequently  produced.  Let  us  take  as 
an  example  a  cell  of  the  outer  skin  or  cuticle,  and  examine  its  life- 
history.  The  cuticle  is  made  up  of  numerous  layers  of  cells  laid 
one  on  the  other,  and  the  surface  cells  are  constantly  being  rubbed 
or  worn  off.  We  find  that  these  cells  have  their  origin  from  the 
cells  of  the  deepest  layer,  which  is  next  to  the  supply  of  nutri- 


REPEODUCTION. 


79 


raent.  This  layer  is  made  of  soft  protoplasmic  units,  with  certain 
specific  inherited  characteristics  no  doubt,  but  to  all  appearances 
the  satBG  as  the  motile  sentient  growing  protoplasm  of  an  indif- 
ferent cell.  By  a  process  of  fission  or  budding  constantly  going 
ou  in  this  deepest  layer  of  these  cells,  new  protoplasmic  units  are 
produced.  These  become  distinct  individuals,  and  occupy  the 
position  of  the  parent  cell,  which,  having  produced  offspring,  is 
moved  one  place  nearer  the  surface,  away  from  the  supply  of  food. 
The  new  cell  in  time  gives  rise  to  offspring,  and  having  attained 
reproductive  maturity,  is  in  turn  moved  onward  towards  the  sur- 


Fjg.  39 


Division  of  egg-cell.     (Gegenbauer.) 

face.  The  result  of  this  is  that  its  supply  of  nutrition  diminishes, 
the  evidences  of  reproductive  activity  disappear,  aud  at  a  certain 
[)oint  all  signs  of  protoplasmic  life  are  lost.  But  on  its  way  from 
the  seat  of  its  origin  to  the  surface,  it  makes  use  of  its  limited 
supply  of  nutrition  for  the  purpose  of  manufacturing  a  special 
kind  of  material  which,  if  present  at  all,  only  occurs  in  the 
minutest  traces  in  ordinary  protoplasn).  As  the  cell  moves  towards 
the  surface,  it  loses  its  protoplasmic  characters,  becomes  tougher 
and  drier,  and  finally  nothing  but  the  special  horny  material 
remains.     Thus  from  the  birth  of  the  cell,  its  energies  are  devoted, 


80  MANUAL   OF    PHYSIOLOGY. 

first  to  its  own  growth,  then  to  the  reproduction  of  its  like,  and 
finally  to  the  formation  of  a  material  fitted  to  act  as  a  niechauical 
protection  to  the  surface  of  the  skin.  Having  manufactured  a 
certain  amount  of  this  material,  the  protoplasm  dwindles,  and 
finally  quite  disappears,  so  that  the  cell  may  be  said  to  die.  Its 
horny  insoluble  and  impermeable  skeleton,  however,  has  yet  to  do 
good  service  in  the  outer  layer  of  the  skin  while  it  is  passing  to 
reach  the  surface,  and  in  its  turn  is  rubbed  off. 

It  has  already  been  stated  that  the  material  which  forms  all 
active  cells,  protoplasm,  is  capable  of  carrying  on  the  many 
functions  required  fur  the  independent  existence  of  simple  crea- 
tures. It  will  be  found  in  the  subsequent  pages  that,  not  only  can 
protoplasm  perform  all  the  activities  necessary  for  the  life  history 
of  unicellular  organisms,  but  that  it  can  also  work  out  all  the 
functions  of  the  most  complex  animals.  Indeed,  the  cells  whicii 
accotnplisli  the  most  elaborate  functions  in  man,  are  but  [)rot<»- 
plasm  more  or  less  modifitd  for  the  special  purpose  to  be  attained. 

The  different  functions  of  an  independent  unicellular  organism 
can  be  muchuKire  completely  watched  than  the  chatiges  which 
take  place  in  any  of  the  cells  of  the  higher  animals,  both  on  ac- 
count of  the  greater  size  of  th«  former,  and  the  more  obvious 
character  of  the  changes  taking  place  in  them.  The  i-tudent  is 
therefore  earnestly  advised  to  spend  a  few  moments  in  contem- 
plating the  operations  which  go  on  in  some  simple  organisms, 
whose  life  is  not  complicated  by  structural  or  functional  elabo- 
ration, before  attempting  to  solve  the  diflBcult  question  of  the 
mechanism  of  man's  life. 

The  lowest  forms  of  living  creatures  that  we  are  acquainted  with 
{micrococcus  and  bacterium),  are  placed  among  the  fungi  in  the 
vegetable  kingdom.  On  account  of  their  extremely  minute  size 
— being  hardly  visible  as  spherical  or  elongated  specks  with  a 
powerful  microscope — we  can  say  but  little  about  their  structure. 
They  appear  to  be  translucent  and  homogeneous. 

Since  we  use  the  term  protoplasm  to  mean  the  material  of 
which  the  active  parts  of  the  simplest  forms  of  living  beings  are 
composed,  we  must  assume  that  bacteria  are  small  particles  of 
that  material,  but  the  characters  commonly  attributed  to  proto- 


BACTEEIA.  81 

plasm  cauuot  be  fielected  in  the  raiuute  glistening  mass  which 
makes  up  their  body. 

They  are  so  certain  to  appear  in  a  couple  of  days  in  organic 
infusions,  or  in  any  fluid  prone  to  putrefaction,  and  they  multiply 
with  such  astounding  rapidity,  that  they  have  been  supposed  by 
some  to  develop  spontaneously.  But  this  is  now  known  not  to 
be  a  fact.  Bacteria  do  not  appear  without  progenitors,  any  more 
than  any  other  form  of  living  thing.  They  float  lifeless  and  dry 
in  multitudes  through  our  atmosphere,  and  adhere  to  all  sub- 
stances to  which  the  air  has  free  access.  However,  the  moment 
they  light  upon  a  suitable  soil,  they  burst  into  prodigious  activity, 
at  first  forming  masses  or  colonies,  which  may  be  seen  as  a  jelly- 
like scum  on  the  fluid.  Such  a  soil  is  supplied  by  any  organic 
substance  capable  of  spontaneous  decomposition,  for  which  pro- 
cess, as  is  well  known,  the  great  requirements  for  life,  moisture 
and  warmth,  to  a  certain  degree  are  necessary.  Vast  varieties 
of  these  organisms  are  now  known.  They  differ  slightly  in  shape, 
in  their  habitat,  and  in  their  properties.  Some  are  obviously 
composed  of  two  distinct  layers,  some  are  provided  with  a  fine 
hair-like  process,  by  the  lash -like  motions  of  which  they  move 
rapidly  in  a  definite  direction. 

They  are  known  to  be  inseparable  from  putrefactive  changes 
in  organic  materials,  in  fact  without  them  no  putrefaction  can 
go  on,  since  this  process  is  but  the  product  of  their  living  activity. 
Intense  heat  kills  them,  too  great  cold  or  dryness  checks  their 
activity  and  stops  putrefaction.  When  an  organic  substance  is 
absolutely  protected  from  their  presence  by  exclusion  of  the  air, 
etc.,  no  putrefaction  occurs,  even  though  it  be  prone  to  spontane- 
ous decomposition,  and  be  placed  under  favorable  circumstances 
as  to  warmth  and  moisture. 

Bacteria  would  not  deserve  so  much  notice  here  were  it  not  for 
the  remarkable  influence  they  have  on  the  higher  forms  of  life. 
We  do  not  know  that  they  are  necessary  for  any  of  the  more 
important  processes  that  normally  go  on  in  the  human  body, 
though  they  are  constantly  present  in  the  intestinal  tract,  and 
are  inseparable  from  at  least  one  change  taking  place  there  that 

7 


82  MANUAI-   OF   PHYSIOLOGY. 

may  be  regarded  as  physiological.  It  is  their  relation  to  the  dis- 
eased state  that  makes  a  knowledge  of  these  creatures  impera- 
tive to  medical  men. 

So  long  as  the  tissue  of  a  higher  animal  is  healthy  and  well 
nourished,  bacteria  cannot  thrive  in  immediate  contact  with  it. 
They  cau  only  exist  in  the  intestine,  etc.,  because  there  they  find 
accumulations  of  lifeless  fluids  which  offer  them  a  suitable  nidus. 
Active  living  tissues  have  antiseptic  power,  i.  e.,  are  able  to  de- 
stroy bacteria,  and  it  is  only  owing  to  this  bactericide  power  of 
our  textures,  that  we  can  with  immunity  breathe  into  our  lungs 
the  atmospheric  air,  and  swallow  nuiltitudes  of  these  organisms. 
But  for  it  every  wound  would  become  putrid,  every  breath  would 
admit  deadly  germs  to  our  blood.  But  when  the  vitality  of  the 
part  or  of  the  body  generally  is  lowered,  the  vital  activity  of  the 
tissue  may  fall  below  that  of  the  bacteria,  and  their  victory  is 
signalled  by  unwonted  and  often  fatal  changes.  Morbid  fluids 
allowed  to  accumulate  in  the  textures  facilitate  the  growth  of 
bacteria,  and  give  rise  to  various  grades  of  "  wound  infection." 
But  if  all  accumulations  be  avoided,  the  bacteria  brought  into 
relation  with  the  living  tissue  can  only  irritate  it,  and  cause  gen- 
eral fever  and  local  suffering  to  the  patient.  They  cannot  pro- 
pagate in  live  tissue  as  in  lifeless  fluids.  As  a  rule,  the  injurious 
effect  of  bacteria  is  in  inverse  proportion  to  the  vital  power  of 
the  textures  which  they  invade.  This  is  seen  in  many  cases 
familiar  to  the  physician  and  the  surgeon.  For  instance,  even 
the  bronchial  mucous  membrane  may  be  unable  to  resist  the 
attacks  of  the  atmospheric  organisms.  A  person  whose  vital 
powers  are  probably  already  low  from  repeated  debauch,  falling 
asleep  in  the  open  air  after  excessive  intemperance,  and  being 
exposed  to  the  reducing  chill  of  night,  may  become  so  lowered 
in  vital  activity,  that  putrefactive  changes  may  begin  in  his  lung 
tissue.  Indeed  this  is  not  an  uncommon  history  in  the  beginning 
of  gangrene  of  the  lung. 

We  next  come  to  forms  of  fungus,  which  set  up  a  process  very 
like  putrefaction,  such  as  the  yeast  plant,  Torula  cerevisia,  which 
causes  alcoholic  fermentation  in  sugar  solutions.  In  the  torula 
an  external  case  containing  protoplasm  may  readily  be  seen,  and 


AMCEB^.  83 

raultiplication  of  the  cells  goes  on  rapidly  by  a  process  of  bud- 
ding. Torulie,  however,  like  bacteria,  though  called  vegetables, 
have  not  the  power  of  assimilating  as  ordinary  green  plants  do, 
but  require  nutriment  to  be  supplied  to  them  which  already  con- 
tains organic  or  complex  compounds.  Structurally  but  little 
different  from  torula  is  a  one-celled  plant,  the  green  protococcus, 
which,  like  a  higher  plant,  can  build  up  its  texture  from  the  sim- 
plest foodstuffs,  and  carry  on  its  functions.  It  consists  of  a  case 
made  of  cellulose,  within  which  lies  a  mass  of  protoplasm  with 
a  nucleus.  The  protoplasm  is  commonly  colored  green  by  a  pecu- 
liar substance  called  chlorophyll.  We  shall  see  presently  that  it 
is  to  protoplasm  containing  chlorophyll,  that  plants  owe  all  their 
most  characteristic  and  wonderful  properties,  viz.,  the  property 
of  assimilating  so  as  to  construct  complex  carbon  compounds  out 
of  simple  inorganic  materials. 

The  smallest  and  simplest  organisms  classed  as  animals  are 
generally  larger  than  the  vegetable  cells  just  alluded  to.  They 
consist  of  protoplasm  without  any  nucleus,  and  only  sometimes 
with  a  structural  difference  between  any  part  of  their  substance. 
As  an  example  we  may  take  Frotamceba.  This  is  a  small  mass 
of  protoplasm  without  any  nucleus,  but  its  outer  layer  is  clearer 
and  less  granular  than  the  central  part.  It  can  move  by  send- 
ing out  protoplasmic  processes,  in  which  currents  can  be  observed 
resembling  those  in  the  vegetable  cells.  Except  as  regards  the 
nucleus,  it  is  much  the  same  as  the  Amceha,  which  can  be  more 
readil}'  watched,  and  will  therefore  be  more  accurately  described. 

The  amoeba  is  a  single  cell  or  mass  of  uncovered  protoplasm, 
containing  a  well-defined  portion  of  substance  or  nucleus,  within 
which  is  a  small  speck  or  nucleolus.  The  central  part  of  the 
protoplasm  is  densely  packed  with  coarse  granules,  but  the  outer 
more  active  part  is  structureless,  and  translucent-looking,  some- 
what like  a  fine  border  of  muffed  glass,  incasing  the  coarsely 
granular  middle  portion.  Such  a  one-celled  animal  has  no  spe- 
cial parts  differentiated  for  special  purposes,  the  requirements  of 
its  functions  being  so  small  that  the  protoplasm  itself  can  accom- 
plish them  all. 

Thus  the  processes  of  protoplasm,  which  flow  out  with  consid- 


84 


MANUAL   OF   PHYSIOLOGY, 


erable  rapidity  from  the  body,  commonly  encircle  particles  of 
nutrient  material,  and  then  closing  in  around  them,  press  them 
into  the  midst  of  the  granular  central  mass.  Here  they  sojourn 
some  little  time,  and  during  this  period,  no  doubt,  any  nutritive 
properties  they  possess  are  extracted  from  them,  and  they  are 
then  ejected  from  the  plastic  substance.  This  form  of  assimila- 
tion demands  no  previous  preparation  of  the  food  such  as  we  shall 
see  takes  place  in  the  alimentary  tract  of  man,  and  in  the  special 
organs  of  the  higher  animals  ;  yet  it  is  a  form  of  digestion  ade- 
quate at  least  to  the  requirements  of  this  simple  organism.  The 
repeated  alteration  of  the  different  parts  of  the  protoplasm  in 
relation  to  one  another  and  the  surrounding  medium  during  the 


Fig.  40. 


Two  different  forms  of  Amoeba?  in  different  phases  of  movement.     Those  on 
the  left  after  Cadlat.     A  and  B  show  an  outer  clear  zone  (Gegenbaur). 

flowing  hither  and  thither  of  the  currents,  produces  not  only  a 
change  in  the  shape  and  position  of  the  animal,  but  also  acts  as 
a  means  of  distributing  the  nutriment  to  the  different  parts  of 
the  body,  and  of  collecting  and  carrying  to  the  surface  the  vari- 
ous products  of  tissue-decomposition  ;  thus  the  streaming  proto- 
plasm does  the  work  of  a  circulating  fluid  such  as  we  see  in  the 
more  elaborate  organisms  for  the  distribution  of  nutriment  and 
elimination  of  waste  materials.  The  surface  of  the  amoeba  is 
sufficient  to  allow  of  the  gas-interchange  necessary  for  life,  and 
by  means  of  the  ever-changing  material  exposed,  sufficient  oxy- 
gen is  taken  for  its  tissue  combustions,  and  so  a  function  of  respi- 
ration is  established.     The  growth  that  results  from  the  perfect 


PARAMCECIUM.  85 

performance  of  these  vegetative  functions  proceeds  until  the 
maximum  size  is  attained,  and  further  nutritive  activity  is  then 
devoted  to  reproduction.  When  growth  ceases,  commonly  the 
cell  divides  and  forms  two  distinct  individuals.  The  movements 
which  form  the  most  striking  operations  of  the  amoeba  are  the 
same  as  those  which  take  place  in  protoplasm,  except  that  they 
are  more  rapid  and  obvious.  The  clear  outer  layer  first  flows  out 
as  a  bud-like  process,  and,  as  it  is  gradually  enlarging,  some  of 
the  central  granular  part  of  the  cell  suddenly  tumbles  into  its 
midst,  where  it  remains,  while  other  pseudopodia  are  being  thrown 
out  in  the  neighborhood,  and  the  same  changes  rejjeated  in  them. 
It  is  difficult  to  watch  the  motions  of  an  amoeba  without  being 
impressed  with  the  idea  that  it  is  not  only  endowed  with  sensibility 
but  that  it  also  can  discriminate  between  different  objects,  for  we 
see  it  greedily  flowing  around  some  food  material,  whilst  it  care- 
fully avoids  other  substances  with  which  it  comes  in  contact. 

If  a  glass  vessel,  containing  several  amcebse,  be  placed  in  a 
window,  they  will  be  found  to  cluster  on  the  side  of  the  glass  most 
exposed  to  the  light.  Frqjn  this  it  would  appear  that,  in  some 
obscure  way,  protoplasm  can  appreciate  light,  and  respond  to  its 
influence  by  moving  towards  it. 

This  single-celled  animal — or  nucleated  mass  of  protoplasm — 
can  perform  all  the  functions  of  a  higher  animal.  It  can  move 
from  place  to  place  and  assimilate  nutriment,  apparently  discrim- 
inating between  diflferent  materials.  It  distributes  nutrient  stuffs 
and  oxygen  throughout  its  body  by  a  kind  of  tissue  circulation, 
and  it  can  appreciate  and  respond  to  the  most  delicate  form  of 
stimulus,  namely,  light,  which  subtle  motion  has  no  effect  on  the 
sensory  nerve-fibres  of  the  higher  animals. 

In  some  unicellular  animals  certain  parts  of  the  cell  are  spe- 
cially modified  for  the  performance  of  special  functions,  a  divi- 
sion of  labor  thus  taking  place  which  insures  the  more  perfect 
accomplishment  of  the  different  kinds  of  activity.  In  one  of  the 
commonest  of  the  Infusoria  (Paramoecia  bursaria),  which  swarm 
in  dirty  water,  this  is  well  exemplified.  The  outer  layer  of  the 
flattened  body  is  denser,  and  forms  a  kind  of  fibrillated  corticu- 
lar  case  (ectosarcj,  which  is  covered  over  with  hair-like  pro- 


86 


MANUAL   OF   PHYSIOLOGY. 


cesses  (vihralile  cilia),  which  constantly  move  iu  a  certain  direc- 
tion, so  as  to  propel  the  creature  rajjidly  through  the  water.  The 
internal  part  of  the  cell  is  vejy  soft,  almost 
fluid,  and  coarsely  granular  in  appearance,  con- 
taining n)any  bodies  which  have  obviously  been 
introduced  from  without.  This  soft  internal 
protoplasm  (endosarc)  moves  slowly  round  iu 
a  definite  direction,  completing  its  circuit  in 
one  or  two  minutes,  and  thus  carries  on  a  cir- 
culation which  mixes  the  various  matters  con- 
tained in  it. .  At  one  point  of  the  ectosarc,  or 
cortical  layer,  an  orifice  or  mouth  leading  to 
an  oesophageal  depression  is  found.  This  ori- 
fice is  lined  by  moving  cilia,  which,  by  their 
vibrations,  drive  the  food  into  the  oesophagus, 
whence  it  is  periodically  jerked  into  the  soft 
internal  protoplasm  or  endosarc,  together  with 
some  water,  and  thus  forms  a  food  vacuole, 
which  is  carried  r4)und  in  the  circulation  of  the 
ectosarc.  Besides  a  well-marked  nucleus  and 
nucleolus  in  the  central  part  of  the  cell,  these 


Diagram  of  Par- 
amcecium,  showing 
digestive  cavity. — 
(a  a)  Body  space 
filled  with  soft 
protoplasm,  into 
which  food  is 
taken.    (6)  Mouth. 


(c)  Anus,  {rl)  Con-  paramoecia    have   one   or   more   clear    spaces 

tractile       vesicle,  placed  near  the  surface  at  the  extremities  of 

(After  Lachmann.)  ,i  •        i      rpi  i  ij      i  i.        i. 

^  the  animal,    ihese  vacuoles  suddenly  contract, 

and  disappear  every  now  and  then.  When  this  contraction 
occurs  fine  canals  radiating  from  the  contractile  vacuole  are  dis- 
tended with  the  clear  fluid  which  has  probably  entered  the  vacuole 
from  without.  Thus  a  permanent  set  of  water  vessels  carry  fluid 
from  the  contractile  vacuole  throughout  the  endosarc. 

In  such  an  animal  there  is  a  distinct  advance  of  function  com- 
pared with  the  amojba ;  a  more  elaborate  and  specialized  method 
of  feeding;  a  more  systematic  and  regular  circulation  of  nutri- 
ent matters ;  a  respiratory  distribution  of  water  by  the  contrac- 
tile vesicle  and  its  water  canals ;  more  rapid  motion  ;  and  more 
obvious  sensation. 

In  the  bell  animalcule,  or  vorticella,  the  same  kind  of  division 
of  labor  exists,  but  in  one  of  its  commonest  conditions  it  is  at- 


PARAMCECIUM.  87 

tached  by  a  thin  stalk  to  the  stalk  of  some  weed  or  other  object. 
Besides  the  ciliary  raovemeut  we  here  fiud  that  the  general  mass 
of  the  protoplasm  can  suddenly  and  forcibly  contract,  so  as  to 
completely  alter  its  shape,  and  change  the  bell  into  a  rounded 
mass.  This  spasm  of  the  body  is  commonly  associated  with  a 
wonderfully  rapid  contraction  of  the  stalk.  This  stalk  consists 
of  a  delicate  transparent  sheath,  in  the  centre  of  which  is  a  thin 
thread  of  pale  protoplasm.  The  rapid  contraction  of  the  proto- 
plasm of  the  stalk  and  the  spasm  of  the  bell  occur  on  the  appli- 
cation of  the  least  mechanical  excitation,  such  as  a  touch  to  the 
cover-glass.  Here  in  a  single  cell  we  have  certain  portions  set 
apart  for  special  purposes,  most  of  which  are  the  same  as  in  para- 
moecia.  But  the  animal  being  attached  requires  a  special  way  of 
escaping  from  its  enemies,  and  hence  we  find  it  endowed  with 
three  special  forms  of  motion.  Besides  the  ciliary  and  stream- 
ing protoplasmic  motion,  its  body  can  spasmodically  change  its 
shape,  and  the  stalk  contracts  with  a  velocity  comparable  with 
that  of  the  most  specially  modified  contractile  tissue  (muscle)  of 
the  higher  animals,  by  means  of  which  their  rapid  and  varied 
movements  are  carried  out. 


CHAPTER  V. 

NUTRITION  AND  FOOD-STUFFS. 

The  continuation  of  protoplasmic  life  depends  on  certain  chemi- 
cal changes  which  are  accompanied  by  a  considerable  loss  of  sub- 
stance. This  loss  must  be  made  good  by  the  assimilation  of 
material  from  without,  and  the  manner  by  which  it  is  obtained 
constitutes  one  great  point  of  difterence  between  Plants  and  Ani- 
mals. In  the  majority  of  the  former  (certain  fungi  form  the 
main  exceptions)  the  cells  in  those  portions  of  the  plant  which 
are  exposed  to  the  light  and  air,  contain  a  peculiar  green  sub- 
stance called  chlorophyll,  and  through  the  agency  of  this  sub- 
stance they  are  able  to  obtain  from  the  inorganic  kingdom  nearly 
all  the  food  they  require.  Water  is  taken  up  by  the  roots  with 
such  salts  as  may  ha})pen  to  be  in  solution,  and  is  carried  through 
the  stem  to  the  leaves;  here  the  active  chlorophyll-bearing  cells, 
under  the  influence  of  the  sun's  rays,  cause  it  to  unite  with  the 
carbon  dioxide  present  in  the  air,  to  form  various  substances,  of 
which  we  may  take  starch  or  cellulose  as  the  simplest  example. 
This  reaction  may  be  represented  chemically,  thus: 

6C0,  -f  5H,0  =  C,H,„0,  +  0,,. 

starch  or  cellulose. 

A  large  proportion  of  oxygen  is  thus  set  free  and  discharged  into 
the  atmosphere. 

The  most  striking  property  of  plant  protoplasm  is,  then,  the 
power  of  using  the  energy  of  the  sun's  rays  to  separate  the  ele- 
ments of  the  very  stable  compounds,  carbon  dioxide  and  water, 
and  from  the  elements  thus  obtained  to  make  a  series  of  more 
complex  and  unstable  compounds,  which  readily  unite  with  more 
oxygen,  and  change  back  to  carbonic  anhydride  and  water. 

The  new  carbon  compounds  made  in  and  by  the  protoplasm  of 
the  green  plants  are  some  of  the  so-called  "  organic  compounds," 


NUTRITION   AND   FOOD-STUFFS.  89 

which  enter  iuto  the  composition  of  both  plants  and  animals,  and 
form  an  essential  part  of  the  food  of  the  latter.  They  may  be 
divided  into  three  groups — 

i.  Carbohydrates — bodies  so  called  from  the  presence  of 
hydrogen  and  oxygen  in  the  proportion  to  form  water ; 
e.g.: 

Starch,  CgH.oO-  =  C/Hp), 

Grape  sugar  (dextrose)  CgHj^Og  =  Cg(H.^O)(; 

Cane  sugar  (sucrose)  C^^.^fi^.^  =  Ci.,(HjO)ji 

ii.  Hydrocarbons — compounds  of  carbon  and  hydrogen  with 
a  less  proportion  of  oxygen  than  Division  i.,  as  oils 
and  fats — 

Olein  (principal  constituent  of  olive  oil),  C^rHj^^Og 

iii.  Albuminous  bodies  which  contain  nitrogen  in  addition 
to  carbon,  hydrogen,  and  oxygen.  These  are  of  very 
complex  composition,  and,  as  yet,  cannot  be  repre- 
sented by  chemical  formulae. 

Animals,  on  the  other  hand,  cannot  thrive  on  the  simple  forms 
of  food  obtainable  from  the  inorganic  kingdom,  which  suffice  for 
the  nutrition  of  a  plant.  They  require  the  materials  for  their 
assimilation  to  be  nearly  allied  in  chemical  composition  to  their 
own  tissues.  In  short  they  require  as  food  the  very  organic  sub- 
stances which  the  plants  spend  their  lives  in  making:  ^iz., 
starches,  fats,  and  albuminous  bodies.  These  substances  must^ 
therefore,  be  supplied  to  animals  ready  made,  as  they  are  pro- 
duced by  plants.  Directly  or  indirectly,  through  the  medium  of 
other  animals,  all  these  complex  substances  which  form  fuel  so 
useful  to  our  economy,  are  derived  from  the  work  done  by  vege- 
table life. 

The  chief  acts  of  animal  protoplasm  are  really  oxidations,  a 
slow  burning  away  of  its  substance,  which  results  in  the  produc- 
tion of  inorganic  materials  like  those  used  by  plants  as  food. 

Plants,  then,  use  simple  food-stuffs,  and  from  them  manufac- 
ture complex  combustible  materials,  and  thus  store  up  the  energy 
of  the  sun's  rays  in  their  textures. 


90  MANUAL  OF   PHYSIOLOGY. 

Animals  use  complex  food-stuffs  to  renew  their  tissues,  which 
they  are  constantly  oxidizing,  and  by  this  means  the  energy  for 
the  performance  of  their  various  active  functions  is  set  free. 

Although  the  various  kinds  of  food-stuffs  used  by  animals  are 
so  highly  organized  in  comparison  with  those  used  by  plants,  yet 
they  cannot  be  admitted  at  once  into  the  economy  without  having 
undergone  a  special  preparation,  which  takes  place  in  the  diges- 
tive tract,  where  the  various  food-stuffs  are  so  changed  as  to  allow 
them  to  pass  into  the  fluids  of  the  body. 

We  shall  first  consider  the  chief  varieties  of  food-stuffs,  next 
their  preparation  for  absorption,  and  then  the  means  by  which 
they  are  distributed  to  the  tissues.  The  last  step  in  tracing  the 
assimilation  of  the  food  is  to  follow  the  intimate  processes  which 
go  on  between  the  blood  carrying  the  nutriment  and  the  different 
tissues.  This  most  interesting  but  difficult  question  shall  receive 
our  attention  in  a  subsequent  section. 

Food. — There  are  two  portals,  namely,  the  lungs  and  the  ali- 
mentary canal,  by  which  new  materials  normally  enter  the  ani- 
mal body. 

Within  the  lungs  the  blood  comes  into  close  relation  with  the 
air,  and  takes  up  oxygen  from  it.  The  oxygen  is  then  carried  to 
the  various  tissues,  where  it  aids  the  combustion  accompanying 
the  life  and  functions  of  these  tissues.  Although  oxygen  is  the 
mogt  abundant  element  in  the  body,  taking  part  in  almost  every 
chemical  change,  and  its  continuous  supply  is  more  immediately 
necessary  for  life  than  that  of  any  other  substance,  yet  it  is  not 
counted  as  food,  because  tissue  oxidation  is  artificially  distin- 
guished from  tissue  nuti'ition. 

The  details  of  the  union  of  oxygen  with  the  blood  will  be  found 
in  the  Chapter  (XIX.)  on  Respiration. 

It  is  then  only  to  the  liquid  and  solid  portions  of  the  material 
income  of  an  animal — that,  in  short,  which  it  must  busy  itself  to 
obtain — that  the  term  "food"  is  applied.  These  are  introduced 
into  the  alimentary  canal,  where  the  truly  nutrient  materials  are 
separated  and  prepared  for  absorption  into  the  blood,  while  the 
portions  which  are  not  useful  for  nutrition  are  carried  away  as 
excrement.     One  is,  therefore,  quite  prepared  to  hear  that  the 


FOOD    REQUIEEMENTS.  91 

really  nutritious  food-stuffs  are  composed  of  materials  which  are 
chemically  like  the  tissues,  although,  as  we  shall  see,  we  have  no 
grounds  for  believing  that  the  different  chemical  groups  of  nutri- 
tive stuffs  are  exclusively  destined  to  replace  corresponding  sub- 
stances in  the  body.  On  the  contrary,  we  have  good  reason  to 
think  that  within  the  body  the  conversion  of  one  group  iuto 
another  is  very  common. 

In  Chapter  III.  the  tissues  of  the  animal  body  were  shown  to 
consist  of  chemical  compounds,  which  have  been  classified  into 
certain  groups.  And  it  has  also  been  stated  that  the  tissues  are 
constantly  undergoing  chemical  changes  inseparable  from  their 
life,  and  that  for  these  changes  a  supply  of  nutritive  material  is 
necessary. 

The  nuti'iment  required  for  an  animal  is,  then,  made  up  of 
substances  which  may  be  divided  into  the  same  chemical  groups 
as  the  tissues  of  the  body  :  viz.,  proteids,  fats,  carbohydrates,  salts, 
and  water.  So  that  each  of  the  various  substances  which  we 
make  use  of  as  food,  contains  in  varying  proportions  several  of 
the  different  kinds  of  nutrient  material,  either  naturally  or  arti- 
ficially mixed  so  as  to  form  a  complex  mass,  the  importaoit  item 
water  being  the  only  one  which  is  commonly  used  by  itself  These 
substances  may  be  considered  to  be  the  chemical  bases  of  the  food, 
as  they  are  also  the  chemical  bases  of  the  animal  body. 

The  following  classification  shows  the  relationships  between 
the  chief  items  of  nutritious  matters,  from  a  chemical  point  of 
view,  and  their  distribution  in  the  various  foods  we  commonly  eat. 

I.  Organic. 

1.  Nitrogenous — 

A.  Albuminous — abundant   in   eggs,   milk,   meat, 

peas,  wheaten  flour,  etc. 

B.  Albuminoid — in  soups,  jellies,  etc. 

2.  Non-Nitrogenous — 

A.  Carbohydrates    (sugar,   starch) — abundant    in 

all  kinds  of  vegetable  food,  and  in  milk,  and 
present  in  small  quantity  in  meat,  fish,  etc. 

B.  Fats — in    milk,  butter,  cheese,  fat    tissues   of 

meat,  many  vegetables,  oils,  etc. 


92  MANUAL  OF   PHYSIOLOGY. 

II.  Inorganic. 

1.  Salts — mixed  with  all  kinds  of  food. 

2.  Water — mixed  with  the  foregoing  or  alone. 

The  nutritive  value  of  any  kind  of  food  depends  upon  a  variety 
of  circumstances,  which  may  be  thus  summed  up : 

I.  Chemical  compontion,  of  which  the  main  points  are — 

(1.)  The  proportion  of  soluble  and  digestible  matters  (true 
food-stuffs)  to  those  which  are  insoluble  and  indi- 
gestible (such  as  cellulose),  etc. 

(2.)  The  number  of  different  kinds  of  nutrient  stuffs 
present  in  it. 

(3.)  The  relative  proportion  of  each  of  these  chemical 
groups. 

II.  Mechanical  Construction. — The  relation  of  the  nutrient  to 
the  non-nutrient  parts  is  of  the  greatest  importance,  as  is  seen 
where  the  nutritious  starch  of  various  vegetables  is  inclosed  in 
insoluble  cases  of  cellulose,  which,  if  not  burst  by  boiling,  prevent 
the  digestive  fluids  from  reaching  the  starch. 

III.  Digestibility. — This  depends  partly  upon  how  the  sub- 
stances affect  the  motions  of  the  intestines,  and  partly  upon  their 
construction.  Thus,  some  substances,  such  as  cheese,  though 
chemically  showing  evidence  of  great  nutritive  properties,  by  their 
impermeability  resist  the  digestive  juices,  and  are  poor  aliments. 

IV.  Idiosyncrasy. — In  different  animals  and  in  different  indi- 
viduals, and  even  in  the  same  individuals  under  different  circum- 
stances, food  may  have  a  different  nutritive  value. 

Chemically,  then,  foods  are  composed  of  a  limited  number  of 
elements  similar  to  those  found  in  the  animal  tissues,  viz.,  carbon, 
oxygen,  nitrogen,  and  hydrogen,  together  with  some  salts.  If 
nothing  more  were  needed  by  the  economy  than  a  supply  of  these 
elements  and  salts  in  a  proportion  like  that  in  which  they  exist 
in  the  tissues, such  could  be  easily  obtained  from  inorganic  sources; 
but,  as  has  already  been  stated,  it  is  necessary  that  an  animal  ob- 
tain these  elements  associated  in  the  form  of  organic  materials  of 
complex  construction  (namely,  proteids,  etc.)  ready  made.  Al- 
lowing the  necessity  of  organic  food,  it  might  be  supposed  that 
since  the  elements  exist  in  proper  proportion  in  the  proteids,  au 


SPECIAL  FORMS  OF  FOOD.  93 

abundant  supply  of  proteids  would  suffice  for  all  nutritive  pur- 
poses, and  alone  form  an  adequate  diet.  Theoretically,  proteid 
alone  ought  to  be  sufficient  for  nutrition.  It,  however,  has  been 
frequently  tested  by  experiment,  and  practically  decided,  that  an 
animal  will  not  thrive  upon  a  free  supply  of  pure  proteid  food 
alone ;  and  in  the  human  subject  such  exclusive  diet  would  induce 
dangerous  abnormal  conditions  in  a  short  time.  Since  nitrogen 
is  an  important  element  in  nearly  all  parts  of  the  body,  we  could 
hardly  expect  that  a  diet  composed  of  non-nitrogenous  food-stuffs 
alone  could  support  the  animal  economy.  In  short,  the  results  of 
numerous  experiments  show  that  no  group  of  the  food-stuffs  al- 
ready enumerated  can  alone  sustain  the  body,  but  rather  that  a 
certain  proportion  of  each  is  absolutely  necessary  for  life. 

Fig.  42. 
Proteids.  Fats.         Carbolivd  rates.         Water. 


Human  milk,    .  .  . 
Cow's  milk,    .... 

Meat, 

Fish,    . 

Leguminous  fruits, 
Potatoes, 

Green  vegetables,    . 

Bread,    

Diagram  showing  the  percentage  of  the  principal  food-stuffs  in  a  few  typ- 
ical comestibles.  The  numbers  indicate  the  peixentages.  Indigestible  ma- 
terials are  omitted. 

Special  Forms  of   Food. 

The  articles  of  diet  we  make  use  of  are  animal  or  vegetable,  ac- 
cording to  the  source  from  which  they  are  derived.     It  will  be 


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94 


MANUAL   OF   PHYSIOLOGY. 


seen  that  a  varying  quantity  of  all  chemical  classes  of  food-stuffs 
are  generally  present  in  most  kinds  of  food,  whether  animal  or 
vegetable.  The  above  diagram  shows  the  proportion  of  the 
more  important  food-stuffis  in  some  examples  of  the  materials 
commonly  used  as  food. 

Among  animal  ibods  are  included  milk,  the  flesh  of  various 
animals,  and  the  eggs  of  birds.  These  may  be  more  fully  described 
as  typical  examples. 

Milk. — For  a  certain  period  of  their  lifetime  the  secretion  of 
the  mammary  gland  forms  the  only  food  of  all  mammals,  and  it 
is  the  one  natural  product  which  when  taken  alone  affords  ade- 
quate nutriment. 

It  consists  of  a  slightly  alkaline  watery  fluid,  containing: 

1.  Proteids,  in  solution. 

2.  Fats,  finely  divided  to  form  perfect  emulsion. 

3.  Sugar,  in  solution. 

4.  Salts,  in  solution. 

Owing  to  the  action  of  certain  organisms  which  readily  propa- 
gate in  milk  if  exposed  to  the  air  at  a  warm  temperature  for  some 
time,  it  loses  its  alkaline  reaction,  and  becomes  sour  from  the  for- 
mation of  lactic  acid  from  the  milk  sugar,  by  a  kind  of  fermen- 
tation, the  probable  equation  for  which  may  be  written  thus : 

Milk  sugar.  Lactic  acid. 

If  fresh  good  milk  be  allowed  to  stand,  the  fatty  particles  tend 
to  float  to  the  surface,  thus  forming  a  layer  of  cream. 

The  milk  of  different  animals  is  similar  in  all  essential  points? 
but  differs  slightly  in  the  relative  proportion  of  the  ingredients, 
as  may  be  seen  in  the  following  table : 


Human. 

Cow. 

Goat. 

Ass. 

Water 

Casein ) 

Albumin J 

Butter 

Milk  Sugar 

Salts 

Solids 

889.08 

39.24           1 

26.66 
43.64 

1.38 

110.92 

857.05 
48.28 

5.76 
43.05 
40.37 

5.48 

142.95 

863..58 
33.60 
12.99 
43.57 
40.04 
6.22 

136.42 

910.24 
j  20.18 
12.56 
1 57.02 

89.76 

loofi.              1000.              icon. 

1000. 

MILK. 


95 


Milk  varies  both  in  the  araouut  of  solids  in  solution,  and  fat, 
according  to  the  age  and  general  condition  of  the  animal,  period 
of  lactation,  time  of  day,  etc. 

Since  human  milk  is  much  poorer  in  proteid,  fat,  and  salts  (see 
Table),  and  richer  in  sugar,  than  that  of  the  cow  and  other  do- 
mestic animals,  it  is  necessary  to  dilute  the  latter  with  water,  and 
add  sugar,  when  it  is  substituted  for  human  milk  in  feeding  infants. 

The  great  value  of  milk  as  nutriment  depends  upon  the  fact 
that  it  contains  every  class  of  food-stuff,  viz.,  proteids,  fat,  carbo- 
hydrates, salts,  and  water,  in  the  proportion  demanded  by  the 

Fig.  43. 


o  — 


0&^ 


Microscopic  appearance  of  milk  in  the  early  stage  of  lactation,  showing 
colostrum  cells  (a). 

economy;  the  salts  in  milk  being  those  required  for  building  up 
the  bones  of  the  infant,  viz.,  phosphates  and  carbonates  of  lime,  etc. 

The  normal  variations  in  these  proportions  are  not  very  great, 
but  as  artificial  modifications  of  the  percentage  of  water  are  com- 
mon, a  knowledge  of  the  method  of  testing  the  purity  of  milk  is 
necessary. 

Milk  Tests. — The  specific  gravity  of  milk  gives  an  easy  measure 
of  the  solids  in  solution,  but  unfortunately  it  gives  no  estimate  of 
the  amount  of  fat  suspended  in  the  emulsion.  Therefore,  to  test 
milk  adequately  two  methods  must  be  employed  :  one  to  estimate 


96  MANUAL   OF   PHYSIOLOGY. 

the  degree  of  density  of  solution,  and  the  other  the  degree  of 
opa.eity  of  the  emulsion. 

I.  To  test  the  density,  a  specially  graduated  form  of  hydrometer 
is  generally  used.  This  is  graduated  so  as  to  indicate  specific 
gravities  from  1042  to  1014.  The  former  being  the  maximum 
density  of  pure  milk,  the  average  beiug  about  1030,  and  the  latter 
being  about  the  density  of  pure  milk  when  mixed  with  an  equal 
bulk  of  water.  Every  reduction  of  3  in  the  specific  gravity  may 
be  said  to  correspond  to  about  10  per  cent,  of  water. 

II.  The  degree  of  opacity  is  estimated  by  the  amount  of  water 
required  to  render  a  small  quantity  of  milk  sufficiently  translucent 
to  allow  a  candle-flame  to  be  seen  through  a  layer  of  the  mixture 
one  centimetre  thick.  One  cubic  centimetre  of  the  milk  (which 
has  been  shown  by  the  microscope  and  the  iodine  test  not  to  con- 
tain chalk  or  starch)  is  placed  in  a  test-glass  with  flat  parallel 
sides,  just  one  centimetre  apart,  and  water  is  cautiously  added 
from  a  graduated  pipette.  The  more  water  required  the  richer 
the  milk  is  in  fat ;  good  fresh  milk  requires  about  70  times  its 
bulk  of  water  to  become  translucent. 

Another  method  employed  for  the  same  purpose  consists  in  the 
comparison  of  the  color  produced  by  a  thin  layer  of  milk  in  a 
black  cell  with  a  previously  prepared  standard  of  grayish  colors. 

The  quantity  of  fat  may  also  be  estimated  by  placing  the  milk 
in  a  tall  graduated  vessel  for  twenty-four  hours,  at  the  end  of 
which  time  it  should  show  at  least  10  per  cent,  of  cream. 

Butter  is  made  from  milk,  or  better  from  cream,  by  breaking 
by  agitation  the  coating  of  proteid  which  before  churning  prevents 
the  oil  globules  from  running  together.  It  is  almost  completely 
composed  of  fat,  the  larger  globules  having  run  together  to  form 
the  solid  butter,  which  can  be  removed,  leaving  some  small  fat 
globules  with  the  proteids,  milk  sugar,  lactic  acid,  and  salts  in 
the  water  forming  "  buttermilk."* 

Cheese  is  another  form  of  food  made  from  milk  by  precipitating 
the  proteid  either  by  lactic  fermentation,  or  the  addition  of  rennet 
— an  extract  of  calves'  stomach  which,  without  the  presence  of 

*  For  the  details  of  secretion  of  milk,  etc.,  see  Mammary  Gland. 


MEAT,    ETC.  97 

any  acid,  curdles  milk — and  draining  off  the  solution  of  milk 
sugar  and  salts  ("  whey").  It  contains  most  of  the  proteid,  and 
a  great  deal  of  the  fat  of  the  milk.  During  the  ripening  of  the 
cheese  more  fat  is  formed,  apparently  from  the  proteid,  while  leucin 
and  tyrosin  also  appear. 

Meat. — We  use  the  flesh  of  the  vegetable-feeding  mammals  and 
birds  that  are  most  easily  obtainable,  and  many  kinds  of  fish. 
The  invertebrate  animals,  mostly  shell-fish,  need  hardly  be  men- 
tioned in  a  physiological  dietary,  and  are  not  spoken  of  as  meat. 

As  it  comes  from  the  butcher,  meat  consists  of  many  of  the 
animal  tissues,  the  chief  ones  being  flesh  (muscle  tissue),  fat,  and 
some  sinews  (fibrous  tissue).  The  fleshy  or  lean  part  of  meat  is 
chiefly  made  up  of  nitrogenous  materials,  and  contains  :  (1)  Sev- 
eral proteids,  chiefly  the  globulin,  viyosin;  (2)  gelatine  yielding 
substances ;  (3)  carbohydrates,  glycogen,  and  sugar ;  (4)  small 
quantities  of  fat ;  (5)  several  inorganic  salts  ;  (6)  extractives. 

Meat  may  be  eaten  raw,  but  as  it  is  impossible  to  impart  to  it 
the  various  flavors  which  our  artificial  tastes  demand  without  some 
special  preparation,  it  is  generally  cooked  before  use.  Moreover, 
the  not  infrequent  occurrence  in  muscle  of  parasites  which  would 
prove  injurious  if  swallowed  alive,  makes  the  exposure  of  meat 
to  a  temperature  high  enough  to  insure  their  destruction  advisable. 

Apart  from  pleasing  the  taste,  it  is  of  great  importance  so  to 
prepare  meat  as  to  preserve  in  it  all  the  nutrient  parts,  many  of 
which  arc  soluble  in  water,  and  therefore  are  apt  to  be  removed  if 
that  solvent  be  injudiciously  used.  Thus,  the  process  of  roasting, 
in  which  all  its  nutrient  parts  are  retained,  ought  to  be  more  satis- 
factory than  boiling,  by  which  the  salts,  extractives,  carbohydrates, 
gelatin,  and  some  albumin  may  be  dissolved  by  the  water.  How- 
ever, if  the  meat  be  plunged  into  water  which  is  already  boiling, 
the  proteids  near  the  surface  are  rapidly  coagulated,  and  the  water 
cannot  reach  the  central  parts  insufficient  quantity  to  remove  evefi 
the  soluble  ingredients.  The  whole  of  the  albuminous  parts  may 
be  thus  coagulated  as  the  temperature  of  the  inner  parts  rises  to 
boiling  point.  In  treating  meat  to  obtain  "stock"  ("bouillon") 
for  the  foundation  of  soups,  the  opposite  procedure  is  adopted. 
Cold  water  is  used,  and  the  temperature  slowly  and  gradually 


98  MAiSUAI.   OV   PHYSIOLOGY. 

raised,  but  not  quite  to  boiliug  poiut,  in  order  that  as  much  as 
possible  of  tiie  soluble  materials  may  be  extracted,  and  a  tasteless 
friable  muscle  tissue  remains  ("bouilli").  As  the  fluid  is  gener- 
ally allowed  to  boil  in  order  to  clear  it,  much  of  the  proteid  ma- 
terial which  was  dissolved  in  the  earlier  stage,  is  coagulated  and 
removed  with  the  scum.  Although  "stock"  cannot  contain  any 
great  proportion  of  the  most  important  constituents  of  meat,  it  is 
of  much  value  as  a  nutriment  in  medical  practice,  possibly  on 
account  of  some  stimulating  action  of  its  ingredients  upon  the 
motions  of  the  intestines  and  heart.  A  strongly  albuminous  ex- 
tract of  meat,  "beef-tea,"  may  be  made  by  digesting  flesh  in  a 
small  quantity  of  water,  and  keeping  the  temperature  below  that 
at  which  albumin  coagulates,  and  adding  vinegar  and  salt  to 
facilitate  the  formation  of  syntoniu  and  the  solution  of  myosin. 
The  salt  can  be  then  removed  by  dialysis. 

Eggs. — Eggs  consist  of  two  parts,  one  the  white,  composed  of 
albumin,  and  the  other,  the  yolk,  chiefly  made  up  of  fat. 

The  white  is  a  concentrated  watery  solution  of  albumin,  held  to- 
gether by  delicate  structureless  membranous  meshworks.  Besides 
the  albumin  it  contains  traces  of  fat,  sugar,  extractives,  and  salts. 

The  yellow  fat  emulsion  of  the  yolk  contains  a  peculiar  proteid, 
vitellin,  some  grape  sugar,  and  some  inorganic  salts,  in  which 
combinations  of  phosphoric  acid  and  potassium  are  conspicuous. 
Raw  eggs  are  difficult  of  digestion,  as  is  all  uncooked  albumin. 
Hard  boiled  eggs,  if  not  finely  divided  by  mastication,  are  also 
very  difficult  to  digest,  for  the  gastric  juice  cannot  penetrate  the 
hard  masses  of  coagulated  albumin  which  are  so  easily  and  com- 
monly swallowed.  Eggs,  when  lightly  cooked,  are  easily  digested, 
as  the  albumin  is  already  coagulated,  and  cannot  be  introduced 
into  the  stomach  in  large  masses.  Eggs  are  of  very  great  nutritive 
value,  as  they  contain  so  large  a  percentage  of  proteid,  fats  and  salts. 

Vegetable  Food. — Vegetables  differ  from  animal  food  : 

(1.)  In  containing  a  much  greater  proportion  of  material  which 
for  man  is  indigestible  (cellulose),  and  a  less  proportion  of  real 
nutritive  material. 

(2.)  The  percentage  of  proteid  is  below  that  of  animal  food, 
and  the  proportion  of  carbohydrates  is  generally  much  greater, 


VEGETABLES. 


99 


while  the  aiuouut  of  fat  is  small  but  varies  considerably.  In  order 
therefore  to  get  the  required  amount  of  nutritive  material  from  a 
purely  vegetable  diet,  it  is  necessary  to  consume  a  much  greater 
quantity,  and  the  amount  of  excrement  indicating  the  indigestible 
matters  is  proportionately  increased. 

Cereals. — The  most  valuable  forms  of  vegetable  food  are  those 
obtained  from  the  seeds  of  certain  kindred  plants  (Grayninacece), 
wheat,  rye,  maize,  oats,  rice,  etc.,  which  when  ground  are  used 
either  as  "whole  meal,"  or,  the  integument  ("bran")  being  re- 
moved, as  flour.  They  contain  different  kinds  of  proteid.  (1.)  A 
native  albumin  soluble  in  water  and  coagulable  by  heat,  and  in 
many  respects  like  animal  albumin  ;  but  as  it  cannot  be  obtained 

Fig.  44. 


Section  of  Pea,  showing  starch  and  aleurone  granules  imbedded  in  the 
protoplasm  of  the  cells.  (After  Sachs.) — a.  Aleurone  granules,  st.  Starch 
granules,     i.  Intercellular  spaces. 


pure  it  is  imperfectly  known.  (2.)  Vegetable  fibrin,  an  elastic 
body,  which  coagulates  spontaneously  and  is  difficult  to  separate. 
(3.)  Vegetable  glue  or  gliadin,  which  gives  the  peculiar  adhesive- 
ness to  the  gluten,  as  the  proteid  mixture  obtainable  from  corn  is 
commooly  called.  Cereals  also  contain  traces  of  fat,  and  a  very 
large  proportion  of  starch  and  some  salts. 

The  following  table  gives  the  percentages  of  the  chief  different 
nutritive  stuffs  in  some  common  cereals : 


100 


MANUAL  OF   PHYSIOLOGY. 


Wheat. 

Barley. 

Oats. 

Maize. 

Rice. 

Water 

13. 
13.53 

1.58 
69.61 

2. 

14.48 
12.26 

2.C3 
67.96 

2.65 

10.88 
9.04 
4. 

73.49 
2.59 

12. 
7.91 
4.83 

73.19 
1.28 

9.20 
5.06 

.75 
84.47 

.5 

Proteid 

Salts....". 

Green  Vegetables. — These  contain  some  starch,  sugar,  dextrin, 
salts,  and  minute  quantities  of  proteid,  and  are  of  small  nutritive 
value. 

Potatoes  contain  very  little  proteid,  but  a  considerable  quantity 
of  starch,  upon  which  their  nutritive  value  almost  entirely  depends. 

The  following  table  gives  the  relative  proportions  of  the  various 
nutritive  materials  contained  in  some  of  the  common  vegetable 
foods : 


Peas. 

Beans. 

Potatoes. 

Cauliflower. 

Water 

Proteid 

Carbohydrates 

14.50 

22.35 

56.61 

1.18 

1.% 

2.37 

12.85 

22. 

56.65 
3.32 
1.59 
2.53 

72.74 

1.32 

23.77 

.97 

.15 

1.05 

79.18 
.50 
18. 

Extractive 

Fats 

Salts 

.7 

The  most  striking  points  are  the  very  large  proportion  of  pro- 
teid in  the  leguminous  fruits,  and  the  comparative  richness  of  all 
vegetables  in  starchy  food-stuffs. 

Water  is  the  great  medium  by  the  solvent  power  of  which  food 
is  made  capable  of  ingestion.  Spring  water  always  has  a  certain 
quantity  of  lime  and  other  salts  in  solution,  and  in  proportion  to 
the  amount  of  salts  is  said  to  be  more  or  less  hard.  Water  is 
tasteless,  inodorous,  and  colorless  when  pure.  Soft  water,  such  as 
rain  water,  is  pure,  but  not  so  agreeable  to  taste  as  spring  water, 
and  is  very  liable  to  contamination  in  its  passage  over  roofs  pre- 
vious to  collection.  Standing  water  should  be  avoided  for  drink- 
ing, owing  to  the  probability  of  its  containing  organic  matter. 

Salts. — Great  varieties  of  salts  are  taken  into  the  system,  of 
which  chloride  of  sodium  forms  the  largest  proportion.  These 
have  no  doubt  very  important  functions  to  perform,  in  entering 


SALTS.  101 

into  combination  with  the  various  tissues,  and  also  probably  in 
aiding  the  chemical  changes  of  parts  of  which  they  do  not  form  a 
normal  constituent.  They  help  to  render  certain  substances  sol- 
uble, and  stimulate  the  cells  of  certain  glands  to  more  active 
secretion,  e.g.,  the  kidney  excretes  more  urea  when  there  is  an 
abundant  supply  of  common  salt  in  the  food. 


CHAPTER  YI. 

THE  MECHANISM  OF  DIGESTION. 

The  acts  of  digestion  may  be  divided  into  mechanical  and 
chemical  processes.  Under  the  mechanical  processes  come  the  ar- 
rangements for  the  subdivision,  onward  movement,  and  general 
mixture  of  the  food.  The  chief  objects  of  the  chemical  changes 
may  be  said  to  be  the  change  from  the  insoluble  to  the  soluble 
form  of  certain  kinds  of  food-stuffs  (starch,  proteids)  and  the 
finer  subdivision  of  others,  such  as  fats,  which  do  not  dissolve  in 
the  intestinal  or  body  juices. 

Attention  has  already  been  called  to  the  fact  that  there  are 
different  kinds  of  contracting  textures,  and  that  they  are  capable 
of  different  kinds  of  motion,  some  slow  and  steady,  some  rhyth- 
mical, some  sharp,  short  and  sudden.  It  must  also  be  remem- 
bered that  the  more  energetic  and  sudden  the  motions  are,  the 
more  marked  becomes  the  differentiation  of  the  tissue.  Thus  the 
active,  quick-contracting  skeletal  muscles  and  the  rhythmically 
acting  heart,  are  made  up  of  tissue  which  is  very  distinct  in 
structure  and  in  mode  of  action  from  that  of  the  contracting  cells 
composed  of  ordinary  protoplasm,  while  in  the  slowly  moving 
internal  organs  we  meet  tissue  elements  which,  in  different  ani- 
mals, show  many  stages  of  gradation  between  simple,  indifferen- 
tiated  protoplasm  and  the  special  striated  muscle  tissue. 

It  is  necessary  that  in  the  first  stages  of  alimentation  the  mo- 
tions should  be  quick  and  energetic;  so  the  mouth,  pharynx,  and 
upper  part  of  the  oesophagus  are  supplied  with  striated  muscle 
tissue,  which  differs  in  function  and  structure  from  that  of  the 
rest  of  the  alimentary  canal.  In  the  stomach  and  intestines  the 
slower  and  more  gradual  kinds  of  motion  are  required,  and  here 
we  find  a  good  example  of  non-striated  muscle  tissue. 


THE   MECHANISM   OF   DIGESTION.  103 

Around  the  extremity  of  the  rectum  is  a  band  of  smooth  mus- 

FiG.  45. 


Diagram  of  Alimentary  Tract.  Angles  of  mouth  slit  to  show  the  back 
of  the  buccal  cavity  and  the  top  of  the  pharynx. — (c)  Cardiac,  (p)  Pyloric 
parts  of  stomach  ;  (d)  Duodennm;  (i)  Jejnnum  and  Ilium;  (ac)  Ascend- 
ing, (ic)  Transverse,  and  (dc)  Descending  Colon  ;  {r)  Rectum  ;  (a)  Anus. 

cle,  which  remains  in  a  condition  of  persistent  or  tonic  contraction. 


104 


MANUAL   OF   PHYSIOLOGY. 


For  further  details  concerning  the  muscle  tissue  the  student 
must  turn  to  the  chapter  on  that  subject.  Here,  however,  it  may 
not  be  out  of  place  to  describe  briefly  the  special  character  of 
the  muscles  found  in  the  wall  of  the  digestive  tube  and  their 
general  arrangement. 

Mastication. — In  man,  the  introduction  of  food  into  the  mouth 
is  generally  accomplished  by  artificial  means,  so  that  the  biting 
teeth  (incisors)  and  the  tearing  teeth  (canines)  are  comparatively 
little  used  for  obtaining  a  suitable  morsel  of  food  (Fig.  46).     In 


Fig.  4G. 


Fig.  47. 


Fig.  46. — Transveise  section  of  the  Canine  Tooth  of  a  man. — (a)  Enamel ; 
(i)  Dentine;  (c)  Pulp  cavity;   (dj  Cnista  petrosa.     (Cadiat.) 

Fig.  47. — Structural  elements  of  the  Enamel  of  Tooth. — A.  Prisms  cut 
across  showing  the  hexagonal  section.     B.  Isolated  prisms.     (Kolliker.) 


the  mouth  the  all-important  act  of  chewing  or  mastication  is  ac- 
complished by  means  of  the  motions  of  the  lower  jaw,  the  tongue, 
and  the  cheeks.  This  process  of  breaking  up  the  solid  parts  of 
the  food  ought  to  be  continued  until  all  hard  substances  are 
ground  into  a  soft  pulp. 

Structure  of  the  Teeth. — The  exposed  part  of  the  teeth  is  cov- 
ered by  a  dense  substance  of  flinty  hardness  called  enamel,  which 
is  developed  from  the  epithelium,  and  consists  of  hexagonal 
prisms  set  on  end,  which  are  really  modified  epithelial  cells  but 
only  contain  about  2  per  cent,  of  animal  matter  (Fig,  47).  The 
bulk  of  the  tooth  is  made  up  of  dentine,  a  substance  like  bone  in 


STRUCTURE   OF   THE  TEETH. 


105 


composition,  pierced  by  numerous  fine  canals — dentine  tubes — 
which  radiate  towards  the  surface,  from  the  pulp  cavity,  in  the 
centre  of  the  tooth.  Processes 
of  protoplasm  run  in  the  den- 
tine tubes  from  the  tooth  cells, 
which  line  the  pulp  cavity  and 
preside  over  the  nutrition  of 
the  tooth.  The  cavity  contains 
vessels,  nerves,  etc.,  vvhich  en- 
ter at  the  root  of  the  tooth 
which  is  inclosed  in  a  kind  of 
modified  bone  tissue  called 
ci'usta  petrosa. 

The  two  rows  of  grinding 
teeth,  molars  and  premolars 
(one  on  each  side)  of  the  lower 
jaw  are  made  to  rub  against 
the  corresponding  teeth  in  the 
upper  fixed  jaw  by  the  com- 
bined vertical  and  horizontal 
movements  induced  by  the  ac- 
tion of  the  powerful  muscles 
of  mastication,  the  temporal 
muscles,  together  with  the 
masseters  and  internal  ptery- 
goids, all  tending  by  their  con- 
traction to  elevate  the  lower  jaw  and  bring  the  teeth  forcibly  to- 
gether. This  action  is  opposed  by  the  digastric,  the  geuio-  and 
mylohyoid  muscles,  which  by  their  combined  force  depress  the 
jaw  and  separate  the  teeth.  The  horizontal  movements  are  in 
the  main  accomplished  by  the  external  pterygoid  muscles,  which, 
acting  together,  pull  the  lower  jaw  forward  so  as  to  make  the 
lower  teeth  protrude  beyond  the  upper.  In  this  action  they  are 
opposed  by  the  digrastric  and  hyoid  muscles.  One  external 
pterygoid  on  either  side  acting  alone,  advances  that  side  of  the 
lower  jaw  only,  and  thereby  causes  the  lower  teeth  to  incline 
towards  the  opposite  side  in  a  lateral  direction.     The  two  muscles 

9 


Section  through  a  portion  of  the 
fang  of  a  tooth. — (a)  Dentine  tubules 
near  the  surface  of  the  fang ;  (6) 
Granular  layer ;  (c)  Crusta  petrosa. 


106 


MANUAL   OF   PHYSIOLOGY. 


acting  alternately  cause  a  horizontal  motion  from  side  to  side. 
Thus,  wiiile  the  lower  teeth  are  pressed  firmly  against  the  upper 
ones  they  are  at  the  same  time  made  to  glide  over  them,  either 
from  side  to  side  or  backwards  and  forwards.  By  these  move- 
ments the  bruised  food  is  soon  pushed  from  between  the  teeth, 
and  passes  tow^ards  either  the  tongue  or  cheek.  The  morsel  is 
soon  replaced  between  the  teeth  by  the  action  of  the  tongue  on 
the  one  hand  and  the  buccinator  muscle  in  the  cheek  on  the 
other. 

While  the  process  of  mastication  is  going  on,  the  food  becomes 
thoroughly  moistened  with  the  fluid  secreted  within  the  mouth. 

Deglutition. — The  next  step  is  swallowing.  When  the  food  is 
sufficiently  triturated  and  moistened  it  is  collected  together  by 
means  of  the  tongue,  and  placed  upon  the  upper  surface  of  that 

Fig.  49. 


Section  througli  a  portion  of  dentine  next  the  pulp  cavity  of  a  growing 
tooth. — (a)  An  isolated  odontoblast ;  (6)  Growing  part ;  (c)  Odontobla.sts  ; 
(d)  Filaments  of  protoplasm  projecting  from  the  tubercles  of  hard  dentine. 
(Beale.) 

orwan,  which  becomes  concave  and  presses  or  rolls  the  soft  pulp 
ao-ainst  the  hard  palate  so  as  to  shape  it  into  an  oblong  mass  or 
bolus  (Fig.  51 ).  The  apex  of  the  tongue  is  now  raised  and  pressed 
against  the  hard  palate,  and  by  the  successive  elevations  of  the 
different  parts  of  the  dorsum  of  the  tongue  the  bolus  is  gradually 
pushed  backwards  towards  the  isthmus  of  the  fauces.     The  root 


DEGLUTITION. 


107 


of  the  tongue  with  the  hyoid  boue  is  at  the  same  time  drawn  up- 
wards and  forwards,  so  that  the  bolus  easily  slips  down  along  the 
retreating  slope  leading  from  the  mouth  cavity,  and  gets  within 
the  reach  of  the  constrictors  of  the  fauces.  Immediately  before 
the  morsel  of  food  is  grasped  by  the  muscles  of  the  fauces  the 
levator  palati  draws  the  soft  palate  upwards  and  backwards  to 
completely  close  the  posterior  openings  of  the  nasal  cavity,  as  is 
shown  by  the  fact  that  during  the  act  of  swallowing  the  pressure 

Fig.  50. 


The  Pterygoid  Muscles  seen  from  without  after  removal  of  the  superficial 
parts,  the  temporal  muscle,  the  zygomatic  arch,  and  a  portion  of  the  lower 
jaw  and  masseter.     (1)  External,  (2)  Internal  pterygoid  muscle. 


in  the  nasal  cavity  is  raised.  At  the  same  moment  the  intrinsic 
muscles  of  the  larynx,  which  surround  the  rima  glottidis  like  a 
constrictor,  firmly  close  that  opening  by  approximating  the  cords 
and  arytenoid  cartilages.  The  entire  larynx  is  at  the  same  time 
drawn  up  behind  the  hyoid  bone  by  the  thyro-hyoid  muscle. 
The  rima  glottidis  is  thus  tucked  in  under  the  cushion  of  the 
epiglottis,  while  the  leaf  of  the  epiglottis  is  pulled  down  over  the 
larynx  by  the  oblique  aryteno-epiglottidean  and  thryo-epiglotti- 
dean  muscles. 

While  the  closure  of  the  nasal  and  pulmonary  air  passages  is 
going  on,  the  bolus  has  passed  out  of  the  cavity  of  the  mouth  and 


108 


MANUAL   OF   PHYSIOLOGY. 


has  been  caught  by  llic  pala to-glossal  and  pahito-pharyngeal 
muscles  which  force  it  iuto  the  pharynx,  and  at  the  same  time 
close  the  isthmus  faucium  behind  the  descending  morsel.  The 
stylo-pharyngeis  and  the  pharyngeal  constrictors  now  grasp  the 


Fm.  51. 


Muscles  of  Tongue  and  Pharynx.— 1,  2,  3,  Muscles  from  styloid  process 
(6)  to  the  tongue,  liynid  bone  (d)  and  pliarynx  res[)cctively  •  4  5  6  7  8 
muscles  of  tongue;  9,  10,  11,  constrictors  of  pharynx;  12,  cesophagus  •  13 
is  placed  on  larynx  (e).    (Allen  Thomson.) 

bolus  spasmodically,  and  the  latter  contract  in  rapid  succession 
moving  the  bolus  onwards,  and  drawing  themselves  over  it,  pass 
it  on  to  the  oesophagus,  where,  by  a  progressing  ring-like  contrac- 
tion of  the  circular  muscles  and  a  simultaneous  shortening  of  the 


DEGLUTITION. 


109 


longitudinal  layer  of  fibres  the  mass  is  slowly  squeezed  dowu  to 
the  cardiac  orifice  of  the  stomach.  The  movements  of  the  oeso- 
phagus are  essentially  peristaltic  in  character,  the  peculiarities 
of  which  form  of  motion  will  be  discussed  when  speaking  of  the 
intestinal  movements. 

Fig.  52. 


Deep  Muscles  of  Cheek,  Pharynx,  etc. — (1)  Orbicularis  oris;  (2)  buc- 
cinator; (3)  superior,  (4)  middle,  and  (5)  inferior  constrictors  of  the 
pharynx;  (6)  CBsophagus;  (7)  styloid  muscles  cut  across;  (8,  9,  10)  mus- 
cles attached  to  the  hyoid  bone  (</)  and  thyroid  cartilage  (e).  (Allen 
Thomson.) 


The  entire  process  of  swallowing  is  a  continuous  series  of  co- 
ordinated muscular  acts,  quite  independent  of  gravitation,  as  may 
be  seen  in  animals  drinking  with  their  heads  downwards.  Al- 
though all  the  complex  sets  of  movements  follow  each  other  per- 


110 


MANUAL   OF   PHYSIOLOGY. 


fec-tly  evenly  and  without  any  check  or  pause,  the  act  of  degluti- 
tion is  commonly  divided  into  three  so-called  stages,  between 
which,  however,  it  is  by  no  means  easy  to  draw  a  hard  and  fast 
line. 

The  j^rs^  stuf/e  is  simply  the  initiatory  step  of  placing  the  morsel 
of  food  or  some  liquid  in  such  a  position  as  to  excite  the  second 

Fig.  53. 


h  -,_ 


.J^ 


Transverse  section  of  ffisoplmgus  (Horsley). — a.  Outer  fibrous  covering. 
b.  Bundles  of  longitudinal  muscle  cut  across,  c.  Transverse  muscular  coat 
cut  obliquely,  d.  Sub-mucous  coat  with  glands  in  section,  e.  Muscularis 
mucosa;.    /.  Mucous  membrane  with  cut  vessels,     g.  Stratified  epithelium. 


or  spasmodic  act  of  deglutition.  This  first  step  is  a  voluntary 
act,  and  it  is  the  only  part  of  the  movements  of  swallowing  over 
which  we  have  any  voluntary  control.  The  progress  of  the 
morsel  between  the  tongue  and  palate  towards  the  fauces  may  be 
as  slow  and  gradual  as  we  wish,  but  the  moment  a  certain  point 


NERVOUS    MECHANISAE    OF    DEGLUTITION.  Ill 

is  reached  volition  is  at  an  end,  and  we  are  unable  to  check  the 
completion  of  the  act. 

By  the  second  stage  is  meant  the  period  occupied  by  the  passage 
of  the  food  bolus  through  the  pharynx  and  past  the  top  of  the 
larynx.  Although  we  are  not  able  to  influence  it  in  any  way  by 
our  will,  we  are  conscious  of  the  food  passing  in  this  region.  It 
is  a  rapid  involuntary  spasm  in  which  a  great  number  of  muscles 
take  part,  all  of  which  are  made  up  of  striated  muscle  tissue. 

The  third  stage  includes  all  the  rest  of  the-tirae  during  which 
the  bolus  is  passing  from  the  grasp  of  the  lower  pharyngeal  con- 
strictor and  along  the  oesophagus.  Not  only  has  our  will  no  in- 
fluence over  this  stage  of  deglutition,  but  we  are  hardly  conscious 
of  its  taking  place,  since  no  sensations  accompany  the  greater 
part  of  it.  Thus  the  more  essential  movements  of  the  act  of 
swallowing  are  purely  reflex  and  involuntary,  though  we  can  call 
forth  this  series  of  reflections  by  voluntary  stimulation  of  a  cer- 
tain part  of  the  fauces  by  means  of  a  morsel  of  food  or  a  drop  of 
liquid.  And  without  such  a  stimulus  as  food  or  liquid  we  cannot 
by  our  will  excite  swallowing.  We  think  we  can  perform  the 
muscular  movements  of  swallowing  when  we  please,  without  any 
food  or  fluid,  but  in  this  we  are  mistaken,  as  careful  observation 
of  our  own  performance  of  the  act  will  show. 

The  pharyngeal  spasm  is  always  preceded  by  the  deposition  in 
the  region  of  the  isthmus  faucium  of  some  drop  of  saliva  collected 
from  the  mouth  or  fauces  themselves.  In  fact,  without  a  slight 
preliminary  movement  of  the  posterior  part  of  the  tongue — which 
might  be  called  the  last  act  of  mastication — the  more  essential 
stages  of  deglutition  cannot  be  excited. 

Nervous  Mechanism. — The  voluntary  influences  which 
regulate  the  motions  of  the  muscles  of  mastication  pass  along  the 
efferent  branches  of  the  fifth  nerves  (trigemini)  which  accompany 
its  inferior  division.  The  muscles  which  depress  the  jaw  to  open 
the  teeth  and  the  intrinsic  muscles  of  the  tongue  are  supplied  by 
the  ninth  pair  of  nerves  (except  the  posterior  belly  of  the  digas- 
tric, which  has  a  branch  from  the  facial,  and  the  mylohyoid  and 
anterior  belly  of  the  digastric,  which  are  supplied  from  the  third 


llli  MANUAI^   OF   PHYSIOLOGY. 

division  of  the  fifth).  The  co-ordination  of  the  movements  of 
mastication  and  suction  seem  to  reside  in  the  medulla  oblongata, 
but  are  obviously  under  the  control  of  the  will. 

The  afferent  impulses  which  excite  the  nerve  centres  in  the 
medulla,  and  give  rise  to  reflex  acts  which  cause  the  swallowing 
movements,  pass  from  the  mucous  membrane  of  the  fauces  along 

(1)  the  descending  palatine  branches  of  the  spheno-palatine 
ganglion  and  the  second  division  of  the  trigeminus,  also  along 

(2)  the  pharyngeal  branches  of  the  superior  laryngeal  branch  of 
the  vagus  to  the  medulla,  where  the  co-ordination  of  the  pharyn- 
geal spasm  and  oesophageal  peristalsis  is  accomplished.  Thence 
the  eflferent  impulses  pass  along  (1)  the  hypoglos.sal  to  the  hyoid 
and  glossal  muscles,  (2)  the  glossopharyngeal  and  vagus  to  the 
pharyngeal  plexus  to  supply  the  constrictors,  and  (3)  along  the 
facial  and  fifth  to  supply  the  fauces  and  palate,  as  indicated  by 
their  anatomical  distribution. 

The  act  of  deglutition  can  be  readily  excited  in  an  animal 
which  is  deprived  of  all  the  nerve  centres  down  to  the  medulla 
oblongata,  and  may  also  be  seen  in  those  human  monstrosities 
(anencephalous  foetus)  which  are  born  without  the  upper  part  of 
the  brain  being  developed,  but  can  both  suck  and  swallow. 

The  movements  of  the  oesophagus  are  reflections  from  the  cen- 
tral nervous  system  (medulla),  both  sets  of  impulses  (possibly 
the  afferent  and  certainly  the  eflTerent)  passing  along  the  branches 
of  the  vagus. 

It  would  appear  that  the  normal  peristaltic  movements  of  the 
oesophagus  are  always  initiated  by  a  pharyngeal  spasm,  and  that 
they  form  an  inseparable  sequel  to  it.  Thus  the  wave  of  con- 
traction pa.¥ses  along  the  entire  length  of  the  oesophagus,  even 
when  the  bolus  is  stopped  mechanically,  and  on  the  other  hand 
a  body  introduced  into  the  oesophagus  without  passing  through 
the  pharynx  excites  no  peristaltic  wave  and  remains  motionless. 

But  it  has  been  observed,  in  apparent  contradiction  to  the  fore- 
going statement,  that  the  oesophagus  when  removed  from  the 
body,  and  therefore  quite  independent  of  the  pharynx  and  its 
nervous  connections,  can  be  excited  to  move  peristaltically.  In 
this  case   the  medulla  or  vagus  can  have  no  part  in  bringing 


MOTION    OF   THE   STOMACH. 


113 


about  this  wave  of  movemeut.  To  explain  this  discrepancy  it 
may  be  urged  that  the  local  nerve  and  muscle  mechanism  in  the 
tissues  of  the  oesophagus  are  capable  by  themselves  of  carrying 
out  peristaltic  contraction  independently  of  the  central  nerve 
organs,  but  that  this  power  is,  under  ordinary  circumstances, 
held  in  check  by  the  vagus.  The  inhibition  is  temporarily  sus- 
pended as  a  sequence  of  pharyngeal  spasm,  and  consequently  a 
wave  of  peristaltic  contraction  is  excited  in  the  oesophageal  mus- 
cles, either  in  response  to  the  direct  stimulus  of  a  passing  bolus, 
or  as  a  result  of  impulses  reflected  along  the  vagus  channels  from 
the  medulla. 

Fig.  54. 


Diagram  of  Wall  of  the  Stomach,  showing  the  relative  thickness  of  the 
mucous  membrane  (a,  b,  c),  and  the  transverse  (e),  oblique  (/),  and  longi- 
tudinal muscle  fibres. 


Motion  of  the  Stomach. — The  stomach  and  greater  part 
of  the  intestinal  tract  move  freely  within  the  abdomen,  being 
covered  by  the  smooth  serous  lining  of  that  cavity,  which  also 
keeps  in  position,  so  as  to  restrict  their  movements,  those  parts, 
such  as  the  duodenum,  into  which  the  ducts  of  large  glands  open. 
When  the  stomach  is  empty  it  hangs  with  the  great  curvature 
downwards,  and  the  muscular  coats  are  quiescent.  On  being 
filled  it  is  passively  rotated  on  its  long  axis,  so  that  the  greater 
curvature  is  turned  forwards,  here  meeting  with  less  resistance, 
and  the  lesser  curvature  is  turned  backwarks  to  its  line  of  attach- 

10 


1  14  MANUAL   OF   PHYSFOIiOGY. 

ment.  In  the  main,  the  motions  of  the  stomach  are  peristaltic. 
They  become  very  active  about  fifteen  minutes  after  the  intro- 
duction of  food,  and  gradually  become  more  and  more  energetic 
until  the  end  of  stomach  digestion,  which  lasts  about  five  hours. 

The  result  of  the  peristaltic  motion  is  to  move  the  food,  par- 
ticularly the  part  next  the  gastric  wall,  along  the  great  curvature 
towards  the  pylorus.  A  back  current  towards  the  cardiac  ex- 
tremity has  been  noticed  running  along  the  lesser  curvature,  and 
trhe  median  axis  of  the  food  mass.  At  the  same  time  a  peculiar 
rotatory  motion  of  the  gastric  wall  takes  place,  similar  to  that  of 
rolling  a  ball  between  the  palms  of  the  hands,  so  that  the  food 
is  twisted  in  a  given  direction,  and  the  deeper  lying  portion  is 
brought  into  contact  with  the  mucous  membrane. 

While  the  fundus  keep  up  considerable  pressure  on  the  con- 
tents of  the  stomach,  the  indistinct  peristaltic  action  of  the  central 
part  is  intensified  on  nearing  the  pylorus  into  a  strong  circular 
contraction,  which  proceeds  as  a  definite  wave  towards  the  pyloric 
valve,  through  which  it  gradually  forces  the  more  or  less  digested 
food.  At  first  only  the  fluid  parts  are  allowed  to  pass,  but  towards 
the  later  stages  of  digestion  the  fatigued  pyloric  muscle  admits 
solid  masses  into  the  duodenum. 

Nerve  Influence  on  Stomach  Motions. — The  stomach 
has  nerve  connections  with  the  cerebro-spinal  axis  through  the 
vagi,  and  the  splanchnic  branches  of  the  sympathetic,  and  in  the 
walls  of  the  organ  itself  are  numerous  ganglion  cells.  The  sym- 
pathetic connections  do  not  seem  to  have  any  influence  on  the 
muscular  coats,  for  neither  their  stimulation  nor  section  has  any 
marked  effect  on  their  movements.  If  the  vagi  be  severed,  stom- 
ach contractions  still  occur,  but  no  form  of  local  stimulation  pro- 
duces the  normal  gastric  motions,  even  if  the  organ  be  quite  full 
of  food,  therefore  it  would  appear  that  the  local  nerve  centres 
are  not  sufficient  to  excite  the  normal  rhythmical  muscular  ac- 
tion. Moreover,  stimulation  of  the  cut  vagi  leading  to  the  stom- 
ach causes  active  movements  when  the  stomach  is  full.  It  is  not 
merely  the  presence  of  food  that  produces  the  movements,  as  is 
shown  by  the  fact  that  the  njotious  increase  as  the  contents  of  the 


VOMITING.  115 

stomach  diminish,  but  conditions  incidental  to  digestion  (hyper- 
a^mia,  etc.)  probably  also  act  as  a  stimulus. 

Vomiting  is  the  ejection  of  the  contents  of  the  stomach  by 
means  of  a  convulsive  action  of  the  respiratory  and  abdominal 
muscles  associated  with  an  abnormal  contraction  of  the  stomach 
wall,  which  aids  in  opening  the  cardiac  orifice  while  it  keeps  the 
pylorus  firmly  closed. 

The  act  of  vomiting  is  commonly  preceded  by  (1)  a  feeling  of 
sickness  or  nausea,  (2)  a  great  seci'etion  of  saliva,  (3)  retching. 
The  latter  consists  in  a  violent  inspiratory  effort  in  the  midst  of 
which  the  root  of  the  tongue  and  larynx  are  raised  and  the  rima 
glottidis  suddenly  closed  so  as  to  prevent  air  entering  the  wind- 
pipe. The  inspiratory  muscles  still  acting,  and  the  pharynx  and 
upper  part  of  the  oesophagus  being  held  open,  air  is  drawn  into 
the  gullet  and  dilates  this  tube  nearly  as  far  as  the  opening  into 
the  stomach.  A  contraction  of  the  muscle  fibres  radiating  from 
the  oesophagus  over  the  stomach,  then  opens  the  cardiac  orifice 
and  alloAvs  some  gas  to  escape.  Now  the  act  of  vomiting  is  com- 
pleted if  at  this  moment — the  mouth  and  pharynx  being  open, 
the  larynx  closed,  the  oesophagus  on  the  stretch,  the  cardiac  ori- 
fice relaxed,  and  the  pylorus  firmly  closed — the  expiratory  mus- 
cles forcibly  contract,  and,  pressing  upon  the  abdominal  cavity, 
give  a  sudden  stroke  to  its  contents  so  as  to  empty  the  stomach. 
The  wall  of  the  stomach  also  contracts  evenly  throughout,  but 
not  ^Yith  any  forcible  anti-peristaltic  action  such  as  would  greatly 
aid  in  the  operation  of  rapidly  ejecting  the  vomit.  The  chief 
object  attained  in  the  adult  by  the  action  of  the  muscular  coat 
of  the  stomach  seems  to  be  the  I'elaxation  of  the  cardiac  orifice. 
In  children,  when  the  fundus  is  little  developed,  and  the  fibres 
radiating  over  the  stomach  from  the  oesophagus  are  numerous  and 
strong,  the  act  of  vomiting  requires  less  effort  on  the  part  of  the 
respiratory  muscles,  the  frequent  puking  of  suckling  infants  being 
accomplished  by  the  gastric  muscle  alone.  When  the  vomit  is 
emitted,  the  hyoidean,  laryngeal,  and  neck  muscles  relax,  and 
the  air  is  forcibly  driven  out  of  the  partially  distended  lungs  so 
as  to  clear  away  any  remaining  particles  from  the  upper  part  of 
the  air-passages. 


116  MANUAL   OF    PHYSIOLOGY. 

Vomiting  is  usually  caused  by  irritation  of  the  stomach  itself, 
and  may  be  induced  by  either  mechanical,  electrical,  or  chemical 
stimulation  of  the  mucous  membrane.  In  this  way  some  emetics, 
such  as  mustard,  sulphate  of  copper,  etc.,  act.  It  may  also  be 
caused  by  intestinal  irritation,  as  when  a  hernia  is  strangulated, 
or  the  mucous  membrane  irritated  by  intestinal  worms. 

Gentle  stimulation  of  the  fauces  and  neighborhood  of  the  root 
of  the  tongue  commonly  induces  vomiting.  In  the  early  stages 
of  pregnancy  the  unusual  condition  of  the  uterus  causes  frequent 
vomiting,  which  is  known  as  "morning  sickness."  The  irritation 
of  a  calculus  passing  through  the  ureter,  or  a  gall-stone  impacted 
in  the  bile  duct,  commonly  excites  vomiting.  Injuries  of  the 
brain,  and  psychical  impressions,  particularly  those  excited  by  the 
sense  of  smell  or  unusual  disturbance  of  equilibrium,  may  give 
rise  to  vomiting.  Moreover,  a  number  of  medicaments,  as  apo- 
morphin,  emetin,  etc.,  cause  vomiting  if  introduced  into  the  blood. 

From  the  foregoing  facts  it  appears  that  vomiting  is  a  complex 
and  irregular  muscular  act,  which  may  be  induced  by  the  stimu- 
lation of  various  parts  of  the  internal  surfaces  of  the  body,  par- 
ticularly those  which  receive  branches  from  the  vagus  nerve. 

One  would  therefore  be  inclined  to  suppose  that  some  afferent 
nerve  channels  exist  in  the  vagus  which  bear  impulses  to  a  vom- 
iting nerve  centre  and  excite  it,  so  as  to  cause  it  to  send  forth 
peculiar  and  irregular  impulses  to  the  respiratory,  gastric,  and 
other  muscles,  and  give  rise  to  their  characteristic  spasm. 

In  short,  it  would  seem  to  be  a  reflex  act,  the  afferent  impulses 
of  which  pass  to  the  medulla  oblongata  by  the  vagus,  and  the 
efferent  impulses  are  conveyed  by  the  ordinary  spinal  nerves  to 
the  respiratory  muscles,  by  the  vagus  to  the  pharyngeal,  laryn- 
geal, and  gastric  muscles,  and  by  the  fifth,  seventh,  and  ninth 
nerves  to  the  palatine,  facial,  and  hyoidean  muscles.  This  vom- 
iting nerve  centre  must  lie  in  the  medulla,  in  very  close  relation- 
ship to  the  respiratory  centre,  with  which  it  nearly  corresponds. 
This  centre  may  bring  about  the  whole  sequence  of  events  known 
as  vomiting  when  stimulated  either  directly  by  poisons,  contained 
in  the  blood,  indirectly  through  the  vagus,  or  even  from  the 
higher  centres  by  emotions  or  ideas.     Section  of  the  vagi  renders 


INTESTINAL    MOVEMENTS.  117 

vomiting  impossible,  as  it  cuts  ofi'  both  the  commonest  source  of 
stimulus  going  to  the  centre,  and  also  the  important  efferent  im- 
pulses which  cause  the  muscle  coat  of  the  stomach  to  contract 
and  to  open  the  cardiac  orifice. 

Movements  of  the  Intestines.— The  muscular  coats  are 
somewhat  differently  arranged  in  the  small  and  the  large  intes- 
tines, but  have  the  same  general  relation  to  each  other,  viz.,  a 
thin  longitudinal  layer  lying  externally,  next  the  serous  mem- 
brane, and  a  layer  of  circular  fibres  considerably  thicker  lying 
internally  under  the  raucous  membrane.  In  the  large  intestine 
the  external  longitudinal  fibres  are  collected  into  three  bands 
placed  at  equal  distances  one  from  another,  which,  being  rather 
shorter  than  the  remainder  of  the  intestine,  throw  the  interme- 
diate part  into  a  series  of  pouches. 

It  is  in  the  small  intestine  that  peristaltic  motion  of  the  most 
typical  kind  occurs.  A  wave  of  contraction  passes  from  the 
pylorus  along  the  circular  fibres,  so  as  to  look  like  a  broad  ring 
of  constriction  progressing  slowly  downwards. 

The  longitudinal  fibres  at  the  same  time  contract  so  as  to 
shorten  the  piece  of  intestine  immediately  below  the  ring  of  con- 
striction, and  also  cause  a  certain  amount  of  rolling  movement 
of  these  loops  of  intestine  which  are  free  enough  to  move. 

This  motion  takes  place  periodically  in  proportion  to  the 
amount  and  character  of  the  contents  of  the  intestine,  the  food 
passing  over  the  raucous  membrane  being  to  all  appearance  the 
stiraulus  which  normally  calls  forth  and  intensifies  the  action. 

The  activity  of  the  peristaltic  movements  varies  with  many 
circumstances  besides  the  contents  of  the  intestines.  Of  these  the 
most  noticeable  is  the  amount  and  character  of  the  blood  flowing 
through  the  vessels  of  the  intestinal  wall.  Thus  stoppage  of  the 
blood  current  by  tying  the  arteries  or  deficiency  of  oxygen  and 
excess  of  carbonic  acid  causes  inordinate  activity  of  the  peristaltic 
action.  Direct  irritation  of  the  serous  surface  of  the  intestine, 
with  mechanical,  chemical,  or  electrical  stimuli  causes  also  in- 
crease in  the  movements  of  the  intestine. 

The  great  activity  of  the  motion  observed  when  the  abdominal 


118 


MANUAIv   OF   PHYSIOLOGY. 


cavity  of  a  recently  killed  animal  is  opened  depends  partly  on 
the  exposure  to  cool  air,  and  partly  on  the  venous  character  of 
the  blood  in  the  vessels  no  longer  oxidized  by  respiration. 

The  irregular  and  impetuous  action  of  the  intestine  which 
follows  the  constriction  or  strangulation  of  a  hernial  protrusion, 
chiefly  depends  on  the  mechanical  effect,  but  also  is  probably 
intimately  related  to  the  interference  with  the  blood  supply  con- 


Diagi'am  of  a  longitudinal  section  of  the  wall  of  the  small  intestine. — «• 
Villi.  6.  Lieberkuhn's  glands,  c.  Muscularis  miicosEe,  below  which  lies 
Anerbach's  nerve  plexus,  d.  Connective  tissue  in  which  many  blood  and 
lymph-vessels  lie.  e.  Circular  muscle  fibres  cut  across,  with  Meissner's  nerve 
plexus  below.    /.  Longitudinal  muscle  fibres,     g.  Serous  coat. 

sequent  on  the  pressure  exerted  by  the  constricting  baud.  Pro- 
longed over-work  often  induces  immobility  of  the  intestinal  wall, 
and  hence  we  find  the  purging  and  vomiting,  which  accompany 
a  temporary  hernial  constriction,  followed  by  inability  of  the 
intestine  to  propel  its  contents.  These  points  have  also  been 
proved  by  results  of  experiments  on  the  lower  animals. 

The  moveiiient  of  the  large  intestines  are  the  same  as  the  small, 
but  not  so  obvious,  owing  to  the  modified  sacculated  shape  of  this 
part  of  the  alimentary  canal.  The  contractions  of  the  colon  begin 
at  the  ileo-ciecal  valve  where  the  peristaltic  wave  of  the  ileum 
ceases.  The  normal  intestinal  motions  thus  pass  in  an  almost 
uninterrupted  wave  from  the  pylorus  to  the  end  of  the  gut,  but 
when  special  sources  of  irritation  exist,  a  wave  may  originate  in 


DEFECATION.  119 

almost  auy  intermediate  part  of  the  intestine.  A  reversed  "  anti- 
peristaltic motion,"  as  it  is  called,  only  occurs  as  a  result  of  some 
intense  local  stimulation,  such  as  the  strangulation  of  a  hernia,  etc. 

The  motion  produced  by  the  substances  contained  in  the  intes- 
tine depends  on  their  character.  The  solid  parts  excite  more 
rapid  movements,  and  the  more  fluid  portions  but  slightly  influ- 
ence the  intestinal  peristalsis. 

Thus  the  solids  which  make  their  way  through  the  pylorus  are 
seldom  to  be  found  in  the  jejunum,  no  matter  at  what  period  after 
a  meal  the  animal  be  killed,  whereas  the  folds  of  the  mucous  mem- 
brane are  always  bathed  in  a  fluid  creamy  material  during  the 
entire  period  of  digestion,  and  even  for  a  considerable  time  after 
all  the  food  has  left  the  stomach. 

Mechanism  of  Defaeeation. — This  is  a  point  of  much 
importance,  for  the  evacuation  of  the  lower  bowel  is  intimately 
connected  with  feelings  of  comfort  and  health,  and  the  insuring 
of  its  accomplishment,  in  illness,  forms  an  essential  part  of  the 
physician's  duty. 

The  movements  of  the  intestine  cause  the  various  excretions 
and  indigestible  parts  of  the  food  to  pass  towards  the  sigmoid 
flexure  of  the  colon,  where  their  onward  motion  is  checked  for  a 
time  by  the  strong  circular  muscle  of  the  rectum  (called  the  su- 
perior sphincter,  or  tertius  by  Hyrtl),  which  does  not  carry  on  the 
peristaltic  wave.  The  materials  here  get  packed  into  a  more  or 
less  solid  mass,  which  is  gradually  augmented  after  each  meal. 

The  lower  outlet  of  the  alimentary  canal  is  closed  by  two  distinct 
sphincter  muscles.  One  thin  external  superficial  muscle,  made 
up  of  striated  fibres,  belongs  to  the  perineal  group,  and  has  little 
influence  on  the  closure  of  the  anus.  The  deep  or  internal  sphinc- 
ter, which  is  much  stronger,  surrounds  the  gut  for  rather  more 
than  an  inch  (3  centimetres,  Henle)  in  height,  and  is  one-quarter 
inch  thick.  It  is  made  of  smooth  muscle,  and  therefore  capable 
of  prolonged  (tonic)  contraction.  It  would  appear,  however,  that 
this  strong  sphincter  is  a  supernumerary  guard  to  the  anal  orifice, 
which  is  but  rarely  called  into  action,  for  during  the  interval  of 
rest  between  the  acts  of  deftecation,  the  faces  do  not  come  in  con- 


120  MANUAL   OF   PHYSIOLOGY. 

tact  witli  the  j)oitioii  of  intestine  surrounded  by  this  muscle.  The 
rectum  for  quite  one  incli  above  tlie  sphincter  is  jierfectly  empty, 
being  kept  free  from  fieculent  particles  partly  by  a  fold  of  the  in- 
testinal wall,  and  partly  by  the  repeated  action  of  the  voluntary 
muscles  in  the  ncit^hborhood,  which  by  intensifying  the  angle  that 
exists  at  this  point,  and  flattening  this  inch  of  rectum,  can  squeeze 
back  the  approaching  matters.  Anyone  fiimiliar  with  the  digital 
examination  of  the  nnevacuated  rectum,  knows  that  no  fteces  are 
met  with  for  about  two  inches. 

Considerable  accumulation  may  take  place  in  the  sigmoid  flex- 
ure without  much  discomfort  ensuing,  but  when  the  rectum  is  dis- 
tended, an  urgent  sensation  of  wanting  to  empty  it  is  experienced, 
and  the  voluntary  movements  mentioned  above,  are  performed  by 
the  levator  ani  and  the  neighboring  perineal  muscles,  with  the 
object  of  preventing  any  substance  reaching  the  part  of  the  rectum 
immediately  above  the  sphincter. 

If  the  rectum  be  distended  with  fluid,  the  occasional  anal  eleva- 
tion does  not  suffice  to  keep  it  back,  and  a  continuous  and  com- 
bined action  of  the  sphincters  and  levator  ani,  etc.,  is  necessary  to 
ward  ofFtiie  expulsion  of  the  contents. 

When  the  lower  bowel  is  habitually  emptied  at  the  same  hour 
daily — a  habit  which  should  be  carefully  exercised — the  sensations 
of  requirement  to  go  to  stool  occur  with  great  punctuality,  or  can 
be  readily  induced  by  the  will,  so  that  normal  defsecation  is  re- 
puted to  be,  and  practically  is,  a  voluntary  act.  But  not  com- 
pletely so,  for  somewhat  like  swallowing,  the  later  stages  of  defse- 
cation  consist  essentially  of  a  series  of  involuntary  reflex  events 
which  we  can  initiate  by  the  will,  but  when  it  is  once  started,  are 
powerless  to  modify  until  the  reflex  sequence  is  completed. 

Under  ordinary  circumstances,  the  evacuation  of  the  fseces  is 
commenced  by  the  voluntary  pressure  exercised  on  the  abdominal 
contents  by  the  respiratory  muscles.  The  diaphragm  is  depressed, 
the  outlet  of  the  air  passages  firmly  closed,  and  the  expiratory 
muscles  thrown  into  action,  while  at  the  same  moment  the  muscles 
which  close  the  pelvic  outlet  relax,  and  allow  the  anus  to  descend, 
so  that  the  inferior  angle  of  the  rectum  is  straightened  and  a  vol- 
untary inhibition  of  the  sphincter  is  brought  about.     This  volun- 


INTESTINAL   NERVE-MECHANISM.  121 

tarv  expiratory  effort  seldom  requires  to  be  continued  for  more 
thau  three  or  four  seconds  before  some  fiecal  matter  reaches  the 
part  of  the  rectum  just  above  the  sphincter.  When  this  has  oc- 
curred, no  further  abdominal  pressure  is  necessary  (except  when 
the  masses  of  fseces  are  large  and  hard),  for  the  local  stimulus 
starts  a  series  of  reflex  acts  which  carry  on  the  operation.  These 
consist  of  an  increased  peristaltic  contraction  of  the  colon  and 
sigmoid  flexure,  the  waves  of  which  pass  along  the  rectum.  These 
waves  are  accompanied  by  synchronous  rhythmical  relaxation  of 
the  sphincter,  which  replaces  its  normal  condition  of  tonic  con- 
traction. 

The  effect  of  the  voluntary  eflTort,  and  the  amount  of  the  "ab- 
dominal pressure  required,  depend  upon  the  consistence  of  the 
ffeces.  When  quite  fluid,  they  constantly  tend  to  come  in  con- 
tact with  the  sensitive  point  of  the  rectum,  and  a  voluntary  effort 
is  required  to  prevent  the  reflex  series  of  events  from  taking 
place,  a  momentary  relaxation  of  the  sphincter  with  voluntary 
abdominal  pressure  is  sufficient  to  eject  the  contents  of  the  bowel. 
On  the  other  hand,  when  the  faeces  are  firm,  time  is  required  in 
order  that  the  slowly  acting  smooth  muscle  may  pass  the  mass 
onward.  In  common  constipation,  the  difficulty  is  to  get  the 
solid  mass  down  to  the  sensitive  exciting  point,  in  which  case  a 
few  drachms  of  warm  fluid,  used  as  an  enema,  may  awaken  the 
necessary  reflex  movements. 

Nervous  Mechanism  of  the  Intestinal  Motion. — 

Many  points  in  the  nervous  control  exerted  over  the  intestinal 
muscles  are  obscure.  We  know  that  intestinal  movements  which 
are  peristaltic  in  their  nature  occur  in  a  portion  of  intestine  re- 
moved from  the  body,  and  thus  separated  from  all  central  nerv- 
ous control.  We  know  also  that  there  are  abundant  nerve  ele- 
ments in  the  walls  of  the  intestines  which  have  all  the  characters 
of  ganglion  cells,  and  therefore  probably  act  as  nerve  centres. 
(Figs.  56,  57.) 

With  regard  to  these  local  nervous  agencies,  anatomists  have 
made  out  two  distinct  sets,  both  of  which  have  the  form  of  a  net- 
work of  nerve  fibrils  studded  with  cell  elements  at  their  nodal 


122 


MANUAL   OF    PHYSIOLOGY. 


points.  Oue  of  these,  a  closely  nie.shed  plexus  with  flattened 
cords  and  ganglionic  masses  at  their  points  of  union,  lies  between 
the  longitudinal  and  circular  layers  of  muscle  (Figs.  56,  57) 
forming  the  plexus  myenierlcns  exterior  of  Auerbach,  and  most 
likely  has  to  do  with  the  movements  of  these  layers  of  muscle. 
The  other  lies  internal  to  the  circular  muscle,  in  close  relation  to 
the  niuscularis  mucosae,  and  is  called  the  plexus  myenterieus  in- 


Fig.  56. 


Fig.  57. 


Fig.  56. — Auerbach's  plexus  from  between  the  muscle  coats  of  the  in- 
testine, with  low  power. 

Fig.  57. — A  nodal  point  of  Auerbach's  plexus  under  high  power,  show- 
ing the  nerve  cells. 


terior  of  Meissner ;  the  meshes  of  which  are  looser  and  more 
irregular,  and  the  cords  and  ganglia  more  rounded  and  finer  than 
those  of  Auerbach's  plexus.     (Figs.  58,  59.) 

The  blood  flowing  through  these  nerve  centres  in  all  probability 
acts  as  a  sufficient  stimulus  under  ordinary  circumstances,  to  pro- 
duce some  peristaltic  motions,  and  hence  we  may  say  that  they 
are  automatic.  When  food  comes  into  the  intestine  it  increases 
the  flow  of  blood,  as  well  as  mechanically  irritating  the  intestinal 
wall.  The  intestinal  vessels  remain  engorged  so  long  as  the  pro- 
cess of  digestion  goes  on.  Food  seems  to  act  more  effectually 
than  insoluble  mechanical  stimuli,  for  when  insoluble  substances 
are  placed  in  the  gut,  they  at  first  call  forth  active  movements; 
but  these  do  not  last  long,  for  the  stimulus  is  not  of  itself  ade- 


INTESTINAL    NERVE-MECHANISM. 


123 


quate  to  excite  prolonged  action,  except  it  be  associated  with 
coutinuiug  congestiou  dependent  upon  other  causes,  such  as  the 
vaso-naotor  changes  accompanying  the  general  digestive  process, 
and  the  absorption  of  the  prepared  food-stuffs. 

With  regard  to  the  influence  of  other  nerves,  it  seems  to  be 
admitted  on  all  sides  that  the  vagus  acts  as  an  exciting  nerve, 
since  stimulation  of  its  peripheral  part  causes  increased  action, 
and  it  is  probable  that  its  great  efferent  channel  for  impulses  is 
reflected  through  the  brain. 

On  the  other  hand,  the  splanchnic  nerves,  which  come  from 
the  thoracic  sympathetic,  are  said  to  be  inhibitors  of  the  myen- 


FiG.  58. 


Fig.  59. 


Fig.  58.— Meissner's  plexus,  low  power. 

Fig.  59. — Meissner's  ijlexus  (high  power),  showing  cells  grouped  at  nodal 
points. 

teric  plexuses.  This  may  be  explained  by  their  effect  on  the 
small  vessels — which  they  no  doubt  control — causing  a  change 
in  the  blood  supply.  Be  this  as  it  may,  the  splanchnic  seem  to 
have  considerable  influence  on  the  intestinal  movements.  When 
stimulated  they  commonly  check  the  intestinal  motions,  but  may 
sometimes  (as  when  the  movements  have  ceased  after  death),  give 
rise  to  new  movements. 

On  account  of  this  double  action,  it  has  been  said  that  there 
are  two  kinds  of  fibres,  (Ij  inhibiting,  which  are  easily  excited, 
and  during  life  have  greater  influence,  and  (2),  exciting,  which, 
though  less  excitable,  retain  their  irritability  longer. 


124  MANUAL    OF    PHYSIOLOGY. 

However,  most  of  these  efiects  may  be  explained  by  referring 
them  to  vaso-motor  changes. 

With  regard  to  deficcation,  we  know  that  a  nerve  centre  exists 
in  the  lumbar  portion  of  the  spinal  cord,  which  governs  the 
sphincter,  and  seems  to  keep  up  its  tonic  contraction.  This 
centre  may  be  either  excited  to  increased  action  or  inhibited,  by 
peripheral  stimuli  or  b}'  central  influences  from  the  brain. 

Thus  the  local  application  of  warmth  causes  inhibition  of  the 
centre,  and  thereby  relaxation  of  the  sphincter,  while  cold  gives 
rise  to  increased  central  action,  causing  contraction  of  the  sphinc- 
ter muscle  (a  point  to  be  remembered  when  examining  or  op- 
erating within  its  grasp).  Besides  the  voluntary  variations  which 
we  can  bring  about  in  the  activity  of  this  lumbar  centre,  many 
other  central  influences,  such  as  emotions,  may  operate  upon  it. 
Thus  terror  inhibits  the  centre,  and  loosens  the  sphincter  inde- 
pendently of  our  will. 


CHAPTER  YII. 


MOUTH  DIGESTION. 


Fig.  60. 


The  cavity  of  the  mouth  is  lined  by  a  bright  red  raucous  mem- 
brane, which  is  continuous  with  the  skin  at  the  lips.  It  varies 
in  structure  in  different  parts  of  the  buccal  cavity,  and  in  its  gen- 
eral construction  more  resembles  the  outer  covering  of  the  body 
than  the  raucous  membrane  lining  the  alimentary  tract.  It  con- 
sists of  (1)  a  superficial  part  composed  of  thick  stratified  epithe- 
lium, the  upper  cells  of  which  are  flat,  scaly,  and  tough,  and 
are  placed  horizontally,  while  in  the  deeper  ones  the  cells  are 
soft,  rounded  or  elongated,  having 
their  long  axis  perpendicular  to 
the  surface  ;  and  (2)  a  deeper  part 
composed  of  fibro-elastic  tissue, 
which,  over  the  alveoli  of  the 
teeth,  is  amalgamated  with  the 
periosteum  and  forms  the  dense 
tough  gums. 

The  mucous  membrane  of  the 
mouth  is  covered  with  papillae, 
which  on  the  dorsum  of  the  tongue 
attain  great  magnitude  and  va- 
riety of  shape  and  epithelial  cov- 
ering.    In  man,  three  kinds  are 

described  :  (1.)  Narrow  po'mted,  filiform.  (2.)  Blunt  and  clubbed 
at  the  apex,  fu7igifo7"tn.  (3.)  Broad  complex  papillse,  surrounded 
by  a  fossa,  circumvallate,  of  which  there  are  but  a  limited  num- 
ber (about  a  dozen). 

The  special  secreting  organs  or  glands,  which  pour  their  juices 
into  the  mouth,  have  all  the  same  general  type  of  structure, 
though  they  vary  much  in  the  detail  as  to  the  variety  and  char- 
acter of  their  cells.     They  are  known  as  the  acinous  or  sacculated 


Diagram  taken  from  a  small 
portion  of  sacculated  gland  from 
Cockroach,  showing  branching 
duct  and  saccules. 


126 


MANUAL   OF    PHYSIOLOGY. 


glands,  from  their  being  made  up  of  numerous  acini,  or  minute 
elongated  sacs  or  tubules,  arranged  at  the  end  of  a  re[)eatedly 
branching  duct,  like  grapes  on  the  terminals  of  the  successive 
little  branches  growing  iVom  the  central  stalk  to  form  a  bunch. 
lu  the  glands  the  saccules  are  packed  together  closely  around 
the  ducts,  and  by  mutual  pressure  are  made  to  assume  various 
shapes.     The  wall  of  the  saccule  is  formed  of  a  very  delicate 


Section  of  the  Submaxillary  Gland  of  the  Dog,  showing  the  commence- 
ment of  a  duct  in  the  alveoli.  X  '125.  (Schiifer.) — a.  One  of  the  alveoli, 
several  being  grouped  round  the  ductlet  (d/).  b.  Basement  membrane  in 
section,  d.  Larger  duct  with  columnar  epithelium,  s.  Half-moon  group  of 
cells. 


clear  transparent  membrane,  on  the  outside  of  which  are  numer- 
ous flattened  branching  stellate  cells,  the  branches  of  which 
anastomose  one  with  another,  and  appear  also  to  penetrate  the 
membrane  in  order  to  reach  the  inside  of  the  acini. 

The  cavity  of  the  little  sacs  is  almost  completely  filled  with 
large  polygonal  gland-cells,  so  that  only  a  very  narrow  space  exists 
iu  the  centre.    (Fig.  61.)   From  this  space,  there  is  free  communi- 


MUCOUS   AND   SALIVARY    GLANDS.  127 

cation  to  the  main  duct  of  the  gland  by  means  of  the  proper  ductlet 
of  each  saccule.  In  the  saccules  of  some  glands,  the  so-called 
mucous  salivary  glands,  another  kind  of  cell  element  is  seen 
between  the  gland  cells  just  described  and  the  wall  of  the  sac, 
their  outer  side  following  accurately  the  concave  boundary  of  the 
saccule,  their  inner  side  impinging  upon  the  gland  cells.  They 
thus  acquire  a  more  or  less  half-moon  shape.  These  demilune 
cells  will  be  again  referred  to  (page  138). 

Between  the  saccules  are  numerous  bloodvessels  winch  branch 
and  form  a  network  of  capillaries  on  the  outside  of  each  little  sac. 

Fig.  G2. 


'6 

A  dissection  of  the  side  of  the  face,  showing  the  Salivary  Glands. — a.  Sub- 
lingual gland,  b.  Submaxillary  glands  with  their  ducts  opening  on  the  floor 
of  the  mouth  beneath  the  tongue  at  d.  c.  Parotid  gland  and  its  duct, 
wiiich  opens  on  the  inner  side  of  the  cheek. 

Numerous  nerves  are  also  found,  which,  according  to  some  ob- 
servers, have  ganglionic  cell-connections  in  the  gland  substance, 
and  send  terminals  into  the  gland-cells  direct. 

Although  this  account  of  the  nerve  terminations  in  the  secreting 
cells  of  other  glands  has  met  with  doubt,  it  is  certain  that  in  the 
lower  animals  nerve  terminals  have  been  traced  into  gland-cells, 
and  upon  physiological  grounds,  as  will  presently  appear,  we  are 
forced  to  believe  that  a  similar  connection  must  exist  in  mammalia. 

The  ducts  are  lined  with  ordinary  cylindrical  epithelium  which 


128  MANUAL   OF   PHYSIOLOGY. 

does  not  appear  to  have  auy  secreting  I'miction.  All  the  glands 
are  made  up  of  numerous  packets  of  lobules  bound  together  in 
one  mass.  Each  of  these  lobules  is  itself  a  perfect  gland,  so  that 
even  the  smaller  mouth-glands  are  separable  into  lobules,  and 
hence  are  called  compound  acinous  glands. 

The  mouth-glands  are  divided  into  two  sets,  which  produce 
different  kinds  of  secretion  :  (1.)  Mucous  glands,  which  secrete 
mucus,  and  (2.)  Salivary  glands,  which  produce  watery  saliva- 
The  functional  distinction  is  seldom  absolute,  for  most  salivary 
glands  have  a  mixed  secretion,  and  various  gradations  of  the 
transition  from  purely  salivary  to  purely  mucous  glands  can  be 
made  out. 

The  proper  mucous  glands  are  small,  varying  in  size  from  a 
pin's  head  to  a  pea.  They  are  found  in  groups  under  the  mucous 
membrane  in  various  parts  of  the  mouth,  and  from  their  positions 
are  called  labial,  buccal,  etc.  Their  cells  contain  a  clear  muci- 
ginous  substance. 

The  great  Salivary  glands  are  the  three  large  glands  which  are 
known  as  the  parotid,  submaxillary  and  sublingual.  On  account 
of  their  great  size  they  form  striking  anatomical  objects,  being 
large  masses  of  irregularly  arranged  glandular  packets,  which 
might  be  spoken  of  as  lobes  to  distinguish  them  from  the  smaller 
packets  or  lobules.  Their  ducts  are  of  considerable  size,  and 
have  strong  walls  made  of  dense  fibrous  tissue,  containing  many 
elastic  fibres,  and  in  one  of  them,  the  submaxillary,  smooth  mus- 
cle tissue  has  been  demonstrated. 

The  parotid  duct  (Steno's)  opens  into  the  mouth  about  the 
middle  of  the  cheek  just  opposite  the  second  molar  tooth.  The 
submaxillary  has  also  a  single  duct  (Wharton's)  which  opens 
beneath  the  tongue  beside  the  frreuum.  The  sublingual  gland 
has  several  ducts,  some  of  which  open  into  that  of  the  submaxil- 
lary, and  others  unite  to  enter  the  mouth  beside  Wharton's  duct. 

In  different  animals  and  in  different  glands  of  the  same  animal 
a  variable  amount  of  mucus  is  secreted  by  these  glands,  which, 
however,  are  all  called  salivary,  though  the  parotid  alone  de- 
serves the  name. 


COMPOSITION   OF  SALIVA. 


129 


Fig 


The  Characters  of  Mixed  Saliva, 

The  liquid  in  the  mouth  is  a  mixture  of  the  secretion  of  the 
salivary  glands  as  well  as  of  the  small  purely  mucous  glands. 

It  is  a  slightly  turbid  tasteless  fluid  of  a  distinctly  alkaline 
reaction,  of  1004-1008  specific  gravity,  and  so  tenacious  that  it 
can  be  drawn  into  threads.  The  amount  secreted  by  an  adult 
human  being  during  twenty-four  hours  varies  greatly  according 
to  circumstances,  and  has  been  variously  estimated  by  different 
authors,  by  whom  the  wide  limits  of  200-2000  grms.  (7-70  oz.) 
have  been  assigned  as  the  daily  amount. 

The  saliva  contains  about  5  per  cent,  of  solids.     Of  these  the 
greater  part  are  organic,  namely  :  (1.)  Mucin,  from  the  submax- 
illary, sublingual,  and  small  mucous  glands,  which  can  be  pre- 
cipitated by  acetic  acid.     To  this 
substance    the    viscidity    of    the 
saliva  is  due.     (2.)  Traces  of  al- 
bumin   precipitable    by    concen- 
trated   nitric   acid   and    boiling 
(3.)  Traces  of  globulin,  precipi- 
tated by  carbonic  acid.     (4.)  Pty- 
alin,  a  peculiar  ferment. 

The  inorganic  constituents  are 
salts,  among  which  an  inconstant 
amount  of  potassium  sulpho-cya- 
nate  is  found,  a  substance  which 
does  not  exist  in  the  blood. 

There  are  also  many  morpho- 
logical elements  :  of  these  the  ma- 
jority are  accidental,  being  the  re- 
mains of  food,  etc. ;  others  are  more  or  less  characteristic,  namely : 
(1.)  Salivary  corpuscles  which  are  rounded  protoplasmic  masses 
containing  nuclei  and  coarse  granules  which  show  Brownian 
movements.  (2.)  Epithelial  scales  from  the  surface  of  the  mu- 
cous membrane  of  the  mouth.  (3.)  Various  forms  of  protophyta, 
which  propagate  readily  amid  the  decaying  particles  of  food  in 

11 


Tlie  form  elements  from  mixed 
saliva  from  tip  of  tongue,  showing 
(e)  large  irregular  scaly  epithelial 
cells,  (c)  round  salivary  corpuscles, 
several  {b)  bacteria,  and  (hi)  mi- 
crococci. 


130  MANUAL    OF    PHYSIOLOGY. 

the  mouth.  But  no  bacteria  or  other  fuugi  exist  in  the  ducts  of 
the  glands  or  saliva  taken  from  the  ducts  with  the  necessary 
aseptic  precautious. 

The  ordinary  mixed  saliva  may  be  easily  collected  by  chewing 
some  insoluble  material,  such  as  a  bit  of  rubber  tubing,  and  col- 
lecting the  fluid  which  the  motion  causes  to  be  poured  into  the 
mouth. 

The  collection  of  the  secretion  of  the  different  glands  requires 
more  delicate  methods.  It  may  be  collected  separately  by  placing 
a  canula  in  the  duct  of  each  gland. 

The  parotid  saliva  obtained  in  this  way  is  found  to  have  no 
structural  elements  nor  mucus,  and  is  a  thin  fluid  dropping  easily, 
and  not  capable  of  being  drawn  into  threads.  It  contains  some 
serum  albumin  and  globulin,  pota.ssiura  sulpho-cyanate,  and  pty- 
alin.  The  portion  first  secreted  is  commonly  acid,  and  it  never 
becomes  strongly  alkaline.  Its  specific  gravity  is  1003-1004.  On 
standing  it  becomes  turbid  from  the  precipitatiou  of  carbonate  of 
lime,  which  existed  as  bicarbonate. 

The  submaxillary  secretion  is  more  strongly  alkaline  than  that 
of  the  parotid  ;  it  contains  structural  elements  and  mucin,  but  is 
not  so  viscid  as  the  general  mouth-fluid. 

The  sublingual  is  much  more  viscid  than  either  of  the  others, 
and  is  more  strongly  alkaline,  and  contains  much  mucus  and 
many  salivary  corpuscles. 

The  Method  of  Secretion  of  Saliva. — Under  ordinary 
circumstances  very  little  saliva  is  secreted,  only  sufticient  being 
poured  into  the  mouth  to  keep  the  surface  moist.  When,  however, 
food  is  introduced  into  the  mouth,  and  the  process  of  mastication 
commences,  the  secretion  goes  on  more  or  less  rapidly,  according 
to  the  stimulating  or  non-stimulating  character  of  the  food. 

The  activity  of  a  salivary  gland  is  at  once  brought  about  by 
means  of  special  nervous  agencies  when  a  stimulus  is  applied  to 
the  mouth.  We  know  that  the  nervous  mechanism  with  which 
we  have  here  to  do,  is  what  is  called  a  reflex  act.  The  stimulus 
travelling  from  the  surface  of  the  mouth  to  the  nerve  centres  is 
reflected  thence  to  the  glands.      We  speak,   then,  of  afferent 


SECRETIOX    OF   SALIVA.  131 

nerves,  which  carry  the  impulses  towards  the  centre,  and  efferent 
nerves  which  carry  thera  from  the  centre. 

If  we  review  the  ordinary  circumstances  giving  rise  to  a  flow 
of  saliva,  there  will  be  no  difficulty  in  determining  the  nerves 
which  act  as  the  afferent  channels  in  the  simple  reflex  act. 

Stimulation  of  the  mucous  membrane  of  the  tongue  and  mouth, 
whether  chemically,  as  with  irritating  condiments,  or  mechani- 
cally, as  by  the  motions  of  mastication,  is  generally  transmitted 
to  the  centre  by  the  sensory  branches  of  the  fifth  cranial  nerve, 
which  supply  the  mouth,  and  by  the  branches  of  the  glosso- 
pharyngeal. 

The  stimulus  of  the  sense  of  taste  is  sent  by  the  nerves  of  that 
sense,  mainly  the  glosso-pharyngeal,  to  the  taste  centre  in  the 
cortex  cerebri,  and  from  thence  to  the  secreting  centre  by  means 
of  intercentral  fibres. 

The  stimulating  of  the  olfactory  region  with  certain  odors  in- 
duces salivation  through  a  channel  of  a  similar  kind  passing 
along  the  olfactory  nerve  to  the  brain,  and  thence  to  the  special 
salivary  centre.  Even  in  the  absence  of  taste  or  smell,  mental 
emotion  may  be  excited  by  seeing  or  thinking  of  food,  and  may 
cause  activity  of  the  salivary  glands,  here  the  inter-central  chan- 
nel is  the  only  one  occupied  in  bearing  the  impulse  to  the  special 
secreting  centre. 

Irritation  of  the  gastric  mucous  membrane  stimulates  the  sali- 
vary glands  as  may  be  seen  with  a  gastric  fistula,  or  by  the  sud- 
den flow  of  saliva  which  commonly  precedes  vomiting.  In  this 
case  the  impulses  are  carried  by  the  gastric  bi-anches  of  the  vagus. 

The  stimulation  of  the  central  end  of  the  cut  sciatic  is  said  to 
cause  an  increase  in  the  flow  of  saliva,  so  that  it  would  appear 
that  even  an  ordinary  sensory  nerve  can  excite  the  centre  to  ac- 
tion. Lastly,  many  drugs,  when  introduced  into  the  blood,  cause 
a  flow  of  saliva,  among  these  are  pilocarpin,  physostigma,  and 
curara,  while  atropia  and  some  others,  on  the  other  hand,  check 
the  action  of  the  glands. 

From  this  we  learn  that  the  nerve  centre,  controlling  the  ac- 
tivity of  the  salivary  glands,  may  receive  impulses  from  many 
distant  and  diverse  nervous  sources,  or  may  be  influenced  directly 


132 


MANUAL   OF   PHYSIOLOGY. 


by  the  quality  of  the  blood  flowing  through  the  special  nerve 
centre  itself. 

The  channels  traversed  by  the  efferent  impulses  going  to  the 
salivary  glands  have  been  demonstrated  by  experiment.  In  the 
case  of  the  submaxillary,  the  route  is  es[)ecially  distinct  and  in- 
structive, so  that  from  this  gland  we  obtain  most  of  our  knowl- 

Fio.  64. 


Diagram  of  Nerves  of  tlie  Submaxillary  Gland.  The  dark  lines  show 
the  course  of  the  nerves  going  to  the  gland. — (vii.)  Portio  dura;  (v.)  Infe- 
rior maxillary  division  of  the  fifth  cranial  nerve;  (g)  Submaxillary  gan- 
glion; (s)  Sympathetic  round  facial  artery  (a);  (s.  c.  g.)  Superior  cervical 
ganglion. 


edge  concerning  the  direct  influence  of  nerve  impulses  on  the 
gland  cells.  This  question,  therefore,  will  be  treated  somewhat 
in  detail. 

There  are  two  sets  of  nerves  going  to  the  salivary  glands,  one 
belonging  to  the  sympathetic  and  the  other  to  the  cerebrospinal 
systems,  both  of  which  have  been  proved  to  exert  a  certain 
amount  of  influence  on  the  action  of  the  glands,  the  share  taken 
by  each  apparently  differing  in  different  animals. 


NERVE-MECHANISM   OF  SALIVARY  SECRETION. 


133 


The  sympathetic  branches  for  the  submaxillary  and  sublingual 
gland  come  from  the  plexus  which  embraces  the  facial  artery, 
those  for  the  parotid  come  from  the  plexus  surrounding  the  in- 
ternal maxillary  as  that  artery  traverses  the  gland.  Both  of 
these  nervous  plexuses  are  derived  from  the  superior  cervical  part 
of  the  sympathetic  nerve. 

Fig.  65. 


Diagram  of  Nerves  supplying  the  Parotid  Gland.  Tlie  dark  lines  indi- 
cate the  course  of  the  nerves  of  the  gland. — (v.)  Inferior  division  of  fifth 
nerve  and  its  (a  t)  auriculo-temporal  branch,  (vil.)  Portio  dura.  (s.  c  G.) 
Superior  cervical  ganglion  sending  a  branch  to  the  carotid  plexus  around 
the  artery. 

The  cerebro-spiual  fibres  for  the  submaxillary  and  sublingual 
glands  lie  in  the  complex  nerve  known  as  the  cborda-tympaui, 
which  comes  from  the  portio  dura  of  the  seventh,  and  joins  the 
lingual  branch  of  the  fifth.  They  pass  thence  through  the  sub- 
maxillary ganglion  to  the  glands- 


134  MANUAL   OF   PHYSIOLOGY. 

The  cerebro-spinal  parotid  branches  pass  through  the  lesser 
superficial  petrosal  nerve  from  the  tympanic  plexus  to  the  otic 
ganglion,  and  thence  to  the  auriculo-temporal  nerve  which  sends 
twigs  to  the  gland.  (Fig.  65.) 

I. — The  effects  of  experimental  stimulation  of  the  cerebrospinal 
glandular  branches,  so  far  as  we  know  are  alike  for  all  the  glands. 
But  owing  to  the  greater  facility  with  which  tlie  submaxillary 
gland  can  be  reached  and  its  nerve  isolated,  research  has  been 
chiefly  devoted  to  it,  by  operating  on  the  chorda  tympani  and 
the  other  nerves  supplying  the  gland. 

It  has  been  found  that  section  of  this  nerve,  or  of  the  portio 
dura  near  its  origin,  removes  the  possibility  of  exciting  the  glands 
to  action  by  stimulating  the  mouth,  so  that  the  cerebro-spinal  and 
not  the  sympathetic  are  the  channels  traversed  by  the  reflected 
impulse  on  its  way  to  the  gland  from  its  centre. 

The  reflex  stimuli  which  were  supposed  to  be  elicited  through 
the  medium  of  the  submaxillary  ganglion,  probably  depended 
on  the  escape  of  the  stimulating  electric  current  used,  and  the 
reflection  from  a  sporadic  ganglion,  such  as  the  submaxillary, 
has  never  been  satisfactorily  demonstrated. 

It  has  further  been  shown  that  direct  stimulation  of  the  chorda 
tympani  nerve,  although  it  be  cut  off  from  its  central  connections, 
causes  a  copious  secretion  of  thin  watery  saliva,  and  this  increased 
secretion  is  accompanied  by  a  great  dilatation  of  the  small  arte- 
ries going  to  the  gland,  so  that  a  pulsation  may  be  seen  in  the 
small  veins,  and  the  blood  retains  its  bright  arterial  color  when 
leaving  the  organ. 

These  two  chief  results  of  stimulation,  activity  of  the  secreting 
cells  and  vascular  dilatation,  are  brought  about  by  different  nerve 
aDfencies,  as  appears  from  the  action  of  atropia  which  stops  the 
secretion  of  saliva,  but  does  not  prevent  the  dilatation  of  the 
vessels  on  stimulation  of  the  chorda  tympani,  from  which  we 
conclude  that  its  effect  is  restricted  to  a  mechanism  engaged  ex- 
clusively in  controlling  the  activity  of  the  gland  cells. 

Stimulation  of  the  chorda  tympani  causes  the  secretion  to  be. 
carried  on  with  great  energy.  The  fluid  was  found  to  enter  the 
duct  with  a  pressure  equal  to  200  m.m.  (about  8  inches)  of  mer- 


NERVE-MECHANISM    OF   SALIVARY   SECRETION.         135 

cury,  while  the  blood  pressure  iu  the  carotid  artery  of  the  animal 
was  only  112  m.m.  (about  4i  inches)  mercury;  that  is  to  say,  the 
force  by  means  of  which  secretion  is  driven  outwards,  is  nearly 
twice  as  great  as  the  pressure  in  the  bloodvessels  in  the  gland. 
The  secretion  of  saliva  cannot  then  be  a  question  of  mere  filtra- 
tion, for  if  the  physical  agency — pressure — alone  were  acting,  the 
saliva  would  be  forced  into  the  bloodvessels  as  soon  as  the  pres- 
sure in  the  duct  exceeded  that  in  the  vessels. 

The  force  and  rate  with  which  the  secretion  is  produced  vary 
with  the  strength  of  the  stimulation.  The  flow  of  saliva  steadily 
increases  within  certain  limits  as  the  stimulus  gets  stronger.  It 
is  not  only  the  quantity  of  the  secretion  that  depends  on  the 
amount  of  nerve  impulse,  but  also  its  quality  ;  that  is  to  say,  with 
a  fresh  gland,  not  wearied  by  previous  experiment,  the  amount 
of  solids  in  the  saliva  increases  as  the  stimulus  is  increased,  so 
that  not  only  the  activity  of  the  gland  cells  is  under  the  control 
of  nerve  influence,  but  the  kind  of  work  they  perform  is  also 
regulated  by  the  intensity  of  nerve  impulse  they  receive. 

It  has  been  found  that  the  increase  in  the  blood  flow  is  secon- 
dary to  the  secretion  called  forth  by  stimulation  of  the  chorda 
tympani.  This  is  shown  by  the  fact  that  even  when  the  blood 
supply  is  cut  off"  by  any  means  (strong  sympathethic  stimulation, 
ligature  of  the  vessels,  or  even  decapitation)  an  amount  of  saliva 
can  be  made  to  flow  from  the  gland  which  could  not  have  been 
stored  up  in  its  cells  prior  to  the  stimulation  of  this  nerve. 

II. — With  regard  to  the  influence  exerted  by  the  sympathetic 
branches,  the  most  obvious  result  of  stimulation  of  these  is  a  con- 
traction in  the  arterioles,  and  a  consequent  diminution  of  the 
amount  of  blood  flowing  through  the  gland.  The  glands  look 
pale,  and  the  blood  leaving  them  is  intensely  venous  in  character ; 
the  exact  opposite,  in  fact,  to  the  result  obtained  by  stimulation 
of  the  cerebro-spinal  nerves.  But  the  sympathetic  has  also  an 
effect  on  the  gland  cell,  as  it  produces  an  increased  flow  of  saliva. 
In  the  dog  the  secretion  of  "sympathetic  saliva"  is  only  tempo- 
rary and  scanty,  having  high  specific  gravity,  and  being  over- 
loaded with  the  solids.  In  the  cat  and  rabbit  "sympathetic 
saliva"  is  scanty,  and  not  thicker  than  the  "chorda  saliva"  of 


136  MANUAL  OF   PHYSIOLOGY. 

the  same  animal,  80  far  as  regards  the  bloodvessels,  theu,  the 
chorda  is  directly  opi)osed  to  the  sympathetic.  To  explain  this 
antagonism  we  may  either  assume  the  existence  of  local  nerve 
centres  governing  the  muscular  coats  of  the  arterioles,  and  sup- 
pose that  the  sympathetic  stimulates  and  the  chorda  inhibits  the 
activity  of  these  centres,  or,  what  seems  more  simple,  in  the  ab- 
sence of  anatomical  evidence  that  such  a  centre  exists,  we  may 
attribute  to  the  arterial  muscle  cells  themselves  an  automatic 
tonic  power  of  contraction  which  can  be  increased  by  the  sym- 
pathetic and  diminished  by  the  chorda  tympani.  It  is  singular 
that  if  all  the  nerves  leading  to  the  gland  be  cut,  after  some 
hours  a  pretty  copious  watery  secretion  of  saliva  occurs  and  lasts 
for  some  weeks,  after  which  the  cells  undergo  atrophic  changes, 
and  the  gland  becomes  reduced  in  size.  The  explanation  of  the 
appearance  of  this  so-called  "paralytic  saliva"  is  not  clearly 
made  out.  Possibly  the  removal  of  some  trophic  nerve-influences 
induces  abnormal  nutritive  changes  which  cause  stimulation  of 
the  cells,  and  ultimately  lead  to  their  degeneration. 

The  histological  investigation  of  the  elements  of  these  glands 
in  the  various  stages  of  secretion  throws  considerable  light  on  the 
behavior  of  the  cells  during  their  periods  of  activity  and  rest. 

It  is  now  certain  that  the  different  stages  are  accompanied  by 
constant  structural  changes  in  the  cells,  which  doubtless  are  inti- 
mately connected  with  secretory  activity.  During  the  period  of 
rest,  that  is,  the  time  when  the  gland  is  not  discharging  its  secre- 
tion, the  cells  slowly  undergo  a  change  in  their  appearance,  which 
is  obvious  in  proportion  to  the  ease  with  which  the  material  they 
secrete  is  recognized  in  the  protoplasm  of  a  cell.  Thus,  in  mucous 
glands,  or  in  mucus-yielding  salivary  glands,  the  changes  are 
conspicuous ;  whilst  in  those  which  give  a  watery  sacretion  they 
are  less  easily  seen. 

As  an  example  we  may  take  a  mucous  gland,  such  as  the  orbital 
gland  of  the  dog,  and  follow  the  changes  which  occur  in  its  cell 
from  the  time  it  ceases  to  discharge  its  function.     (Fig.  66.) 

Immediately  after  the  prolonged  and  active  discharge  of  the 
secretion  of  the  gland,  the  cells  have  all  the  characters  of  ordinary 
protoplasmic  units,  and  the  distinction  between  the  polygonal  cells 


CHANGES   IN    THE    GLAND-CELLS. 


137 


and  those  next  the  wall  of  the  acinus  (derai-lune  cells)  is  made 
out  with  great  difficulty,  because  all  the  cell^  stain  evenly  with 


Sections  of  Orbital  Gland  of  the  Dog.  (Heidenhain.)— (a)  After  pro- 
longed period  of  rest,  (b)  After  a  period  of  activity.  In  (a)  the  secreting 
cells  are  clear,  being  swollen  up  with  raucigen,  and  the  half-moon  cells  are 
very  distinct  and  darkly  stained.  In  (b)  the  accnmulated  material  has  been 
discharged  from  the  cells,  and  the  alveoli  are  shrunken, 

carmine,  and  have  no  special  characters  except  those  belonging  to 

active  protoplasm. 

12 


138 


MANUAL   OF   PHYSIOLOGY. 


During  rest  certain  changes  gradually  appear  in  those  gland- 
cells  which  are  next  the  lumen  of  the  saccule.  They  appear  to 
swell  towards  the  lumen,  and  at  the  same  time  become  clear  and 
resist  staining  with  carmine,  their  protoplasm  becoming  impreg- 
nated with  mucous-like  material  (mucigen),  while  the  demi-lune 
cells  remain  protoplasmic  and  stain  easily,  and  are  thereby  readily 
distinguished  from  the  cell  in  the  cavity  of  the  saccule. 

Now,  if  the  discharge  of  secretion  be  called  for,  either  by  normal 
reflex  excitation,  or  by  direct  stimulus  of  the  chorda  tympani 
nerve,  the  cells  discharge  the  contained  specific  material,  some  of 
them   most  probably  being  destroyed  by  the  act.     If  the  active 

Fig.  67. 


Cells  of  the  Alveoli  of  a  Serous  or  Watery  Salivary  Gland.  (Langley.) — 
(a)  After  rest,  (b)  After  a  short  period  of  activity,  (c)  After  prolonged 
period  of  activity. 

secretion  be  continued  for  some  time,  the  cells  return  to  their 
former  protoplasmic  state,  and  the  cells  which  have  been  worn 
out  are  replaced  by  others  from  the  demi-lune  or  marginal  cells. 

In  the  glands  which  do  not  produce  any  mucus  the  brilliant 
look  of  the  cells  after  rest  is  wanting,  but  a  corresponding  change 
occurs.  The  secreting  protoplasm  becomes  extremely  granular 
during  the  resting  period,  and  again  clear  after  the  discharge  of 
the  secretion.     (Fig.  67.) 

Thus  it  would  appear  that  during  the  so-called  period  of  rest, 
when  little  or  no  fluid  is  poured  into  the  duct,  the  gland-cells  are 
busy  at  their  manufacturing  process,  diligently  adding  to  their 
stock  in  hand  in  order  to  be  ready  for  a  certain  demand  which 
they  could  not  meet  by  merely  concurrent  work. 

To  sura  up,  then,  we  may  conclude : 


FUNCTIONS   OF   THE   SALIVA.  139 

I.  That  the  manufacture  of  the  specific  materials  of  the 
secretion  is  accomplished  as  the  result  of  the  intrinsic 
power  of  the  protoplasm  of  the  gland  cells. 
II.  That  a  vital  process  is  called  forth  in  the  gland  cells  by 
the  action  of  nerve  impulses,  because — (1.)  The  force 
with  which  the  secretion  is  expelled  cannot  be  ac- 
counted for  by  the  blood  pressure.  (2.)  The  quantity 
and  quality  of  the  secretion  is  modified  by  the  inten- 
sity of  the  nerve  stimulation.  (3.)  The  temperature 
of  the  blood  is  raised.  (4.)  Structural  changes  in  the 
cells  can  be  observed. 

III.  The  normal  stimulus  to  secretion  passes  from  the  centre 

in  the  medulla  oblongata  to  the  salivary  glands  along 
cerebro-spinal,  not  sympathetic,  nerves. 

IV.  This  centre  for  salivary  secretion,  which  at  ordinary 

times  is  moderately  active,  may  be  excited  to  energetic 
action  by  impulses  coming  from  taste,  smell,  and  or- 
dinary sensory  nerve  terminals  (particularly  in  the 
mouth),  as  well  as  by  those  which  emanate  from  men- 
tal emotions. 

Changes  undergone  by  the  Food  in  the  Mouth. 

Food  when  taken  in  the  mouth  undergoes  two  processes,  which 
are  inseparable  and  simultaneous  in  action  ;  viz.,  mastication  and 
insalivation. 

The  mechanism  of  mastication  has  already  been  discussed,  so 
far  as  its  triturating  power  is  concerned.  In  its  final  object  of 
forming  the  subdivided  food  into  a  bolus  which  can  be  easily  swal- 
lowed, it  is  much  aided  by  insalivation,  particularly  in  chewing 
dry  food ;  and  in  this  latter  the  moistening  of  the  particles,  so  as 
to  make  them  adhere  together,  is  the  most  necessary  act  of  mouth 
digestion,  and  is  next  in  importance  to  the  subdivision  accom- 
plished by  the  teeth.  Moreover,  the  saliva  covers  the  bolus  with 
a  coating  of  viscid  fluid,  so  that  it  can  more  easily  slip  down  the 
oesophagus.  Deglutition  of  solids  is  difficult  during  a  scanty  sup- 
ply of  saliva. 


140  MANUAL   OF   PHYSIOLOGY. 

Our  food  is  generally  composed  of  materials  some  of  which 
are  soluble  in  water,  and  some  are  not. 

While  iu  the  mouth  the  saliva  dissolves  a  great  quantity  of  the 
more  readily  soluble  materials,  such  as  sugar  and  salt,  which  may 
be  either  mingled  with  the  insoluble  substances,  and  swallowed 
together  with  the  bolus,  or  swallowed  separately  iu  a  fluid  form. 
Solution,  then,  is  an  important  item  in  mouth-digestion. 

In  many  carnivorous  animals  the  use  of  the  mouth-fluid  is 
chiefly  mechanical,  dissolving  some  insignificant  part  of  the  food, 
and  aiding  mastication  and  deglutition.  In  man,  however,  and 
other  animals  that  make  use  of  much  vegetable  food,  it  has  a 
chemical  function,  and  acts  on  the  insoluble  starch,  converting  it 
into  soluble  sugar. 

The  active  principle  which  brings  about  this  change  is  Ptyalin. 
This  is  one  of  a  series  of  ferments  which  exist  in  the  body,  and  to 
which  most  of  the  chemical  changes  in  digestion  are  due.  Although 
each  possesses  certain  peculiarities,  yet  as  a  group  they  may  be 
described  as  effecting  by  their  presence  various  alterations  in  the 
substances  on  which  they  act,  while  they  themselves  do  not  undergo 
any  perceptible  change,  and  a  small  quantity  will  do  as  much 
work  as  a  larger  quantity  if  allowed  a  proportionately  longer  time. 

Ptyalin  acts  exclusively  on  starch,  and  hence  is  spoken  of  as  an 
amylolytic  ferment,  its  action  consisting  in  causing  the  starch  to 
unite  chemically  with  one  molecule  of  water  thus  : 

starch.  Grape-sugar. 

During  this  process,  which  takes  at  the  least  a  few  minutes  to 
complete,  various  stages  can  be  detected :  first,  two  substances 
are  formed  which  together  are  commonly  spoken  of  as  dextrin  ; 
one,  erythro-dextrin,  which  gives  a  red  color  with  iodine,  and 
easily  passes  into  soluble  sugar;  and  the  other,  achroo-dextrin, 
gives  no  color  with  iodine,  and  is  with  diflBculty  converted  into 
sugar.  As  it  gives  no  color  with  the  ordinary  test,  its  presence  is 
often  overlooked. 

The  sugar  thus  formed  has  been  called  Ftyalose,  which,  how- 
ever, can  be  converted  into  ordinary  grape-sugar  (glucose)  by  the 
action  of  snlphuric  acid.     Some  say  the  product  is  all  maltose. 


AfOUTH    DIGESTION.  141 

The  presence  of  starch,  either  iu  its  soluble  or  insoluble  form, 
is  easily  recognized  by  the  blue  color  given  by  free  iodine,  which 
color  disappears  on  heating  to  about  100°  C,  but  reappears  on 
cooling. 

Very  many  tests  have  been  recommended  for  the  detection  of 
sugar.  The  most  generally  applicable  one  is  Trommer's.  The 
liquid  is  made  strongly  alkaline  with  potash,  and  a  few  drops  of  a 
dilute  solution  of  cupric  sulphate  are  added  ;  a  clear  blue  solution 
results,  which,  on  being  raised  to  the  boiling  point,  deposits  a 
yellow  precipitate  of  cuprous  oxide.  Fehling's  and  Pavy's  solu- 
tions are  modifications  of  the  above  test  adapted  for  quantitative 
analysis. 

When  yeast  is  added  to  grape-sugar  and  placed  in  an  inverted 
test-tube,  the  sugar  is  converted  into  alcohol  and  carbon-dioxide. 
The  latter  rises  to  the  top  of  the  test-tube,  and  can  be  used  as  an 
indication  of  the  quantity  present.  Experiments  may  be  carried 
out  with  saliva  obtained  from  any  of  the  glands  directly,  but  the 
mixture  of  the  secretion  of  all  is  found  to  be  more  efficacious  than 
that  of  any  single  one.  The  ordinary  mouth-fluid  filtered  serves 
well  for  ordinary  experiment. 

An  effective  glycerin  solution  of  ptyalin  may  be  obtained  by 
steeping  chopped  salivary  glands  in  alcohol,  and  then  extracting 
for  some  days  with  glycerin  and  water. 

The  following  facts  must  be  borne  in  mind  concerning  the 
araylolytic  action  of  ptyalin  : 

1.  The  extremely  small  amount  of  the  ferment  required  to 

make  the  fluid  effective. 

2.  There  is  no  appreciable  diminution  in  the  amount  of  fer- 

ment, so  that  it  cannot  be  said  to  be  used  up  in  the 
process. 

3.  The  action  takes  place  most  readily  in  alkaline  solutions, 

such  as  the  saliva,  slowly  in  neutral  solution,  and  not  at 
all  in  acids  of  the  strength  of  .2  per  cent,  of  hydrochloric 
acid. 

4.  Temperature  has  a  marked  effect  on  the  process.     Cold 

(0°  C.)  quite  checks  the  action  ;  heat  (75°  C.)  destroys 


142  MANUAL   OF   PHYSIOLOGY. 

the  power  of  the  ferment,  which  is  most  active  at  the 
body  temperature  (35°-40°  C). 

5.  Strong  acids  or  alkalies  destroy  the  amylolytic  power  of 

ptyalin. 

6.  The  ferment  has  but  little  effect  on  raw  starch,  its  cellulose 

coating  protecting  it;  but  it  acts  rapidly  on  well-boiled 
starch. 

7.  Ptyalin  is  more  active  in  weak  solutions,  and  seems  to  be 

impeded  in  its  action  by  an  accumulation  of  sugar. 

To  recapitulate,  we  find  that  the  following  changes  take  place 
1  n  the  mouth  : 

(1)  Solid  food  is  or  should  be  finely  subdivided  ;  (2)  dry 
food  is  moistened,  (3)  rolled  into  a  bolus,  (4)  and 
lubricated  ;  (5)  the  soluble  part  is  dissolved,  and 
rendered  capable  of  being  tasted;  (6)  and  part  of  the 
starch  is  converted  into  soluble  sugar  by  the  action 
of  a  ferment  called  ptyalin. 

In  the  short  time  occupied  by  the  passage  of  food  through  the 
oesophagus  no  special  change  takes  place  in  it,  so  we  may  pass  on 
at  once  to  the  gastric  digestion,  which  will  occupy  the  next 
chapter. 


CHAPTER  VIIT. 


STOMACH  DIGESTION. 


The  general  surface  of  the  stomach  is  covered  by  a  single 
layer  of  cylindrical  epithelial  cells  which  also  line  the  orifices 
of  the  numerous  glands  with  which  the  mucous  membrane  is 
thickly  studded.  This  single  layer  of  cylindrical  cells  is  marked 
off  from  the  stratified  squamous  cells  lining  the  oesophagus  by  a 
sharp  line  of  demarcation.     The  glands  of  the  stomach  are  tubes 

Fig.  68. 


Diagram  of  a  section  of  the  Wall  of  the  Stomach. — a.  Orifices  of  glands 
with  cylindrical  epithelium,  b.  Fundus  of  glands  with  spherical  and  oval 
epithelium,  c.  Muscularis  mucoste.  d.  Submucous  tissue  containing  blood- 
vessels, etc.  e.  Circular,  (/)  oblique,  and  (</)  longitudinal  muscle  coats. 
h.  Serous  membrane. 


of  which  the  orifices  are  conical  depressions  which  divide  into 
two  or  three  tubular  prolongations.  The  outlet  or  orifice  is  cov- 
ered by  the  common  cylindrical  epithelium  of  the  surface  of  the 
stomach,  and  the  fundus  is  filled  with  specific  granular  cells. 
The  glands  dip  down  into  the  delicate  submucous  tissue,  the 
branching  tubes  lying  parallel  and  exceedingly  close  together. 


144 


MANUAL   OF   PHYSIOLOGY, 


Betweeu  them  an  injection  demonstrates  a  dense  network  of 
capillary  vessels  which  surrounds  the  tube  and  closely  invests  the 
delicate  basement  membrane  which  forms  the  boundary  of  the 
glands  and  the  basis  of  attachment  of  the  glandular  cells. 

In  the  cardiac  end  of  the  stomach  two  distinct  kinds  of  cells 
are  found  in  the  deeper  part  of  the  gland  tubes.  One  kind,  which 
is  much  the  more  numerous,  consists  of  small   pale  spheroidal 

Fig.  G9. 


Diagram  sliowing  tlie  relation  of  the  ultimate  twigs  of  the  bloodvessels 
(v  and  a),  and  of  the  absorbent  radicals  to  the  glands  of  the  stomach  and 
the  diflferent  kinds  of  epithelium,  viz.,  above  cylindrical  cells;  small  pale 
cells  in  the  lumen,  outside  which  are  the  dark  ovoid  cells. 


cells,  which  occupies  the  lumen  of  the  gland  and  forms  the  regu- 
lar cell-lining  of  its  cavity.  These  cells  have  been  called  the 
"chief  cells"  (Hauptzelleu),  or  "central  spheroidal  cells." 

The  cells  of  the  other  form  are  comparatively  few,  being  alto- 
gether wanting  in  some  of  the  glands.  They  are  larger  and 
more  striking  than  the  central  cells,  between  which  and  the  base- 


GASTRIC   JUICE.  145 

ment  meaibraue  they  lie  scattered  here  and  there  over  the  fundus 
of  the  gland,  making  the  delicate  membrane  bulge.  They  stain 
more  easily, and  have  darker  granules  than  the  central  cells.  On 
account  of  their  position  they  have  been  called  "  parietal," 
"marginal  or  border  cells"  (Belegzellen),  and  from  their  oval 
shape,  which  equally  well  distinguishes  them  from  the  other, 
"  ovoid  cells."     (See  fig.  69.) 

There  is  a  different  class  of  glands,  the  so-called  mucous,  found 
chiefly  near  the  pyloric  end  of  the  stoiuach,  in  which  there  is  but 
one  kind  of  cell  throughout,  and  this  seems  to  differ  in  character 
from  both  the  varieties  in  the  other  glands,  resembling  rather  the 
cylindrical  epithelium  covering  the  surface  of  the  stomach  and 
dipping  into  the  conical  orifices  which  lead  to  the  glands. 

The  difference  between  the  two  kinds  of  glands  found  in  the 
stomach,  both  as  regards  their  distribution  and  way  of  branching, 
and  the  cells  which  line  the  deeper  parts  of  the  tubes,  is  found  to 
vary  in  different  animals.  The  diflSculty  of  obtaining  fresh 
specimens  of  the  human  stomach  makes  it  still  uncertain  whether 
the  same  differences  exist  in  the  human  subject.  The  varieties 
of  opinion  and  drawings  published  suggest  that  various  stages  of 
gradation  from  one  kind  of  gland  to  another  are  met  with  in  the 
stomach  of  even  the  same  animal. 

Experimental  research  does  not  show  decisively  that  the  ana- 
tomical differences  denote  differences  of  function. 

The  Characters  of  Gastric  Juice. 

The  gastric  juice  is  a  clear  colorless  fluid  with  strongly  acid 
reaction.  It  contains  .5  per  cent,  of  solids,  its  specific  gravity 
being  1002.  The  amount  secreted  in  the  day  is  extremely  vari- 
able, and  depends  upon  the  amount  and  character  of  the  food;  in 
well-fed  dogs  it  has  been  estimated  to  be  one-tenth  of  the  body 
weight. 

1.  It  contains,  in  man,  about  .2  per  cent,  of  free  hydrochloric 
acid,  in  the  dog  considerably  more.  The  lactic,  formic, 
butyric,  and  other  acids  which  have  been  found  in  the 
gastric  juice  probably  depend  on  the  decomposition  of 
some  of  the  iugesta. 


146  MANUAL   OF    PHYSIOLOGY. 

2.  Pepsin,  the  specific  substance  which  gives  the  gastric  juice 

its  digestive  qualities,  is  a  nitrogenous  ferment  which, 
with  the  foregoing  acid,  acts  on  proteids.  About  .3 
per  cent,  is  present  in  the  secretion  of  the  human 
stomach.  It  is  probably  associated  with  other  less 
known  ferments,  one  of  which  curdles  milk  without  the 
presence  of  any  acid. 

3.  A  variable  quantity  of  mucus  is  found  in  the  secretion  of 

the  stomach. 

4.  It  contains  .2  per  cent,  of  inorganic  salts,  chiefly  chlorides 

of  sodium,  potassium  and  calcium. 

Method  of  Obtaining  Gastric  Secretion. — Formerly 
attempts  were  made  to  obtain  gastric  juice  by  obliging  a  dog 
while  fasting,  to  swallow  a  sponge,  and  withdrawing  it  when 
saturated  with  the  gastric  secretion  ;  or  a  fasting  dog,  allowed  to 
swallow  insoluble  materials,  was  killed,  and  the  secretion  pro- 
duced was  collected  from  the  stomach.  It  is  best  obtained  di- 
rectly from  a  fistulous  opening  in  the  abdominal  wall  communi- 
cating with  the  stomach. 

A  gastric  fistula  was  first  made  by  accident.  It  was  a  case  in 
which  the  surgical  treatment  of  a  gunshot  wound  of  the  stomach 
of  a  man  left  a  permanent  fistula,  by  means  of  which  the  gastric 
secretion  was  carefully  investigated.  Thus  a  man  proved  a  valu- 
able subject  for  experimental  research. 

It  is  not  a  difficult  matter  to  reach  the  stomach  by  making  an 
artificial  opening  through  the  wall  of  the  abdomen,  and,  having 
brought  the  serous  surface  of  the  gastric  wall  into  firm  connec- 
tion with  the  serous  lining  of  the  abdominal  wall,  to  open  the 
stomach.  The  juxtaposition  of  the  parts  as  well  as  the  patency 
of  the  fistula  can  be  secured  by  a  suitable  flanged  canula  closed 
with  a  well-fitting  cork.  By  removing  the  cork  the  gastric  juice 
may  be  obtained  in  small  quantities,  and  various  kinds  of  food 
may  be  introduced  through  the  canula,  and  the  changes  occur- 
ring in  them  studied. 

For  experimental  purposes  an  artificial  gastric  juice  may  be 
used.     This  can  be  made  from  the  gastric  raucous  membrane  of 


GASTRIC  SECRETION.  147 

a  dead  anin)al  (pig)  by  extracting  the  pepsin  from  the  finely 
divided  ghindular  membrane,  with  a  weak  acid  (less  than  .2  per 
cent.),  or,  better,  with  a  large  quantity  of  glycerin,  and  subse- 
quently adding  HCI  to  the  extent  of  .2  per  cent. 

Mode  of  Secretion. — The  gastric  juice  is  not  secreted  in 
large  quantity  when  the  stomach  is  empty,  but  only  when  the 
mucous  membrane  is  irritated  with  some  chemical  or  mechanical 
stimulus.  The  swallowing  of  alkaline  saliva  acts  as  a  gentle 
stimulus  and  causes  secretion,  so  that  the  surface  of  the  stomach 
becomes  acid.  When  the  lining  membrane  of  the  stomach  is 
mechanically  stimulated  through  a  fistula  it  becomes  red,  and 
drops  of  secretion  appear  at  the  point  of  stimulation,  but  the 
amount  of  secretion  thus  produced  is  very  scanty  when  compared 
with  that  called  forth  by  chemical  irritants. 

Thus  ether,  alcohol,  and  pungent  condiments  produce  copious 
secretion.  Weak  alkaline  solutions  also  cause  secretion,  but  the 
most  perfect  form  of  stimulant  seems  to  be  the  normal  one, 
namely,  a  consistent  mass  of  food  saturated  with  the  alkaline 
saliva. 

In  all  probability  the  secretion  of  the  gastric  juice  is  under  the 
control  of  a  special  nerve  mechanism,  and  the  way  in  which  the 
state  of  activity  follows  stimulation  of  the  part  seems  to  point  to 
its  being  a  simple  reflex  act.  However,  the  nervous  connections 
(vagi  and  splanchnics)  between  the  stomach  and  central  nervous 
system  may  all  be  severed  without  any  marked  effect  on  the  secre- 
tion, other  than  that  which  would  naturally  follow  the  changes 
in  the  amount  of  blood  supply,  which  of  course  is  profoundly 
altered  by  cutting  the  vasomotor  nerves  —  the  splanchnics. 
Whether  this  be  so  or  not,  there  must  be  some  connection  with 
the  nerve  centres,  for  sudden  emotions  check  the  secretions,  and 
the  sensations  caused  by  the  sight  or  smell  of  food  give  rise  to 
gastric  secretion. 

It  has  been  suggested  that  Meissner's  submucous  ganglionic  net- 
work may  act  as  a  reflex  centre  and  regulate  the  secretion.  But 
since  reflection  from  local  ganglionic  centres  has  not  yet  been 
definitely  demonstrated,  we  are  hardly  entitled  to  assume  that  it 


148  MANUAT>   OF    PHYSIOLOGY. 

occurs  here,  and  since  the  stimulus  comes  into  close  contiguity 
with  the  secreting  cells,  it  seems  quite  as  probable  that  these  ele- 
ments are  excited  to  activity  by  direct  stimulation  of  their  pro- 
toplasm. 

As  in  the  salivary  glands,  so  in  the  gastric  tubes,  the  cells  show 
some  structural  changes  which  accompany  with  great  regularity 
their  periods  of  rest  and  activity,  and  therefore  may  be  concluded 
to  be  the  indications  of  the  internal  processes  belonging  to  the 
production  of  the  specific  materials  of  their  secretion. 

It  appears  probable  that  the  chief  secretory  activity  resides  in 
the  small  central  cells,  and  not  in  the  large  ovoid  border-cells, 
since  no  distinct  changes  can  be  seen  in  the  latter,  and  the  smaller 
gland  cells  seem  to  contain  the  pepsin,  for  if  the  mucous  mem- 
brane be  treated  with  weak  hydrochloric  acid,  these  central  gland- 
cells  are  rapidly  dissolved  by  a  process  of  digestion,  while  the 
border-cells  simply  swell  up  and  become  more  transparent.  So 
that  the  outer  ovoid  cells  have  no  title  to  their  former  name  of 
"  peptic  cells." 

The  central  cells  of  the  gastric  glands  are  finely  granular,  pale, 
protoplasmic  masses,  and  continue  so  during  the  time  when  the 
stomach  is  empty  and  the  glands  not  secreting.  In  the  earlier 
stages  of  digestion  these  cells  swell  up  and  become  turbid  and 
coarsely  granular,  and  stain  more  readily  with  the  aniline  dyes. 
As  the  digestive  process  goes  on  the  cells  again  diminish  in  size, 
but  are  found  to  contain  a  large  quantity  of  peculiar  granules, 
which  are  discharged  from  the  cell  before  its  return  to  the  ordi- 
nary state  of  rest.  The  cells  are  said  to  be  rich  in  pepsin  in  pro- 
portion to  their  size  ;  Avhen  swollen  during  active  digestion  they 
contain  much  pepsin,  when  small,  during  hunger,  they  contain 
but  little. 

It  would  therefore  appear  that  the  pepsin  of  the  gastric  juice 
is  produced  as  a  distinct  and  new  manufacture  by  the  central 
cells  of  the  peptic  glands,  and  not  by  the  other  cells.  Structural 
changes  have  also  been  followed  out  in  the  so-called  mucous 
glands  and  in  glands  without  any  of  the  ovoid  border  cells  which, 
taken  with  the  fact  that  the  alkaline  secretion  of  the  pyloric  end 
of  the  stomach,  where  the  mucous  glands  abound,  is  capable  of 


I 


GASTRIC   DIGESTION.  149 


rapily  digesting  proteid  if  acid  be  added  to  it,  tends  to  show  that 
in  these  so-called  mucous  glands  pepsin  is  also  produced. 

The  acid  is  found  chiefly  on  the  surface  of  the  stomach.  The 
mode  of  its  production  seems  distinct  from  that  of  pepsin,  but  is 
not  well  understood.  Although  the  deeper  parts  of  the  glands  do 
not  give  an  acid  reaction,  while  the  neck  and  orifices  of  the  gland 
are  distinctly  acid,  there  is  good  reason  for  believing  that  this 
manufacture  of  acid  from  the  alkaline  blood  is  really  an  active 
process  carried  out  by  some  glandular  cells. 

It  has  been  suggested  that  the  cell  elements  which  produce  the 
acid  are  the  ovoid  border-cells,  from  whence  it  rapidly  passes  to 
the  orifice  of  the  glands.  This  view  is  supported  by  the  alka- 
linity of  the  pyloric  end  of  the  stomach  where  the  border  cells 
are  not  found.  In  some  animals  the  distinct  distribution  of  the 
different  cell  elements  and  the  accompanying  reaction  of  the 
secretion  are  well  marked. 

Action  of  the  Gastric  Juice. 

The  gastric  juice  has  in  the  absence  of  mucus  no  effect  on  the 
carbohydrates,  and  probably  the  araylolytic  fermentation  set  up 
by  the  saliva  is  impeded,  if  not  completely  checked,  by  the  free 
acid  in  the  stomach. 

The  gastric  juice  has  no  effect  on  pure  fats,  but  it  dissolves  the 
proteid  frame-work  of  adipose  tissue  and  thus  sets  the  fats  free, 
which  are  then  turned  by  heat  to  a  liquid  mass  like  oil.  Upon 
the  albuminous  bodies  the  gastric  digestion  produces  a  marked 
effect.  The  proteids  being  colloid  bodies  cannot  pass  through  an 
animal  membrane  by  the  process  called  dialysis ;  it  has  therefore 
been  assumed  that  they  cannot  be  "absorbed  through  the  lining 
membrane  of  the  stomach.  They  are  also  often  eaten  in  an  in- 
soluble form.  To  convert  the  insoluble  and  iudiffusible  albumins 
into  a  soluble  and  diffusible  substance  would  obviously  be  a  great 
step  towards  their  absorption.  This  power  is  ascribed  to  the  gas- 
tric juice.  The  steps  of  the  process  may  be  accurately  followed 
in  a  suitable  glass  vessel,  irrespective  of  the  stomach,  by  using 
artificial  gastric  juice,  and  attending  to  the  various  conditions 


150  MANUAL  OF   PHYSIOLOGY. 

necessary  for  its  action.  The  power  of  artificial  gastric  juice 
carefully  prepared  from  the  mucous  membrane  of  an  animal's 
stomach  diHers  in  no  essential  respect  from  that  of  the  natural 
secretion  in  the  stomach,  if  all  the  circumstances  which  aid  the 
action  of  the  gastric  ferments  be  applied  in  the  experiment.  This 
action  consists  in  a  conversion  of  coagulated  albumins  into  the 
peculiar  soluble  uud  diflusible  form  of  proteid  known  as  ■'  pep- 
tones." 

The  change  is  not  effected  immediately,  but  certain  stages  may 
be  recognized  in  which  the  two  chief  constituents  of  the  gastric 
juice,  the  acid  and  the  pepsin,  seem  to  have  a  separate  action. 

Shortly  after  the  introduction  of  a  proteid,  such  as  boiled  fibrin, 
into  gastric  fluid  at  the  temperature  of  the  body,  the  masses  of 
fibrin  swell  up,  become  transparent,  and  eventually  are  easily 
shaken  to  pieces  and  dissolved. 

The  first  step  in  the  process  seems  to  be  brought  about  by  the 
free  acid,  and  consists  in  the  formation  of  acid  albumin.  This 
can  be  shown  by  neutralizing  the  fluid  during  the  process  and 
thereby  causing  a  precipitate  of  acid  albumin  (v.  p.  61).  The 
amount  of  this  precipitate  will  depeud  upon  how  far  the  conver- 
sion into  peptone — which  is  not  precipitated  by  neutralization — 
has  progressed.  Thus,  in  the  earlier  stages,  nearly  all  the  proteid 
used  will  be  thrown  down  by  neutralization,  while  only  a  com- 
paratively small  amount  is  precipitated  in  the  later  stages. 

The  formation  of  acid  albumin  may  be  effected  with  acid  alone 
without  the  other  constituents  of  the  gastric  juice,  and  therefore 
the  preliminary  step  may  be  attributed  to  the  unaided  action  of 
the  acid  ;  but  since  this  stage  in  the  formation  of  peptone  is  con- 
stant, and  the  material  may  possibly  be  distinguishable  from  the 
ordinary  acid  albumin,  it  has  been  called  parapepto)ie. 

While  the  parapeptone  is  being  formed  by  the  acid,  the  pepsin 
is  engaged  in  changing  it  into  the  final  soluble,  diffusible,  and 
uncoagulable  product — peptone.  The  pepsin  by  itself  cannot 
convert  proteid  into  peptone,  as  may  be  seen  in  the  want  of  effi- 
cacy of  a  neutral  solution  of  pepsin,  in  which  neither  peptone 
nor  parapeptone  is  formed.  In  other  words,  pepsin  solution  can 
only  change  parapeptone  or  acid  albumin  into  peptone.    It  would 


GASTRIC   DIGESTION.  151 

appear  probable,  however,  that  it  possesses  this  property  to  an 
unlimited  extent,  since  it  undergoes  no  change  itself,  and  with 
fresh  supplies  of  acid  a  very  minute  quantity  of  pepsin  can  con- 
vert an  indefinite  amount  of  proteid  into  peptone. 

The  rapidity  with  which  proteid  is  converted  varies  according 
to  the  circumstances  under  which  it  is  placed  as  well  as  the  kind 
of  proteid  used.  If  the  same  proteid  be  used  the  following  cir- 
cumstances will  be  found  to  influence  the  rapidity  of  the  process : 

1.  The  temperature.     As  already  stated,  the  op^imn?n  degree 

of  heat  for  the  change  is  about  that  of  the  body,  38°- 
40°  C. 

The  activity  of  the  gastric  juice  diminishes  when  the 
temperature  either  rises  above  or  falls  below  this  standard. 
The  minimum  at  which  it  is  capable  of  action  at  all  is 
about  1°  C.  and  the  maximum  is  about  90°  C.  Boiling 
permanently  destroys  the  function  of  pepsin. 

2.  The  percentage  of  acid  as  well  as  the  kind  of  acid  has  a 

marked  eflfect.  Though  the  action  will  go  on  with  other 
acids,  hydrochloric  is  the  most  effective,  and  that  of  a 
strength  of  .2  per  cent. 

3.  Large  quantities  of  salts  in  solution  or  a  condensed  solu- 

tion of  peptone  impede  the  process,  a  certain  degree  of 
dilution  being  necessary  for  the  process.  In  strong  solu- 
tions of  proteid,  the  peptones  must  be  removed  by  dialy- 
sis in  order  to  allow  of  the  continuance  of  the  action. 
This  occurs  in  the  stomach  by  means  of  the  blood  and 
absorbent  vessels. 

4.  The  degree  of  subdivision  to  which  the  proteid  has  been 

subjected  materially  influences  the  rapidity  of  its  con- 
version into  peptone.  The  more  finally  subdivided  the 
substance  the  greater  will  be  the  relative  extent  of  sur- 
face exposed  to  the  action  of  the  digestive  fluids.  When 
large  masses  are  introduced  into  the  stomach,  the  gastric 
fluid  cannot  reach  the  central  portions,  and  their  diges- 
tion must  await  the  completion  of  that  of  the  exterior 
part. 

5.  Motion  aids  the  action  of  the  foregoing  factors. 


152  MANUAL    OF    I'HYSIOLOGY. 

All  these  requisites  are  found  in  the  normal  act  of  digestion. 
The  temperature  of  the  stomach  is  38°  to  39°  C.  (  =  100°F.). 
Hydrochloric  acid  is  present  in  the  proportion  of  about  .2  per  cent. : 
as  quickly  as  the  peptones  are  formed  they  can  be  removed  by 
absorption  from  the  stomach,  and  thus  the  needful  dilution  is  ac- 
complished :  and  finally,  if  the  mouth  has  done  its  duty,  the  pieces 
of  proteid  have  been  reduced  to  a  pulp,  composed  of  minute  par- 
ticles: these  are  kept  in  constant  motion  by  the  gastric  walls,  and 
thus  are  repeatedly  brought  in  contact  with  fresh  supplies  of  the 
digestive  fluid. 

There  can  be  little  doubt  that  the  conversion  of  proteid  into 
peptone  is  normally  brought  about  by  the  pepsin,  which  acts  as  a 
ferment,  in  some  way  or  other  facilitating  a  process  which  without 
it  is  extremely  difficult  to  accomplish.  Proteids  may,  however, 
give  rise  to  peptone  without  the  presence  of  any  pepsin  at  all,  if 
they  be  treated  with  strong  acids,  alkalies,  boiling  under  high 
pressure,  putrefactive  and  other  fermentative  actions.  This,  to- 
gether with  the  analogy  suggested  by  the  chemical  details  of  the 
amylolytic  action  of  saliva,  which  one  may  say  depends  on  an  atom 
of  water  being  taken  up,  suggests  that  the  change  of  proteid  into 
peptone  is  also  hydrolytic,  the  peptones  being  simply  an  extremely 
hyd rated  form  of  proteid.* 

So  far  we  have  found  that  the  action  of  the  gastric  juice  affects 
proteids  alone.  Its  action  on  other  coustituents  of  food  varies. 
Gelatinous  material  is  dissolved  by  the  gastric  digestion  and  ren- 
dered incapable  of  forming  a  jelly  ;  its  conversion  into  peptone 
has,  however,  not  been  established.  The  connective  tissue  of  meat 
is  therefore  soon  removed,  and  the  muscle  fibres  fall  asunder,  the 
sarcolcmma  is  dissolved,  and  the  muscle  substance  is  converted 

*  Though  proteids  will  not  diffuse  through  a  dead  animal-membrane  when 
distilled  water  is  used,  a  fair  amount  of  diffusion  takes  place  if  a  suitable 
solution  of  common  salt  be  employed  instead  of  water.  It  must  also  be  re- 
membered that  the  gastric  mucous  membrane  is  a  living  active  structure, 
and  that  the  fluid  into  which  the  albumins  have  to  diffuse  may  be  regarded 
as  a  salt  solution.  It  is  therefore  quite  probable  that  a  considerable  quantity 
of  albumin  may  be  absorbed  as  such.  The  fact  that  i)eptone  cannot  be  found 
in  any  quantity  in  chyle  or  portal  blood  tends  to  prove  that  the  albumin 
does  pass  through  the  stomach  wall  without  being  changed  into  peptone. 


GASTRIC    DIGESTION.  153 

into  true  peptone.  The  delicate  sheets  of  elastic  tissue,  such  as 
basement  membranes  and  those  of  small  vessels,  are  dissolved,  but 
larger  masses  of  yellow  elastic  tissue  are  not  affected  by  the  gas- 
tric digestion.  The  horny  part  of  the  epidermis,  hairs,  etc.,  are 
quite  unaltered,  and  also  the  mucus,  which  passes  along  the  ali- 
mentary tract  without  change.  Bone  dissolves  slowly,  the  auimal 
part  being  attacked  at  the  surface  by  the  gastric  juice  and  the 
acid  slowly  removing  the  salts. 

The  action  of  the  gastric  juice  on  milk  is  peculiar.  On  reaching 
the  stomach  milk  is  curdled  by  a  special  ferment  formed  in  the 
gastric  mucous  membrane.  This  is  known  as  "Rennet,"  which  is 
made  from  the  stomach  of  the  calf,  and  used  in  the  manufacture 
of  cheese.  The  precipitation  of  the  Casein  (alkali  albumin), 
which  gives  rise  to  the  curdling  of  the  milk,  is  not  brought  about 
by  the  hydrochloric  acid  (although  the  acidity  would  be  sufficient 
cause),  because  neutralized  gastric  juice  has  the  same  effect.  It 
appears  that  a  special  ferment  (not  pepsin)  which  directly  affects 
the  casein  aud  causes  its  coagulation,  must  exist.  It  is  not  due 
to  common  lactic  ferment,  for  though  lactic  acid  is  produced,  it  is 
formed  too  slowly  to  account  for  the  very  rapid  coagulation  of 
milk  which  occurs  in  the  stomach. 

The  gastric  juice  has  little  effect  on  vegetable  food  in  general, 
though  well-masticated  bread  may  be  very  materially  altered, 
owing  to  the  action  of  the  saliva  on  the  starch  continuing  until 
the  mass  is  broken  up,  and  the  gastric  juice  then  dissolving  the 
proteids  (gluten).  The  greater  part  of  the  substance  of  bread, 
however,  leaves  the  stomach  in  an  imperfectly  digested  state. 

In  short,  the  amount  of  change  which  any  given  form  of  food 
will  undergo  in  the  stomach  will  depend  on  the  amount  and  ex- 
posed condition  of  the  proteid  it  contains. 

In  recapitulating  the  chief  events  of  gastric  digestion,  it  must 
be  remembered  that  while  the  food  is  yet  in  the  mouth  the  secre- 
tion of  the  gastric  juice  commences,  and  is  greatly  increased  by 
the  arrival  of  a  bolus  of  food  and  a  quantity  of  frothy  alkaline 
saliva.  As  the  stomach  is  filled,  more  and  more  secretion  is  pro- 
duced, and  as  some  food  is  absorbed  an  additional  stimulus  is 

13 


154  MANUAL   OF    PHYSIOLOGY. 

applied.  Being  kept  iu  motion  in  a  large  quantity  of  liquid 
which  dissolves  the  eases  iu  which  the  food  particles  are  con- 
tained, the  bolus  of  food  soon  falls  asunder  and  each  of  its  in- 
gredients is  fully  exposed  to  the  action  of  the  gastric  juice.  The 
acid  reaction  of  the  gastric  fluid  neutralizes  the  alkalinity  of  the 
saliva,  so  that  the  action  of  the  ptyalin  is  hindered,  and  the 
starch  granules  float  about  quite  unaffected  by  the  pepsin  or  hy- 
drochloric acid.  The  heat  of  the  stomach  melts  the  fats,  and  the 
motion  breaks  up  the  oily  fluid  into  smaller  masses.  They  are 
then  mingled  with  the  general  fluid,  which  becomes  more  and 
more  turbid  owing  to  the  admixture  of  starch  granules,  fat  glo- 
bules, dissolved  parapei)toues,  and  minute  particles  of  partially 
digested  proteids.  This  dull  gray  turbid  fluid  is  called  chyme. 
The  proteids  (which  class  of  food-stuffs  are  most  profoundly 
affected  by  the  gastric  digestion)  are  changed  more  or  less  rapidly 
according  as  their  particles  are  small  and  uncovered,  or  large 
and  massed  together,  so  that  they  are  more  or  less  readily  reached 
by  the  gastric  juice,  and  also  in  proportion  to  the  facility  with 
which  they  form  acid  albumin.  The  chyme  contains  but  little 
peptone,  so  that  we  may  conclude  that,  when  formed,  it  is  rapidly 
absorbed  as  are  also  the  soluble  sugar  and  ordinary  fluids  taken 
with  the  food.  The  chyme  begins  to  leave  the  pylorus  soon  after 
gastric  digestion  has  begun,  some  passing  into  the  duodenum  in 
about  half  an  hour.  The  materials  which  resist  the  gastric  secre- 
tion, or  are  aflfected  very  slowly  by  it,  are  retained  many  hours  in 
the  stomach,  and  the  pylorus  may  refuse  exit  to  such  materials 
for  an  indefinite  time,  so  that  after  causing  much  uneasiness  they 
are  finally  removed  by  vomiting.  However,  many  solid  masses, 
unchewed  vegetables,  etc.,  escape  through  the  pylorus  when  it 
opens  to  let  out  the  chyme. 


CHAPTER  IX. 

PANCREATIC  JUICE. 

Second  only  to  the  stomach  in  importance  as  a  digestive  cavity, 
is  the  duodenum,  into  which  the  copious  secretion  of  two  of  the 
largest  glands  of  the  body — the  pancreas  and  the  liver — is 
poured. 

The  pancreas  is  a  large  compound  sacculated  or  acinous  gland, 
being  composed  of  numerous  irregular  packets  of  gland  tissue 
attached  by  its  lateral  branchlets  to  the  main  central  duct.  The 
saccules  are  rather  elongated,  but  have  the  same  general  con- 
struction as  those  of  the  serous  salivary  glands  already  described. 
A  single  layer  of  irregular  or  slightly  conical  cylindrical  cells  in 
the  saccule,  shows  a  difference  of  structure  in  its  central  and 
peripheral  sides,  so  that  an  external  or  homogeneous  zone,  and 
an  internal  granular  zone  may  be  distinguished.  Each  zone  cor- 
responds to  one-half  of  the  cells,  the  clear  half  being  next  the 
boundary,  and  the  granular  half  next  the  lumen  of  the  saccule. 
The  relative  width  of  these  zones  varies  with  the  digestive  pro- 
cess, so  that  the  nuclei  which  are  situated  between  them  some- 
times appear  to  be  in  the  outer  clear  zone,  and  sometimes  in  the 
inner  granular  zone.  The  outer  zone  colors  readily  with  car- 
mine, whilst  the  inner  zone  remains  unstained. 

The  large  duct  which  passes  down  the  axis  of  the  gland,  re- 
ceiving tributaries  on  all  sides,  is  surrounded  with  a  layer  of 
loose  connective  tissue  which  forms  an  outer  coat.  The  proper 
coat  of  the  duct  is  composed  of  elastic  tissue,  lined  by  a  single 
layer  of  cylindrical  epithelium. 

Collection  of  Pancreatic  Juice. — From  a  temporary 
fistula  the  secretion  of  the  pancreas  can  be  obtained  in  sufficient 
quantity  to  determine  its  character  and  properties.  A  permanent 
fistula  is  established  with  difficulty,  and  the  secretion  soon  alters 


156  MANUAL    OF    PHYSIOLOGY. 

its  characters,  becoming  thin  and  losing  its  efficacy,  most  prob- 
ably being  altered  by  an  abnormal  state  of  the  gland. 

An  artificial  pancreatic  juice  may  be  extracted  by  water  from 
the  minced  gland  taken  a  few  hours  after  death  from  an  animal 
which  has  been  killed  during  active  digestion  (a  couple  of  hours 
after  eating).  This  extract,  used  with  proper  precautions,  will 
have  the  same  effect  as  the  secretion  itself. 

A  glycerin  solution  containing  the  active  principles  of  the 
pancreatic  secretion  may  also  be  made  from  the  pancreas  of  a  dead 
animal  by  treating  the  minced  gland  for  a  couple  of  days  with 
absolute  alcohol,  removing  the  alcohol,  and  substituting  sufficient 
glycerin  to  cover  it,  in  which  it  should  remain  a  week  or  so.  This 
extract,  filtered,  contains  but  little  else  than  pancreatic  ferments. 

Characters  of  the  Secretion. — The  pancreatic  juice  is  a 
very  thick,  transparent,  colorless,  strongly  alkaline  fluid,  which 
turns  to  a  jelly  if  cooled  to  0°  C  It  often  contains  about  ten 
per  cent,  of  solids  when  obtained  from  a  temporary  fistula,  but  it 
may  have  as  little  as  two  per  cent. 

Of  these  a  considerable  proportion  are  organic,  namely : 

1.  Albumin  which  is  coagulated  by  boiling. 

2.  Alkali  albumin,  precipitated  by  acetic  acid  or  by  adding 

magnesium  sulphate  to  saturation. 

3.  Leucin  and  tyrosin. 

4.  Fats  and  soaps. 

5.  Salts,   particularly   sodium    carbonate,  which    makes   it 

alkaline. 

6.  Three  ferments,  to  which  it  owes  its  specific  action  on  the 

food-stuffs. 

Mode  of  Secretion. — The  pancreas  does  not  continue  in  a 
state  of  activity  during  the  interval  between  the  periods  of  active 
digestion.  When  the  gland  is  at  rest  it  is  of  a  pale  yellow  color 
and  is  flaccid,  but  during  active  digestion  it  becomes  more  turgid, 
and  assumes  a  pinkish  color  from  the  increased  flow  of  blood. 
The  secretion  commences  immediately  after  taking  food,  and  rises 
rapidly  for  a  couple  of  hours,  then  falls  and  rises  again  in  the 


CHANGES   IN    PANCREATIC   CELLS.  157 

later  hours  of  digestion,  five  to  seven  hours  after  a  meal ;  then  it 
gradually  falls  for  eight  to  ten  hours,  and  ceases  completely  wheu 
digestion  is  at  an  end.  The  first  rise  which  accompanies  the  in- 
troduction of  food  into  the  stomach,  is  certainly  brought  about 
by  nervous  agencies  of  a  similar  nature  to  that  of  the  stomach, 
the  secretion  of  which  follows  closely  upon  mastication.  The 
second  accompanies  the  passage  of  the  undigested  food  through 
the  small  intestines,  and  may  also  be  most  conveniently  explained 
as  the  I'esult  of  reflex  nervous  stimulation  of  the  gland  cells. 

The  great  complexity  of  the  nerve  distribution  to  the  glands  of 
the  intestinal  tract  makes  it  difficult  to  ascertain  the  exact  chan- 
nels traversed  by  the  afferent  and  efferent  impulses.  The  follow- 
ing observations,  if  accurate,  would  tend  to  prove  that  certain 
inhibitory  impulses  pass  from  the  stomach  along  the  vagus  to  the 
medulla,  and  are  thence  reflected  to  the  gland  by  its  vaso  motor 
nerves.  During  vomiting,  or  when  the  central  end  of  the  divided 
vagus  is  stimulated,  the  secretion  of  the  pancreas  ceases.  Section 
of  the  nerves  which  surround  the  bloodvessels  distributed  to  the 
pancreas  causes  considerable  (paralytic)  flow  of  secretion  which 
stimulation  of  the  vagus  cannot  check. 

No  nerve  channels  have  been  demonstrated  to  carry  exciting 
impulses  direct  to  the  glands,  as  the  chorda  tympani  does  to  the 
submaxillary  ;  but  the  direct  stimulation  of  the  gland  itself,  or  of 
the  medulla  oblongata,  is  said  to  induce  activity  of  the  gland. 

During  the  period  of  rest  of  the  pancreas,  i.e.,  when  the  alimen- 
tary tract  is  not  in  activity,  no  secretion  flowing  from  the  duct 
and  the  gland  being  pale,  the  gland  cells  in  the  acini  undergo  a 
change  which  may  be  compared  with  that  observed  in  the  cells 
of  the  serous  salivary  glands.  The  division  of  the  row  of  cells 
lining  the  acinus,  into  a  central  granular  and  outer  clear  zone, 
has  already  been  mentioned. 

Immediately  after  very  active  secretion,  the  central  granular 
zone  is  reduced  to  a  minimum  owing  to  the  paucity  of  granules ; 
and  the  outer  zone  occupies  the  greater  part  of  the  cell,  the  entire 
substance  of  which  stains  readily  and  looks  like  ordinary  proto- 
plasm. After  rest,  however,  the  granules  reappear,  and  after  the 
lapse  of  a  short  quiescent  period,  the  inner  granular  zone  has 


158 


MANUAL   OF    PHYSIOLOGY, 


again  encroached  ou  the  outer,  owing  to  the  accumulation  of  gran- 
ules which,  rapidly  increasing,  fill  the  greater  part  of  the  cells, 
and  cause  them  to  bulge  inwards  and  occlude  the  lumen  of  the 
gland.  When  digestion  commences,  the  cells  undergo  a  slight 
change  in  form,  so  that  each  individual  cell  is  more  distinctly 
seen,  and  its  angles  are  retracted,  giving  a  notched  appearance  to 
the  margin  of  the  acinus.  The  blood  supply  during  this  period 
is  much  increased,  red  arterial  blood  flowing  from  the  veinlets  of 
the  gland.     At  the  same  time  the  granules  are  diminished  in  num- 

FiG.  70. 


A.  B. 

One  Saccule  of  tlie  Pancreas  of  the  Kabbit  in  different  states  of  activity. 
— A.  After  a  period  of  rest,  in  which  case  the  outlines  of  the  cells  are  indis- 
tinct, and  the  inner  zone,  i.e.,  the  part  of  the  cells  (a)  next  the  lumen  (c), 
is  broad  and  filled  with  fine  granules,  u.  After  the  gland  has  poured  out  its 
secretion,  when  the  cell  outlines  (c/)  are  clearer,  the  granular  zone  («)  is 
smaller,  and  the  clear  outer  zone  is  wider.     (Kiihne  and  Lea.) 


ber,  escaping  at  the  free  central  margin  of  the  cells  into  the  lumen 
towards  which  they  appear  to  crowd,  leaving  the  outer  zone  once 
more  clear  and  free  from  granules,  while  the  lumen  of  the  saccule 
and  of  the  ducts  are  filled  with  secretion. 

Let  us  then  examine  a  single  cell ;  during  the  period  of  rest 
with  a  comparatively  poor  supply  of  blood,  the  cell  receives  its 
normal  nutrition,  which  is  accompanied  by  an  accumulation  of 
granules  in  the  protoplasm  next  the  free  side  of  the  cell.  During 
secretion  these  granules  are  pushed  out  of  the  cell,  and  seem  in 
some  way  to  form  the  secretion. 


CHANGES    IN    PANCREATIC    CELLS.  159 

It  will  be  seen  immediately  that  one  of  the  most  important 
functions  of  the  pancreatic  juice  is  the  formation  of  peptone  from 
proteid,  which  operation  is  carried  out  by  a  special  ferment  called 
trypsin.  It  has  been  found  that  this  ferment  cau  only  be  obtained 
from  the  active  pancreas,  and  that  the  wider  the  inner  granular 
zone  of  the  cells  is,  the  richer  in  ferment  is  the  glycerin  extract 
made  from  the  gland.  But  it  has  been  found  that  if  a  glycerin 
extract  be  rapidly  made  from  an  actively  secreting  absolutely 
fresh  gland,  i.e.,  removed  from  the  dead  animal  while  still  warm, 
the  extract  is  found  to  be  quite  inert  towards  proteids,  while  an 
extract  made  from  a  portion  of  the  same  pancreas  which  has  been 
kept  some  hours  after  death  is  very  active ;  and  a  portion  of  the 
fresh  pancreas  pounded  in  a  mortar  with  a  little  weak  acid  so  as 
to  develop  the  trypsin  in  it,  acts  in  an  alkaline  solution  and  forms 
peptone  energetically. 

We  must  therefore  conclude  that  the  special  proteolytic  fer- 
ment of  the  pancreas  does  not  exist  prior  to  the  period  at  which 
the  secretion  is  poured  out  from  the  gland  cells. 

Although  a  definite  relation  seems  to  exist  between  the  amount 
of  granules  in  the  active  cells  and  the  degree  of  efficacy  of  the 
secretion,  the  ferment  does  not  appear  in  full  force  for  some  time 
after  that  the  height  of  the  gland  activity  has  been  established, 
and  it  is  likely  that  the  presence  of  an  acid  helps  in  the  birth  of 
the  ferment. 

It  has  therefore  been  assumed  that  the  granules  of  the  gland 
cells  give  rise,  not  to  the  proteolytic  ferment,  but  to  a  ferment- 
producing  substance  which  is  called  Zymogen. 

So  that  if  we  trace  the  history  of  the  pancreatic  proteolytic  fer- 
ment, we  shall  find  that,  so  far  as  this  trypsin  is  concerned,  there 
can  be  no  question  as  to  whether  it  pre-exists  in  the  blood  and  is 
removed  thence  by  the  gland  or  not,  because  by  studying  the  process 
we  find  that  the  final  elaboration  of  the  secretion  takes  place  after 
it  has  got  into  the  ducts  or  the  intestinal  cavity.  Thus  the  blood 
gives  to  the  protoplasm  of  the  gland-cells  nutriment.  The  proto- 
plasm of  the  cells,  by  its  intrinsic  chemical  processes,  manufac- 
tures peculiar  granules.     These  granules  give  rise,  among  other 


100  MANUAL   OF   PHYSIOLOGY. 

things,  to  zymogen,  which   iu  the  presence  of  an  acid   begets 
trypsin. 

Pancreatic  Digestion. — The  pancreatic  juice  is,  of  all  the 
digestive  fluids,  the  most  general  solvent.  It  acts  upon  the  three 
great  classes  of  food-stuffs  which  require  modification  to  enable 
them  to  pass  through  the  barrier  that  intervenes  between  the  in- 
testinal cavity  and  the  blood  current.  It  changes  proteids  into 
peptones,  it  profoundly  modifies  fatty  substances,  and  converts 
starch  into  soluble  sugar.  The  ferments  to  which  its  activity  is 
due  may  be  separately  described. 

I.  Action  of  pancreatic  juice  on  proteids. — The  ferment  which 
produces  peptones  is  trypsin.  Some  of  the  conditions  required  for 
its  perfect  operation  are  the  same  as  those  necessary  for  the  action 
of  the  gastric  ferment  pepsin  ;  namely,  a  certain  degree  of  dilu- 
tion, and  a  temperature  of  about  40°  C.  But  itdiflfers  from  pep- 
sin in  the  most  important  characteristic  of  its  action.  Whilst  the 
presence  of  an  acid  is  absolutely  necessary  for  peptic  proteolysis, 
we  find  that  an  alkaline  reaction  is  required  for  this  action  of  the 
pancreatic  ferment,  and  as  the  peptic  peptone  has  to  pass  through 
preliminary  stages  in  which  it  closely  resembles  acid  albumin,  so 
the  tryptic  peptone  is  first  produced  from  alkali-albumin,  which  has 
been  formed  as  a  preliminary  step  by  the  alkali  of  the  pancreatic 
juice.  The  addition  of  the  sodium  carbonate  aids  the  action,  and 
indeed  seems  to  play  a  part  which  closely  corresponds  to  that 
taken  by  the  hydrochloric  acid  in  gastric  digestion. 

The  change  to  alkali  albumin  and  peptone  as  accomplished  by 
the  trypsin,  is  not  accompanied  by  any  swelling  of  the  albumin 
such  as  occurs  in  the  formation  of  the  acid  albumin  in  the 
stomach,  but  the  proteid  is  gradually  eroded  from  the  surface 
and  thus  diminished  iu  size. 

Moreover,  the  alkali  albumin  is  not  made  directly  into  peptone, 
but  passes  through  a  stage  in  which  it  resembles  globulin,  and  is 
soluble  in  solutions  of  sodium  chloride. 

Besides  these  differences  between  the  mode  of  action  of  pepsin 
and  trypsin  in  producing  peptones,  trypsin  has  a  peculiar  power 
upon  proteids,  which  has  no  analogue  in  the  peptic  action.  While 


rAXCREATIC   DIGESTION.  161 

the  pancreatic  peptone  is  being  produced,  a  further  change  occurs, 
which  gives  rise  to  the  formation  of  two  crystallizable  nitrogenous 
bodies  known  as  leucin  and  tyrosiu,  the  former  belonging  to  the 
fatty  acid,  and  the  latter  to  the  aromatic  acid  group.  These 
substances,  which  are  commonly  found  together  as  a  result  of  the 
decomposition  of  peptones,  seem  inseparable  from  pancreatic  di- 
gestion, aud  increase  in  amount  towards  the  later  stages  of  the 
action. 

The  amount  of  peptone  produced  reaches  a  maximum  in  about 
four  hours,  after  which  the  proportion  of  the  different  unknown 
decomposition  products  appears  to  increase  at  the  expense  of  the 
peptone.  Among  these  substances  must  be  named  indol  and 
skatol,  the  materials  from  which  the  process  of  pancreatic  diges- 
tion derives  its  peculiar  odor. 

This  breaking  up  of  the  surplus  pi'oteid  food  into  bodies  which 
cannot  be  of  much  utility  in  the  economy,  and  which,  as  will 
appear  hereafter,  are  but  a  step  in  the  direction  of  their  elimina- 
tion, is  probably  an  important  part  of  the  pancreatic  function, 
as  it  relieves  the  economy  of  a  surcharge  of  albuminous  sub- 
stances. 

Small  quantities  of  phenol  are  also  found  in  conjunction  with 
the  above. 

II.  Action  on  Fat. — The  action  of  the  pancreatic  juice  on  fats 
is  of  two  kinds.  (I.)  Saponification. — By  the  action  of  a  special 
ferment  (sleapsin)  the  neutral  fats  are  split  up  into  glycerin  and 
their  corresponding  fatty  acids.  The  acids  thus  formed  readily 
unite  with  the  alkali  present,  and  thus  form  soap.  The  chemis- 
try of  the  change  will  be  found  at  p.  71,  and  maybe  thus  shortly 
stated,  taking  olein  as  an  example.  Olein  is  a  compound  of 
oleic  acid  and  glycerin.  Olein  in  presence  of  ferment  and  soda 
gives  glycerin  and  oleic  acid,  and  the.  latter  combines  with  soda 
to  form  soap.  This  process  materially  aids  in  the  next.  (II.) 
Emulsification. —  Which  means  that  the  fat  is  reduced  to  a  state 
of  very  fine  subdivision,  as  it  exists  in  milk.  The  production  of 
this  condition  is  facilitated  by  (1),  the  quantity  of  albumin  in 
solution;  (2),  the  alkalinity  of  the  fluid;  (3),  the  presence  of 
soap  alluded  to  above;  and  (4),  the  motion  of  the  intestines. 

14 


162  MANUAL   OF   PHYSIOLOGY. 

III.  Action  on  Starch. — Tins  action  of  the  pancreatic  juice 
seems  to  depend  on  a  separate  ferment  (Anujlopsin),  and  with  the 
exception  that  it  is  much  more  rapid  and  energetic,  and  is  said 
to  affect  raw  as  well  as  boiled  starch,  its  action  seems  to  be  iden- 
tical with  that  of  the  saliva.  This  power  is  found  to  exist  in  the 
extract  of  the  gland,  whether  it  has  been  removed  from  a  fasting 
or  from  a  recently  fed  animal,  and  therefore  does  not  depend  on 
whether  the  gland  is  engaged  in  active  secretion  or  not. 


CHAPTER  X. 

BILE. 

The  liver  has  two  chief  functions,  which  are  so  distinct  in 
their  ultimate  object  that  they  may  be  conveniently  described 
separately,  although  we  are  not  aware  that  any  natural  distinc- 
tion exists  in  the  manner  of  their  performance.  One  is  mainly 
excrementitious,  namely,  the  secretion  of  bile,*  which  belongs  to 

Fig.  71. 


Section  of  tlie  Liver  of  the  Newt,  in  which  the  bile  ducts  have  been  in- 
jected, and  can  be  seen  to  form  a  network  of  fine  caiiillaries  around  the 
liver  cells,  the  outlines  and  nuclei  of  which  can  be  seen. 

the  fluids  connected  with  digestion,  and  therefore  naturally  falls 
into  this  chapter.  The  other  is  purely  nutritive,  consisting  in 
the  formation  of  glycogen.  The  glycogenic  function  of  the  liver 
is  of  the  first  importance  in  the  elaboration  of  the  blood,  and  will 
therefore  be  reserved  for  the  chapter  on  that  subject. 

Among  the  most  striking  anatomical  peculiarities  of  the  liver 


*  Probably  also  the  manufacture  of  urea  should  be  mentioned  here,  for 
there  is  no  doubt,  as  will  be  seen  later  on,  that  the  liver  has  an  important 
share  in  producing  this  substance. 


164  MANUAL  OF  PHYSIOLOGY. 

are:  (1)  The  gall  bladder  is  its  receptacle  for  storing  the  secre- 
tion until  it  is  required.  (2)  It  has  a  double  supply  of  blood. 
Besides  that  coming  from  the  spleen,  pancreas  and  intestinal 
canal,  collected  by  the  tributaries  of  the  great  portal  vein,  and 
distributed  by  its  branches  to  the  liver,  it  receives  by  the  hepatic 
artery  a  small  supply  of  fresh  arterial  blood.  (3)  A  beautiful 
network  is  formed  by  the  minute  ducts  (bile  capillaries)  which 
freely  anastomose  between  the  cells.  (4)  Although  in  the  em- 
bryo, and  in  many  animals  throughout  their  adult  life,  the  liver 
is  a  compound  saccular  gland,  yet  the  relation  of  the  duct  radi- 
cles to  the  saccules  is  so  modified  in  the  higher  animal  and  man, 
that  the  analogy  is  no  longer  apparent,  and  the  structural  ar- 
rangement is  best  understood  by  following  its  vascular  ground- 
work. 

Structure  of  the  Liver. — On  the  surface  of  the  liver  are 
seen  small  rounded  markings  about  the  size  of  a  pin's  head  which 
give  the  organ  a  peculiar  mottled  appearance.  This  is  much 
more  striking  in  some  animals  (girafie,  bear,  pig)  than  others, 
but  easily  recognizable  in  the  livers  of  all  mammalia.  These 
little  areas  mark  out  the  lobules  of  the  liver.  They  are  sur- 
rounded by  a  dark  red  boundary,  and  their  centre  is  marked  by 
a  dark  spot,  between  which  there  is  a  paler  yellowish  zone.  The 
dark  parts  correspond  to  the  bloodvessels,  and  have  a  constant 
relation  to  the  lobules. 

The  entire  liver  is  made  up  of  these  little  lobules,  and  each 
one  of  them  has  the  same  construction  and  blood  supply,  and 
therefore  forms  in  itself  a  little  liver  perfect  in  all  its  structural 
arrangements,  so  that  the  description  of  one  such  unit  will 
suflace  to  give  an  idea  of  the  structure  of  the  liver.  For  other 
details,  anatomical  works  must  be  referred  to. 

The  branches  of  the  large  portal  vein  and  those  of  the  small 
hepatic  artery  pursue  the  same  course  through  the  gland,  and  are 
inclosed  in  a  sheath  of  connective  tissue,  which  also  forms  the 
bed  of  the  hepatic  duct  and  its  numerous  tributaries.  If  these 
branching  vessels  be  followed  to  their  final  ramifications,  they  are 
found  to  pass  around  and  between  the  neighboring  lobules.     The 


STRUCTURE    OF   THE    L,IVEE, 


165 


branches  of  the  portal  vein  in  this  situation  receive  the  name  of 
the  interlobular  veins.  They  anastomose  freely  with  the  terminal 
veinlets   in    the  vicinity,  so  as  to  form    a  network  round  each 


Fig.  72. 


i^/i\\K\y\im 


Section  of  Lobule  of  Liver  of  Rabbit  in  which  the  blood  and  bile  capil- 
laries have  been  injected.  (Cadiat.) — a.  Intralobular  vein.  6.  Interlobular 
veins,    c.  Biliary  canals  beginning  in  fine  capillaries. 

lobule.  From  this  a  number  of  capillary  vessels  pass  into  the 
lobule,  and,  lying  between  the  gland-cell,  form  a  network  with 
long  meshes  radiating  from  the  centre  like  the  threads  of  a 


166 


MANUAL   OF   PHYSIOLOGY. 


spider's  web.  These  are  the  lobular  blood  capillaries.  The  vessels 
of  this  radiated  capillary  network  become  larger  as  they  unite 
and  converge  to  the  centre  of  the  lobule,  where  they  open  into 
a  central  vein  which  lies  in  immediate  apposition  with  the  gland 
cells.  This  vein  is  called  the  intralobnhir  vein,  and  is  the  radicle 
of  the  efferent  or  hepatic  vein,  which  carries  the  blood  of  the 
liver  to  the  inferior  vena  cava. 

The  ultimate  ramifications  of  the  hepatic  artery  can  be  traced 
to  various  destinations.  Some  go  to  the  walls  of  the  accompanying 
vein  and  duct,  and  to  the  connective  tissue  which  surrounds  these 

Fig.  73. 


Cells  of  the  Liver.  OnelargeniassHhows  the  shape  they  assume  by  mutual 
pressure. — (a)  The  same  free,  when  tliey  become  spheroid.  (6)  More  mag- 
nified,    (c)  During  active  digestion,  containing  refracting  globules  like  fat. 


vessels.  Many  of  the  arterial  capillaries  unite  with  offshoots 
from  the  interlobular  venous  plexus  and  thus  reinforce  the  lob- 
ular capillaries.  Other  branches  form  an  interlobular  capillary 
plexus,  which  flows  into  the  interlobular  branches  of  the  vena 
porta,  together  with  those  from  the  walls  of  the  vein  and  duct. 

The  blood  flowing  to  the  liver  in  the  large  vena  porta  and  the 
small  hepatic  artery,  is  thus  conducted  by  those  vessels  to  the 
boundaries  between  the  lobules  (interlobular  veins),  and  thence 
streams  through  the  converging  lobular  capillaries  to  the  intra- 


STEUCTURE   OF   THE   LIVER. 


167 


lobular  vein,  and  is  collected  from  the  latter  by  the  sublobular 
tributaries  of  the  hepatic  vein,  by  which  it  is  conducted  back  to 

Fig.  74. 


Section  of  the  liver  showing  the  relation  of  the  portal  branches  (vp)  and 
of  the  radicles  of  the  hepatic  veins  (hv)  to  the  lobules.  Below  is  a  portion 
of  the  same  highly  magnified. — (a)  Liver  cell  with  (n)  nucleus;  (6)  Blood 
capillaries  cut  across  ])assing  along  angles  of  cells ;  (c)  Bile  capillaries  be- 
tween flattened  sides  of  cells.    (Huxley.) 

the  general  circulation,  and  enters  the  heart  by  the  inferior  vena 
cava. 


168 


MANUAL  OF   PHYSIOLOGY. 


Tightly  packed  between  the  meshes  of  the  lobular  capillaries 
are  the  gland  cells.  These  are  large,  soft  polyhedral  cells,  with 
one,  two,  or  even  more  nuclei,  and  no  trace  of  limiting  membrane. 
Owing  to  the  shape  of  the  capillary  meshes  the  cells  are  placed 
in  rows  radiating  from  the  centre  of  the  lobule  towards  the 
periphery. 

The  capillary  meshes  are  said  to  pass  along  the  angles  and 
edges  of  these  cell  blocks  so  as  not  to  come  into  close  relation 
to  the  smaller  channels  or  bile  capillaries  about  to  be  described 
(Fig.  75).  The  finely  granular  protoplasm  of  the  liver  cells  is 
capable  of  undergoing  some  slight  change  in  form  while  alive. 
In  the  protoplasm  are  commonly  situated  varieties  of  granules, 


Section  of  the  Liver  of  the  Newt,  in  whicli  the  bile  ducts  have  been  in- 
jected, and  can  be  seen  to  form  a  network  of  fine  capillaries  around  the 
liver  cells,  the  outlines  and  nuclei  of  which  can  be  seen. 


the  commonest  being  bright  refracting  fat  globules,  which  vary 
in  amount  with  the  different  stages  of  digestion,  others,  of  a  yellow 
color,  seem  connected  with  the  coloring  matter  of  the  bile,  and  a 
third  variety,  less  refracting  and  colorless,  is  said  to  be  related 
to  the  glycogen. 

Between  the  cells  of  the  lobules  there  can  be  demonstrated 
very  fine  straight  anastomosing  canals,  which  appear  to  be  formed 
by  the  juxtaposition  of  grooves  which  lie  in  the  middle  of  the 
flat  surface  of  two  neighboring  cells.     Every  liver  cell  is  related 


METHOD   OF   OBTAINING'  BILE.  169 

to  such  a  caual,  and  consequently  a  very  dense  network  with 
peculiarly  regular  polygonal  meshes  is  present,  each  mesh  cor- 
responding in  size  to  one  cell. 

These  fine  intercellular  canals  are  called  lobular  bile  capillaries, 
and  must  not  be  confounded  with  lobular  blood  cajnllaries,  the 
diameter  of  which  is  about  ten  times  as  great  as  the  former,  and 
which  have  a  definite  boundary  wall,  whilst  the  bile  capillaries 
have  no  other  boundary  than  the  substance  of  the  liver  cell,  and 
therefore  are  not  really  vessels. 

These  fine  intercellular  bile  passages  are  described  as  commu- 
nicating with  the  interlobular  ducts  directly  opening  into  the 
ducts  without  any  marked  increase  in  the  size  or  change  of  ar- 
rangement. The  interlobular  ducts  which  follow  the  course  of 
the  artery  and  portal  vein  are  composed  of  a  delicate  basement 
membrane  lined  with  a  thin  layer  of  epithelium,  which,  in  the 
larger  vessels,  shows  a  cylindrical  character.  The  larger  bile 
ducts  have  a  firm  fibro-elastic  coat  lined  with  a  definite  mucous 
membrane  covered  with  cylindricalepithelium  lying  upon  a  vas- 
cular submucosa,  in  which  are  scattered  numerous  glands  of  sac- 
cular form. 

The  amount  of  connective  tissue  in  the  liver  of  man  and  most 
domestic  animals  is  very  small,  but  in  the  pig,  bear,  giraflfe,  and 
some  others,  it  is  easily  recognized  around  the  lobules,  sending 
delicate  supporting  processes  between  the  cells  of  the  lobules.  It 
passes  into  the  organ  with  the  portal  system  of  vessels  forming  a 
loose  sheath  derived  from  the  capsule  of  Glisson,  and  is  distributed 
with  the  subdivisions  of  those  vessels  to  the  various  parts  of  the 
gland. 

The  lymphatics  are  known  to  be  very  plentiful,  and  in  intimate 
relation  to  the  bloodvessels. 

Method  of  Obtaining  Bile. — For  most  practical  purposes 
the  bile  obtainable  from  the  gall-bladder  of  dead  animals  is  suffi- 
cient. The  bile  pigments  and  cholesteriu  may  be  conveniently 
obtained  from  the  gall-stones  so  often  found  in  the  human  gall- 
bladder. 

In  order  to  investigate  the  -composition  of  the  bile  as  it  comes 


170  MANUAL   OF   PHYSIOLOGY. 

from  tlic  ducts,  before  it  has  been  modified  by  its  sojourn  in  the 
gallbladder,  it  is  necessary  to  make  a  biliary  fistula,  comniuui- 
cating  either  with  the  gall-bladder  or  with  the  bile-duct.  In  this 
way  the  rate,  pressure,  and  other  points  concerning  the  mode  of 
secretion  may  be  determined. 

Composition  of  Bile. — The  bile  of  man  and  carnivorous 
animals  is  of  a  deep  orange-red  color,  turning  to  greenish-l)rown 
by  decomposition  of  its  coloring  matter.  In  herbivorous  animals 
it  has  some  shade  of  green  when  quite  fresh,  but  turns  to  a  muddy 
brown  after  a  time.  It  is  transparent,  and  more  or  less  viscid 
according  to  the  length  of  time  it  has  remained  in  the  gall- 
bladder. It  has  a  strong  bitter  taste,  a  peculiar  aromatic  odor, 
and  after  remaining  for  some  time  in  the  gall-bladder  it  has  an 
alkaline  reaction.  Its  specific  gravity  is  about  1010  when  taken 
from  the  bile-ducts  directly,  but  it  rises  to  1030  after  prolonged 
slay  in  the  gall-bladder,  owing  to  the  addition  of  mucus  and  the 
absorption  of  some  of  its  fluid. 

The  following  table  gives  approximately  the  proportions  of  the 
chief  constituents  of  the  bile: 


Water, 

.     85.0  per  cent 

Bile  salts, 

.     10.0 

Coloring  matter  and  nuicns,     . 

.       3.0 

Fats, 

.       1.0 

Cholesterin, 

.       0.3 

Inorganic  salts,        .... 

.       0.7 

100.0 

Bile  contains  no  structural  elements  nor  any  trace  of  albumin- 
ous bodies. 

I.  The  bile  acids  are  two  compound  acids,  glyco-cholic  and 
tauro-cholic,  which  exist  in  the  bile  in  combination  with  sodium. 
The  amount  of  each  varies  in  different  animals  and  at  different 
times  in  the  same  animal.  The  bile  of  the  dog  and  other  carni- 
vora  contains  only  tauro-cholate  of  soda.  In  the  ox  the  glyco- 
cholate  of  soda  is  greatly  in  excess.  In  man  both  are  present, 
the  proportion  being  variable,  but  the  tauro-cholate  is  said  to 
preponderate. 


BILE   CONSTITUENTS.  171 

To  separate  these  acids,  bile  is  evaporated  to  one-fourth  its 
volume,  rubbed  to  a  paste  with  animal  charcoal  to  remove  the 
pigments,  and  carefully  dried.  The  black  cake  is  extracted  with 
absolute  alcohol,  which  dissolves  the  bile  salts.  From  the  strong 
alcoholic  solution  after  partial  evaporation  the  bile  salts  can  be 
precipitated  by  ether.  They  first  appear  as  an  emulsion,  and 
then  form  glistening  crystals  which  are  soluble  in  water  or  alcohol, 
but  insoluble  in  ether. 

From  the  solution  of  the  two  salts  the  glyco-cholic  acid  may 
be  precipitated  by  neutral  lead  acetate,  as  lead  glyco-cholate, 
from  which  the  lead  may  be  removed  by  sulphuretted  hydrogen, 
and  the  acid  precipitated  from  its  alcoholic  solution  by  the  addi- 
tion of  water.  The  tauro-cholic  acid  may  be  obtained  subse- 
quently by  treating  with  basic  lead  acetate. 

Glyco-cholic  acid  when  boiled  with  weak  acids,  alkalies,  or 
baryta-water,  takes  up  an  atom  of  water,  and  splits  into  cholic 
acid  and  glycin  (amido-acetic  acid).     (See  p.  66.) 

Tauro-cholic  acid,  under  similar  treatment,  splits  into  cholic 
acid  and  taurin  (araido-ethyl-sul phonic  acid).     (See  p.  65.) 

Cholic  acid  occurs  free  in  the  intestines,  the  bile  salts  being 
split  up  in  digestion  and  tauro-cholic  and  glyco-cholic  acids  sepa- 
rated. 

The  nitrogenous  cholic  acid  is  in  a  great  measure  eliminated 
with  the  fjeces,  while  the  taurin  and  glycin  are  reabsorbed  into 
the  blood  with  many  of  the  other  constituents  of  the  bile,  and 
are  again  probably  utilized  in  the  economy. 

No  traces  of  these  bile  acids  can  be  detected  in  normal  blood, 
and  there  is  no  accumulation  of  them  in  the  body  after  the  re- 
moval of  the  liver ;  hence  it  has  been  concluded  that  they  are 
manufactured  in  the  liver. 

11.  Mucus.  The  greater  part  of  the  mucus  which  the  bile  con- 
tains is  produced  in  the  gall  bladder,  and  there  added  to  the  bile. 
Some  mucus  comes  from  the  mucous  glands  in  the  bile-ducts,  but 
unless  the  bile  has  remained  in  the  gall-bladder  there  is  but  an 
insignificant  amount  of  mucus  present,  as  is  seen  when  a  fistula 
is  made  from  the  hepatic  duct.     The  mucus  passes  in  an  un 


172  MANUAL   OF   PHYSIOLOGY. 

changed  state  through  the  intestine,  and  is  evacuated  with  the 
ficces. 

III.  The  bile  i)igment  of  man  and  carnivora  is  chiefly  the  red- 
dish form  called  hiliruhin.  It  is  insoluble  in  water,  but  soluble 
in  chloroform.  It  can  be  obtained  in  rhombic  crystals,  and  is 
easily  converted  by  oxidation  into  a  green  pigment,  biliverdin, 
which  is  the  principal  coloring  matter  in  the  bile  of  many  ani- 
mals, and  is  not  soluble  in  chloroform,  but  readily  so  in  alcohol. 
Bilirubin  is  supposed  to  be  identical  with  hsematoidin,  a  deeply 
colored  material  found  by  Virchow  iu  old  extravasations  of  blood 
within  the  body,  and  hence  the  bile  pigment  is  said  to  be  derived 
from  the  coloring  matter  of  the  blood.  Probably  the  haemoglobin 
of  some  red  corpuscles  which  have  been  broken  up  in  the  spleen 
is  converted  into  bile  pigment  by  the  liver. 

Under  the  influence  of  decomposition  bilirubin  undergoes  a 
change,  taking  up  water  and  forming  hydro-bilirubin  ;  this  oc- 
curs in  the  intestine,  and  the  bilirubin  is  thus  eliminated  as  the 
coloring  matter  of  the  faeces  (stercobilin),  which  is  probably  iden- 
tical with  the  urobilin  of  the  urine. 

IV.  Fatty  matters,  the  principal  of  which  are  lecithin  (see  p. 
71),  palmitin,  stearin,  olein,  and  their  soda  soaps. 

V.  Cholesterin  (C^gHj^^O)  is  an  alcohol,  and  crystallizes  in  clear 
rhombic  plates,  insoluble  in  water,  but  held  in  solution  by  the 
presence  of  the  bile  salts.  It  can  be  obtained  from  gall-stones, 
the  pale  variety  of  which  are  almost  entirely  composed  of  it.  The 
cholesterin  leaves  the  intestine  with  the  faices. 

VI.  The  inorganic  salts  are  sodium  and  potassium  chloride, 
calcium  phosphate,  some  magnesia,  and  a  considerable  quantity 
of  iron. 

Tests  for-  Bile. — The  most  important  constituents  of  the  bile, 
viz.,  the  bile  acids  and  pigment,  may  be  detected  by  appropriate 
tests,  which  are  of  great  practical  use : 

1.  Pettenkofer's  test  for  the  bile  acids  :  To  a  fluid  contain- 
ing either  or  both  bile  acids  add  some  cane  sugar,  and 
then  slowly  drop  by  drop  strong  sulphuric  acid.  The 
solution  turns  to  a  cherry-red  and  then  changes  to  a 
fine  purple.     As  other  substances,  such  as  albuminous 


BILE   SECRETION.  173 

bodies,  give  under  this  treatment  ^a  similar  color,  in 
order  to  make  the  reaction  a  trustworthy  test  for  bile 
salts,  the  two  characteristic  bands  given  by  the  spectro- 
scope should  also  be  observed. 

The  following  is  said  to  be  a  more  characteristic  test : 
Rinse  out  a  porcelain  capsule  successively  with  the  fluid 
to  be  tested,  then  with  weak  sulphuric  acid,  and  finally 
with  a  weak  solution  of  sugar,  then  heat  to  70°  C,  when 
the  capsule  turns  purple. 
2.  Gmelin's  test  for  the  bile  pigments  depends  upon  the 
fact  that,  during  the  stages  of  oxidation,  the  bili- 
rubin undergoes  a  series  of  changes  in  color  which  fol- 
low the  sequence  of  the  familiar  solar  spectrum.  Place 
a  few  drops  of  the  fluid  to  be  tested  on  a  white  surface 
(a  capsule  or  plate),  and  allow  a  drop  of  nitric  acid, 
yellow  with  nitrous  acid  fumes,  to  run  into  it ;  as  they 
mingle  together  the  rainbow-like  play  of  color  appears. 
This,  when  watched,  will  be  found  to  consist  of  a  series 
of  changes  to  green,  blue,  violet,  red,  and  yellow. 

The  same  can  be  observed  by  allowing  the  acid  to 
trickle  gently  down  the  side  of  a  test-tube  fixed  in  an 
inclined  position  so  that  it  cannot  be  shaken,  the  play 
of  color  can  then  be  seen  starting  from  the  point  of 
junction  of  the  two  fluids. 

Method  of  Secretion  op  Bile. 

The  secretion  of  the  liver  varies  less  in  the  amount  formed  in 
a  given  time  than  that  of  other  digestive  glands.  Although  the 
changes  in  the  rate  of  its  secretion  are  not  so  marked,  they  fol- 
low the  same  general  rule  as  those  of  other  glands,  i.e.,  after 
food  is  taken  there  is  a  sudden  rise,  then  a  gradual  fall,  followed 
by  a  second  rise  in  the  amount  produced,  as  is  so  well  seen  in  the 
case  of  the  pancreas.  Although  huuger  is  said  to  check  the 
secretion  of  bile,  it  is  practically  continuous,  as  is  the  activity  of 
all  glands  whose  duty  it  is  to  eliminate  noxious  substances. 

At  the  end  of  a  period  of  fasting  the  gall-bladder  is  always 


174  MANUAL   OF    PHYSIOLOGY. 

found  greatly  distended,  because  the  secretion  has  continued  to 
flow  into  that  receptacle,  and  there  has  been  no  call  for  its  dis- 
charge into  the  duodenum. 

The  amount  of  bile  produced  by  dog3  is  much  influenced  by 
the  diet.  It  is  very  great  when  meat  alone  is  consumed,  less  with 
vegetable  diet,  and  very  small  with  a  diet  of  pure  fat.  As  a  gen- 
eral rule  the  bile  is  more  abundantly  produced  in  herbivorous 
than  in  carnivorous  animals. 

The  secretion  of  bile  is  also  influenced  by  the  amount  of  blood 
flowing  through  the  organ.  Ligature  of  the  hepatic  artery  causes 
cessation  of  the  secretion,  and  ultimately  death,  from  malnutri- 
tion of  the  tissue  of  the  liver. 

These  variations  in  the  rate  of  secretion  may  depend  on  direct 
nervous  influence,  but  no  special  secretory  nerve  mechanism  has 
been  discovered  for  the  liver,  and  it  is  quite  possible  that  the 
changes  in  the  activity  of  the  gland  which  accompany  the  diflTer- 
ent  periods  of  digestion  may  be  accounted  for  by  changes  in  the 
intestinal  blood  supply,  which  give  rise  to  corresponding  differ- 
ences in  the  amount  of  blood  flowing  through  the  portal  vein. 
W  the  vena  porta  be  ligatured,  an  effect  corresponding  to  the 
magnitude  of  the  vessel  is  produced,  the  secretion  is  arrested,  and 
the  animal  dies;  but  it  has  been  said  that  the  secretion  continues 
in  the  peripheral  part  of  the  lobules.  If  both  the  portal  vein  and 
hepatic  artery  are  ligatured,  the  secretion  at  once  stops. 

The  force  with  which  the  bile  is  secreted  is  very  small.  That 
is  to  say,  the  pressure  in  the  ducts  never  exceeds  that  of  the  blood 
as  occurs  in  the  salivary  glands;  but,  on  the  contrary,  when  a 
pressure  of  about  16  ram.  (.63  in.)  mercury  is  attained,  the  evac- 
uation of  the  bile  ceases,  and  with  a  little  increase  of  opposing 
force  the  fluid  in  the  manometer  retreats  and  finds  its  way  into 
the  blood.  The  low  pressure  which  can  be  reached  in  the  gall 
ducts  does  not  imply  any  want  of  secretory  power  on  the  part  of 
the  liver  cells,  but  merely  that  there  exists  a  great  facility  of 
communication  between  the  duct  radicles  and  the  bloodvessels, 
most  probably  through  the  medium  of  the  lymphatics.  This  is 
made  obvious  by  experiment,  by  which  it  can  be  shown  that  with  a 
comparatively  low  pressure  (200  mm.  =  nearly  8  in.  of  water  for  a 


FUNCTIONS    OF    BILE.  176 

guinea-pig)  any  fluid  can  be  forced  into  the  circulation  from  the 
bile  ducts. 

It  is  observable  also  in  the  stoppage  of  the  bile  ducts  in  the 
human  subject,  when  some  at  least  of  the  bile  constituents  con- 
tinue to  be  formed,  and  pass  into  the  blood,  where  their  presence 
is  demonstrated  by  the  yellow  color  characteristic  of  jaundice. 
The  ready  evacuation  of  the  bile  is  then  a  matter  of  great  import- 
ance for  health,  the  least  check  to  its  free  exit  causing  the  secre- 
tion, or  as  it  might  be  equally  well  called  excretion,  to  be  forced 
into  the  cirulating  blood  instead  of  into  the  gall  passages.  Under 
normal  circumstances,  the  large  receptacle  of  the  gall-bladder 
being  always  ready  to  receive  the  bile  insures  its  easy  exit  from 
the  ducts,  but  the  forces  which  cause  its  flow  are  extremely  weak. 
The  smooth  muscle  in  the  walls  of  the  duct  seems  rather  for  the 
purpose  of  regulating  than  aiding  the  flow. 

When  food  from  the  stomach  begins  to  flow  into  the  duodenum, 
the  muscular  coat  of  the  gall-bladder  contracts  and  sends  a  flow 
of  bile  into  the  intestine,  which  action  is  doubtless  brought  about 
by  a  reflex  nerve  impulse,  for  it  is  only  when  this  part  is  stim- 
ulated that  the  bile  flows  freely  from  the  bladder,  and  the  acid 
gastric  contents  seem  to  be  the  most  eflicacious  stimulus. 

In  the  human  subject  the  quantity  of  bile  secreted  has  been 
found  to  be  about  600  cc.  (21  oz.)  per  diem  in  cases  where  there 
were  biliary  fistulre.  This  would  equal  about  13  grms,  per  kilo 
of  the  body-weight. 

In  the  guinea-pig  and  rabbit  it  has  been  estimated  to  be  about 
150  grms.  per  kilo  body-weight. 

Functions  of  the  Bile. 

1.  As  Excrement. — Although  the  great  bulk  of  the  bile  is  re- 
absorbed from  the  intestinal  tract  into  the  blood,  and  again  used 
in  the  economy,  some  of  its  constituents  pass  off  with  the  fseces, 
and  are  no  doubt  simply  excrementitious  matters  that  must  begot 
rid  of.  Thus  all  the  cholcsterin,  mucus,  and  coloring  matter  are 
normally  eliminated,  and  a  considerable  quantity  of  the  bile  acids 
are  split  up,  the  cholic  acid  being  found  in  the  fseces. 


176  MANUAL  OF   PHYSIOLOGY. 

2.  As  a  stimulant,  the  bile  is  of  considerable  use,  for  it  excites 
the  muscles  of  the  intestine  to  increased  action,  and  thereby  aids 
in  absorption  and  promotes  the  forward  movement  of  the  food, 
and  more  particularly  of  those  insoluble  materials  which  have  to 
be  evacuated  per  anum  :  this  stimulation  may  amount  to  mild 
purging. 

3.  Moistening  and  luhricating. — The  bile  adds  to  the  ingesta 
an  abundant  supply  of  food  and  mucus,  much  of  which  passes 
along  the  intestine  to  moisten  and  lubricate  the  faeces  and  facili- 
tate their  evacuation.  In  cases  of  jaundice,  or  when  the  bile  is 
removed  by  a  fistula,  the  fteces  are  hard  and  friable,  and  with 
difficulty  expelled,  owing  to  the  deficient  fluid  and  mucus,  as  well 
as  to  the  weaker  peristaltic  movements. 

4.  As  an  Antiseptic,  the  bile  is  said  to  have  an  important  func- 
tion to  perform.  Possibly  it  restricts  the  formation  of  certain  of 
the  bye  products,  such  as  the  indol  resulting  from  pancreatic 
digestion,  but  it  is  not  aseptic,  since  bacteria  abound  and  thrive 
in  the  intestine. 

5.  Emuhification  of  fats. — The  bile  has  no  doubt  some  power  of 
forming  an  emulsion,  but  in  a  far  less  degree  than  the  secretion 
of  the  pancreas  ;  however,  the  mixed  secretions  are  probably  more 
efficacious  than  either  separately,  from  the  presence  of  the  free 
fatty  acids  which  form  soaps  and  aid  in  forming  the  emulsion. 

6.  As  an  aid  to  absorption. — The  bile  having  some  soap  in  so- 
lution has  a  close  relationship  to  both  watery  and  oily  fluids,  and 
possibly  on  this  account,  as  well  as  owing  to  a  peculiar  power 
possessed  by  the  bile  salts,  a  membrane  saturated  with  bile  allows 
an  emulsion  of  fat  to  pass  through  it  much  more  readily  than  if 
the  same  membrane  were  kept  moistened  with  water.  This  can 
be  seen  experimentally  with  filter-paper. 

7.  By  neutralizing  acidity  and  p>recipitating  peptones. — When 
the  acid  contents  of  the  stomach  are  poured  into  the  duodenum 
and  meet  with  a  gush  of  alkaline  bile — a  copious  cheesy  precipi- 
tate is  formed  which  clings  to  the  wall  of  the  intestine.  This 
precipitate  consists  partly  of  acid  albumin  (parapeptone)  and 
peptones  thrown  down  by  the  strong  solution  of  bile  salts,  and 
partly  of  bile  acids,  the  salts  of  which  have  been  decomposed  by 


FUNCTIONS   OF    BILE.  177 

the  liydrochloric  acid  of  the  gastric  juice.  With  the  bile  acids 
the  pepsin  is  mechanically  carried  down.  Thus,  immediately  on 
their  entrance  into  the  duodenum  the  peptic  digestion  of  the 
gastric  contents  is  suddenly  stopped  not  only  by  the  precipitation 
of  the  soluble  peptones  and  the  shrinking  of  the  swollen  para- 
peptone,  but  also  by  the  removal  of  the  pepsin  itself  from  the 
fluid  and  the  neutralization  of  the  gastric  fluid  by  the  alkaline 
bile. 

By  thus  checking  the  action  of  the  gastric  ferment  the  bile 
prepares  the  chyme  for  the  action  of  the  pancreatic  juice. 


V) 


CHAPTER  XI. 

FUNCTIONS  OF  THE  INTESTINAL  MUCOUS  MEMBRANE. 

In  the  IMucous  Membrane  of  the  iutestine  arc  found  small 
glands  of  two  distinct  kinds.     The  glands  of  one  kind,  which  are 

Fig.  76. 


Portion  of  the  Wall  of  the  Small  Intestine  laid  open  to  show  the  valvulse 
ooniiiventes.     (Rrinton.) 

commonly  called  Briinner's  gland.s,  and  are  localized  in  the  duo- 
denum, are  insignificant  in   number   when   compared   with   the 

Fig.  77. 


Drawing  of  transverse  section  of  the  duodenum  showing  Briinner's  Glands 
(b)  opening  into  Lieberkiihn's  follicles  (l)  ;  (v)  villi,  (m)  muscular  coats. 

others,  Lieberkiihn's  glands,  which  are  closely  set  and  distributed 
over  the  entire  intestinal  tract  in  enormous  numbers. 


STRUCTURE   OF   THE  SMALL,  INTESTINES. 


179 


Briinuer's  glauds  form,  iu  some  animals,  a  dense  layer  in  the 
submucous  tissue  of  the  beginning  of  the  duodenum  ;  they  are 
small  branched  saccular  glands  resembling  mucous  glands  in 
structure.  Owing  to  their  small  size  the  secretion  cannot  be  ob- 
tained in  sufficient  quantity  to  make  satisfactory  experiments  in 


Fig.  78. 


Section  of  the  Mucous  Membrane  of  small  intestine,  showing  Lieber- 
kiihn's  follicles  (a)  with  their  irregular  epithelium  and  the  villi  (6)  passing 
out  of  view  ;  (c)  Muscularis  mucosse;  {d)  Submucous  tissue.     (Cadiat.) 


respect  to  its  properties.  It  is  said  to  dissolve  albumin  and  to 
have  a  diastatic  fermentative  action,  so  that  probably  the  secre- 
tion is  analogous  to  that  of  the  pancreas,  as  Briinner  originally 
supposed.  The  quantity  of  fluid  secreted  by  these  glands  is  so 
small  that  its  existence  is  not  taken  into  account  in  speaking  of 


180 


irANUAL   OF   PHYSIOLOGY. 


Fig.  79.  tlie   iute.sliual  juice,  by  which 

i.s  meant  the  fluid  poured  out 
by  tlie  innumerable  short  tubes 
or  follicles  of  Lieberkiihu. 

These  intestinal  follicles  be- 
long to  a  very  simple  form  of 
gland,  each  one  being  a  single 
straight  depression  in  the 
mucous  membrane  not  deep 
enough  to  deserve  the  name 
of  a  tube.  In  the  small  intes- 
tine they  are  set  as  closely  as 
the  villi  permit.  In  the  large 
intestine,  where  the  villi  are 
absent,  they  are  more  closely 
set  and  are  also  deeper  (Fig. 
78).  They  are  bounded  by 
a  thin  basement  membrane 
which  is  embraced  by  a  close 
capillary  network  of  blood- 
vessels, and  are  lined  by  a 
single  layer  of  cylindrical  or 
spherical  epithelial  cells. 

The  epithelial  covering  of 
the  processes  known  as  villi, 
which  are  studded  all  over  the 
mucous  membrane  of  the  small 
intestine,  produce  some  mucus. 

Method  of  Obtaining 
Intestinal    Secretion.  — 

Considerable  difficulty  has 
been  found  in  obtaining  the 
proper  intestinal  juice  free 
from  admixture  with  the  secre- 
tions of  the  liver  and  pancreas 

,,.,,        .  ,     ,  .,,...       ,  which  are  carried  along  and 

V  illiis  witli   tlie  caj)illarios  iiijcolea       .       ,      .  ,     . 

,      .      ^1    •     ,  I  .•      *       •»!      mixed  with  it.     A  short  por- 

showing  their  close  reJatinn  to  epitlie-  ^ 

Hum,  some  of  the  cells  of  which  aredis-  tiou  of  the  small  intestine  has, 
tended  with  mucus.     (Cadiat.)  however,  been  successfully  iso- 


FUNCTIONS   OF   THE   INTESTINAL   JUICE.  181 

lated  from  the  rest  without  injuring  the  mesentery  or  its  blood- 
vessels. One  of  the  extremities  of  the  isolated  portion  was  closed, 
and  the  other  was  retained  by  sutures  at  an  opening  in  the  ab- 
dominal wall.  The  cut  ends  of  the  remainder  of  the  intestine 
were  at  the  same  time  united,  so  that  the  continuity  of  the  ali- 
mentary tract  was  preserved.  Thus,  a  limited  piece  of  gut  formed 
a  cul-de-sac  from  which  the  fluid  could  be  collected  through  a 
fistulous  opening. 

Characters  of  the  Secretion. — The  fluid  obtained  from 
such  a  fistula  is  a  thin  opalescent  yellowish  fluid  with  a  strong 
alkaline  reaction  and  a  specific  gravity  of  1011.  It  contains  some 
proteid  and  other  organic  material,  a  ferment  and  inorganic  salts 
in  which  sodium  carbonate  preponderates. 

Mode  of  Secretion. — The  secretion  flows  but  slowly  from 
such  a  fistula,  but  the  amount  increases  during  digestion,  showing 
that  the  secretion  of  the  intestine  is  under  the  control  of  some 
nerve  centre  which  can  call  the  entire  tract  into  action  when  one 
part  is  stimulated.  Moreover,  the  local  stimulation  of  the  mucous 
membrane  makes  it  red,  and  causes  it  to  pour  out  a  more  abundant 
secretion.  Beyond  this  little  is  known  of  the  nervous  mechanism 
or  the  local  cell-changes  which  accompany  the  formation  of  the 
secretion. 

Functions  of  the  Intestinal  Juice. — All  the  properties 
of  the  secretion  of  the  pancreas  have  been  accorded  to  the  intes- 
tinal juice.  It  is  said  to  have  a  ferment,  capable  of  being  ex- 
tracted with  glycerin,  which  can  convert  cane  sugar  and  starch 
into  grape  sugar,  and  bring  about  lactic  fermentation.  It  dis- 
solves fibrin  very  slowly  and  still  less  easily  other  proteids.  It  is 
also  said  to  emulsify  fats.  However,  the  observations  as  to  its  di- 
gestive properties  are  very  discordant,  experiments  giving  opposite 
results  in  different  animals,  and  in  the  hands  of  different  persons 
even  in  the  same  animal.  From  the  foregoing  account  of  the  in- 
testinal secretions  it  may  be  seen  that  the  changes  which  the  va- 
rious kinds  of  food  undergo  on  their  way  through  this  part  of  the 


182  MANUAL   OF   PHYSIOLOGY. 

alimentary  tract  are  numerous ;  a  short  review  may  therefore  be 
useful. 

When  the  acid  gastric  chyme  flows  into  the  duodenum,  a  flow 
of  bile  takes  place  from  the  gall-bladder,  and  at  the  same  time 
the  secretions  of  the  pancreas,  Briinner's  glands,  and  Lieberkiiha's 
follicles  are  poured  copiously  into  the  intestine.  The  bile  meeting 
with  the  turbid  fluid  chyme  causes  it  to  change  to  a  soft  cheesy 
granular  mass,  the  appearance  of  which  depends  chiefly  on  the 
precipitation  and  shrinking  of  the  parapetone  and  peptones.  The 
pepsin  is  rendered  powerless,  both  it  and  the  bile  salts  being  car- 
ried down  with  the  precipitate.  Gastric  digestion  is  thus  arrested 
and  the  onward  flow  of  the  fluid  chyme  checked.  As  the  alkaline 
pancreatic  and  intestinal  juices  meet  this  semi-fluid  cheesy  mass, 
the  conversion  of  starch  into  sugar  proceeds  rapidly,  even  the  raw 
starch  granules  being  thus  changed.  The  small  oil  globules  come 
in  contact  with  the  alkaline  mixture  of  bile  and  pancreatic  juice. 
The  pancreatic  secretion  splits  up  some  of  the  fat  separating  the 
fatty  acid  from  the  glycerin  radicle.  Some  of  the  soda  of  the 
bile  salt  is  substituted  for  the  latter,  and  uniting  with  the  fatty 
acid  forms  a  soap.  In  such  a  mixture  as  this — an  alkaline  fluid 
with  proteid  and  soap  in  solution — a  fine  emulsion  is  readily 
formed,  as  can  be  seen  by  adding  sodium  carbonate  to  some  rancid 
oil.  The  free  acid  (the  cause  of  rancidity  in  the  oil)  unites  with 
some  soda  to  form  a  soap  which  in  the  alkaline  mixture  enables 
the  oil  to  be  converted  into  an  emulsion  by  even  slight  agitation, 
so  that  the  pancreas,  by  setting  free  fatty  acid,  and  the  bile  pos- 
sibly by  contributing  some  soda,  aid  one  another  in  giving  rise  to 
a  definite  but  small  amount  of  soap. 

The  precipitated  parapetone  and  peptone  and  the  finely  divided 
proteid  are  presented  to  the  pancreatic  juice  in  a  form  which  it 
can  most  easily  attack,  and  thus  the  conversion  of  proteid  into 
peptones  goes  on  rapidly. 

How  far  the  peculiar  action  of  trypsin  on  proteids,  converting 
them  further  into  leucin  and  tyrosin,  goes  on  in  normal  digestion 
is  not  known,  but  it  is  probable  that  the  production  of  these  bodies 
is  increased  with  the  over-abundant  ingestion  of  proteid  or  a  purely 


FUNCTIONS   OF   THE   LARGE   INTESTINE.  183 

meat  diet,  aud  is  theu  useful  as  a  means  of  preveutiug  the  inju- 
rious effects  of  too  great  proteid  absorption. 

The  gastric  chyme  is  therefore  completely  changed  in  the  duo- 
denum, aud  ill  the  other  parts  of  the  small  intestines  we  find  in 
its  stead  a  thin  creamy  fluid  which  clings  to  the  mucous  membrane, 
coats  over  its  folds  (valvulte  conniventes)  and  surrounds  the  long 
villi  of  the  jejunum,  etc.  This  intestinal  chyme  is  the  form  in 
which  the  food  is  presented  to  the  mucous  membrane  for  absorp- 
tion. It  resembles  somewhat  by  its  whiteness  the  fluid  called 
chyle  which  flows  in  the  lacteals,  aud  formerly  was  considered  to 
be  identical  with  it.  This  creamy  lining  is  the  chief  material 
found  in  the  upper  part  of  the  small  intestine,  the  coarser  parts 
of  the  food  being  hurried  on  by  peristaltic  action  to  the  large  in- 
testine. 

In  the  large  intestine  the  secietiou  of  the  long  closely-set  Lie- 
berkiihn's  follicles  is  the  only  one  of  importance.  Its  reaction 
aud  that  of  the  mucous  membrane  is  alkaline,  but  the  contents  of 
the  colon  are  acid  owing  to  certain  fermentative  changes  which  go 
on  in  this  part  of  the  intestine. 

Of  the  changes  brought  about  in  the  large  intestine  by  the 
agency  of  the  digestive  juices  we  know  but  little.  Judging  from 
the  large  size  of  the  ciecum  aud  colon  in  herbivorous  animals,  we 
are  prompted  to  conclude  that  vegetable  substances,  possibly  cel- 
lulose, may  be  dissolved  here,  but  we  do  not  know  how  this  is 
accomplished.  Although  devoid  of  villi,  the  large  intestine  can 
certainly  absorb  readily  such  materials  as  are  in  solution.  As 
the  insoluble  materials  pass  along  the  small  intestines  the  supply 
of  fluid  is  kept  up  to  about  the  same  standard,  the  absorption  and  ' 
secretion  being  about  equal;  but  in  the  large  intestine,  the  absorp- 
tion of  the  fluid  so  exceeds  the  secretion  in  amount  that  the  undi- 
gested materials  are  gradually  deprived  of  their  fluid,  and  are 
converted  into  soft  solid  masses  which  pass  on  to  be  added  to  the 
ficces. 

Owing  to  its  absorbent  power  the  large  intestine  is  a  ready  and 
rapid  channel  by  which  materials  can  be  introduced  into  the 
sy.stem  in  cases  in  which  the  stomach  is  too  irritable  to  retain  food. 

The  quantity  of  fteces  evacuated  in  the  day  depends  upon  the 


184  MANUAL   OF   PHYSIOLOGY. 

kind  of  diet,  being  greater  with  a,  vegetable  than  meat  diet, 
averaging  about  150  grammes  a  day  (60-250  grras.).  This 
amount  may  be  greatly  increased  by  largely  partaking  of  indi- 
gestible forms  of  food.  The  more  rapid  the  passage  of  the  ingesta 
through  the  intestine  the  greater  is  the  amount  of  fluid  remaining 
with  the  fteces,  so  that  any  stimulant  to  the  intestinal  movements 
reduces  the  consistence  of  the  faeces  and  facilitates  the  evacuation. 
The  fsetor  depends  in  a  great  measure  on  the  presence  of  indol, 
which  is  an  outcome  of  pancreatic  digestion,  and  also  upon  the 
presence  of  certain  volatile  fatty  acids.  The  color  depends  upon 
the  amount  of  the  bile  pigment  and  the  degree  of  change  the  latter 
has  undergone. 

The  fjBces  are  composed  of  (1)  the  undigested  parts  of  the  food, 
and  (2)  the  useless  or  injurious  parts  of  the  secretions  of  the  various 
glands.  In  the  first  category  we  find  perfectly  indigestible  stuffs 
such  as  yellow  elastic  tissue,  horny  structure,  portions  of  hairs  from 
animal  food,  and  cellulose  woody  fibre  and  spiral  vessels  from 
plants,  and  also  masses  of  digestible  substances  which  have  been 
swallowed  in  too  large  pieces  to  be  throughly  acted  on  by  the  secre- 
tions. All  forms  of  food  may  thus  appear  in  the  feeces,  but  most 
commonly  vegetable  substances  are  conspicuous. 

In  the  second  category  we  find  a  variable  quantity  of  mucus 
and  the  decomposed  coloring  matter  of  the  bile,  together  with 
some  cholic  acid,  cholesterin,  etc. 

A  few  inorganic  substances  are  found,  mainly  those  which  diffuse 
with  diflftculty,  as  calcium  salts  and  ammonio-raagnesium  phos- 
phate. 

Putrefactive    Fermentations   in   the    Intestine. — 

With  the  air  and  saliva  which  are  swallowed  mixed  with  the  food, 
large  numbers  of  the  lower  organisms  existing  in  them  are  intro- 
duced into  the  alimentary  canal. 

The  effect  of  these  organisms  is  to  produce  certain  fermentative 
changes  quite  distinct  from  the  action  of  the  special  ferments 
peculiar  to  the  digestive  fluids. 

This  is  proved  by  the  composition  of  the  gases  found  in  the  in- 
testine.    Atmospheric  air  only  is  introduced  from  without,  and 


PUTREFACTIVE   FERMENTATIONS   IN   THE   INTESTINE.      185 

this  is  not  found  in  any  part  of  the  alimentary  tract,  the  oxygen 
soon  being  absorbed  and  the  nitrogen  left,  while  a  quantity  of 
carbonic  anhydride  and  hydrogen  from  the  fermentation  of  the 
sugar  are  set  free,  lactic  and  butyric  acids  being  produced  at  the 
same  time. 

Indol  and  skatol  are  also  formed  by  putrefactive  fermentation 
of  the  leucin  and  tyrosin,  although  this  is  in  a  great  measure 
held  in  check  by  the  antiseptic  nature  of  the  bile. 

It  is  in  the  large  intestine  that  putrefactive  fermentations  have 
the  greatest  effect,  the  acid  reaction  being  caused  by  the  various 
acids  produced. 

With  regard  to  the  interesting  question, — Why  do  not  the  di- 
gestive fluids  dissolve  the  tissues  of  the  organs  in  which  they  are 
contained,  we  cannot  speak  positively.  Wo  cannot  now  say  that 
the  "vital  principle"  has  a  protective  influence,  for  we  know  the 
fact  that  a  tissue  being  alive  is  not  sufficient  to  ward  oft"  the  diges- 
tive action  of  the  alimentary  juices,  since  the  limb  of  a  living  fnjg 
is  digested  when  introduced  through  a  fistula  into  the  stomach  of 
a  dog;  and  when  the  intestinal  juice  trickles  from  a  fistula  the 
neighboring  skin,  the  snout,  and  the  tongue  of  the  animal  soon 
become  eaten  away  owing  to  its  licking  the  fluid,  which  rapidly 
digests  these  parts  so  as  to  destroy  the  skin  and  even  expose  the 
bloodvessels. 

We  can  however  modify  John  Hunter's  statement  that  the 
resisting  power  was  associated  with  the  life  of  the  structures,  by 
saying  that  it  is  not  the  property  of  an  abstract  "vital  principle" 
but  a  special  resisting  power  dependent  upon  the  specific  character 
of  the  vital  processes  of  certain  textures. 


16 


CHAPTER  XII. 

ABSORPTION. 

Ik  Older  that  the  food-stuffs  when  altered  by  the  various  pro- 
cesses described  under  digestion,  may  be  of  any  real  use  to  the 
economy,  the  nutritive  materials  must  be  distributed  through  the 
textures  and  organs.  For  this  purpose  they  must  pass  through 
t'le  lining  membrane  of  the  alimentary  canal,  and  obtain  ad- 
mission to  the  blood,  which  is  the  common  mode  of  intercom- 
munication between  the  various  parts  of  the  body. 

The  nutrient  part  of  the  food  has  then  to  be  absorbed  out  of 
the  alimentary  canal  by  the  surrounding  tissues,  and  mixed  with 
the  general  circulating  fluid. 

But  the  blood  is  separated  from  the  intestinal  contents  by  a 
barrier,  which  for  it  at  least  is  impassable,  although  it  exerts  con- 
siderable pressure,  and  therefore  tends  to  burst  out  from  the 
vessels. 

The  question  then  arises — How  does  the  elaborated  chyme 
make  its  way  through  this  barrier,  which  is  sufficient  to  prevent 
the  flow  of  blood  into  the  intestinal  tract? 

The  general  answer  is  easily  given,  viz. :  the  blood  cannot  pass 
through  an  animal  membrane.  But  this  is  not  a  satisfactory  so- 
lution of  the  question,  for  sometimes,  under  certain  circumstances, 
the  blood  does  pass  through  the  wall  of  the  vessels,  and  normally 
the  plasma  escapes  from  the  capillaries  into  the  tissues,  in  order 
to  nourish  them.  We  must  further  remember,  in  considering 
this  point,  that  the  wall  of  the  vessels  and  the  membrane  lining 
of  the  intestine  are  both  made  up  of  living  cells  which  are  en- 
dowed with  a  capability,  coincident  with  their  lives,  of  con- 
trolling any  passage  through  or  between  them.  Some  of  these 
cell-guards,  which  we  might  call  secreting  agents,  do  allow,  or 
rather  cause  a  passage  of  fluid  from  the  blood  to  the  intestinal 
cavity,  and,  as  we  shall   presently  see,  others  of  them  induce  a 


ABSORPTION. 


187 


passage  of  the  nutritious  materials  from  the  intestinal  canal  into 

the  surrounding  tissues. 

Fig.  80. 


Diagram  showing  the  Course  of  the  Main  Trunks  of  the  Absorbent  Sys- 
tem. The  lymphatics  of  lower  extremities,  etc.,  meeting  the  lacteals  of  in- 
testines at  the  receptacuhira  chyli  (r.  c),  which  opens  into  the  thoracic 
duct.  The  superficial  vessels  are  shown  in  the  diagram  on  the  left  arm  and 
leg  (s.),  and  the  deeper  ones  on  the  arm  to  the  right  (c).  The  glands  are 
here  and  there  shown  in  groups.  The  small  right  duct  opens  into  the 
veins  on  the  right  side.  The  thoracic  duct  discharges  into  the  union  of 
the  great  veins  of  the  left  side  of  the  neck  (t.). 

In  order  clearly  to  understand  the  method  by  which  absorp- 


188  MANUAL  OF  PHYSIOLOGY. 

tion  18  accomplished,  it  is  necessary  to  have  some  idea  of  the 
absorbent  system  generally ;  it  may  be  well,  therefore,  at  this 
place  to  give  a  brief  account  of  the  construction  of  the  special 
apparatus  which  carries  on  this  function.  Although  the  ab- 
sorbent vessels  form  one  continuous  system,  they  may  be  con- 
veniently divided  into  two  provinces,  namely,  interstitial  and  sur- 
face absorption.  A  certain  modification  of  the  latter  called  the 
lacteal  system  occurs  in  the  alimentary  canal,  and  ia  described 
under  intestinal  absorption. 

I.  Interstitial  Absorption. 
The  blood  flowing  through  the  body  in  delicate  capillary  vessels 
yields  to  the  various  tissues  a  kind  of  irrigation  stream  of  plasma, 
which,  leaving  the  capillaries,  permeates  their  substance  so  that 

Fig.  81. 


Tendon  of  mouse's  tail  treated  with  nitrate  of  silver,  showing  clefts  or 
cell-spaces  around  the  bundles  of  fibrils  as  white  patches.  These  interstices 
may  be  called  the  smallest  lymph-channels  or  spaces.     (SchiifTer.) 

every  texture  is  saturated  with  nutrient  fluid.  The  surplus  of 
this  irrigation  stream  is  collected  and  carried  back  to  the  blood 
current  by  a  special  set  of  fine  vessels  with  slender  walls,  called 
the  lymph  vascular  system,  which  act  as  drains  to  the  tissues,  and 
pour  their  contents  into  the  veins. 

When  the  nutrient  fluid  escapes  from  the  capillaries,  it  lies  in 
the  interstices  in  the  tissue  elements,  and  here  bathes  the  tissue 
cells  which  commonly  occupy  these  interstices.    (Figs.  81  and  86.) 

Communicating  freely  with  the  interstices  of  the  tissues  are  ir- 
regular anastomosing  flattened  channels,  which  convey  the  lymph 
or  any  fluid  forced  between  the  tissues  into  vessels  with  more  defi- 


LYMPHATIC  SYSTEM.  189 

nite  walls.  These  vessels,  which  are  lined  with  characteristic 
endothelium,  form  a  more  or  less  dense  network  of  lymphatic 
capillaries,  from  which  spring  the  tributaries  of  the  lymph  ves- 
sels.    (Figs.  82  and  83.) 

The  lymphatic  vessels  are  throughout  slender  thin-walled  chan- 
nels with  close-set  valves,  usually  in  pairs,  and  with  frequent 
anastomoses.     They  lie  imbedded   in  the  connective  tissue,  and 


Fig 


Lymph  Channels  from  the  thoracic  side  of  the  central  tendon  of  the 
diaphragm  of  the  rabbit,  treated  with  silver  nitrate.  The  fine  lines  indicate 
the  boundaries  of  the  endothelium  cells  lining  the  lymph  channels.  The 
dark  part  shows  the  islets  between  the  lymphatic  network.     (Klein.) 

when  empty  are  difficult  to  see,  owing  to  their  extreme  delicacy. 
They  converge  towards  a  central  vessel  called  the  thoracic  duct, 
which,  passing  from  the  abdominal  cavity,  through  the  thorax, 
reaches  the  left  side  of  the  neck,  and  opens  into  the  angle  of 
junction  of  the  two  great  veins  from  the  head  and  upper  ex- 


190 


MANUAL   OF   PHYSIOLOGY. 


tremity.  (Fig.  80.)  On  the  right  side  a  smaller  trunk  convey- 
ing the  lymph  from  the  right  arm  and  that  side  of  the  head, 
chest,  and  neck,  opens  into  the  corresponding  venous  trunks. 

The  thoracic  duct  is  much  larger  than  any  of  the  numerous 
tributaries  which  enter  it  at  close  intervals  from  all  directions. 

Fig.  83. 


Diagram  of  a  Lymphatic  Gland,  showing  (a  I)  afferent  and  (e  I)  efferent 
lymphatic  vessels;  (c)  Cortical  substance;  (m)  Medullary  substance;  (c) 
Fibrous  coat  sending  trabeculse  (<r)  into  the  substance  of  the  gland,  where 
they  branch,  and  in  the  medullary  part  form  a  reticulum,  the  trabeculse  are 
surrounded  by  the  lymph  path  or  sinus,  which  separates  them  from  the 
adenoid  tissue  {I  h).     (Sharpey.) 


Its  lower  extremity  or  point  of  origin  is  an  irregular  dilata- 
tion called  the  reeeptaculum  chyli,  because  the  lymphatic  vessels 
from  the  stomach  and  intestines,  or  lacteals  as  they  are  called, 
pour  their  contents  into  it.  The  chyle  from  the  intestines  thus 
flows  into  the  same  channel  as  the  lymph  which  is  derived  from 
the  drainage  of  the  tissues  and  organs  of  the  lower  extremity, 


STRUCTURE   OF    LYMPHATIC   GLANDS.  191 

the  trunk  and  left  side  of  the  head,  and  neck,  and  arm  ;  and  the 
two  fluids  are  mixed  in  the  reeeptaculum  chyli,  and  the  other 
parts  of  the  thoracic  duct. 

Along  the  course  of  the  lymphatic  vessels  are  numerous  small 
bodies  called  lymphatic  glands  or  follicles,  which  are  composed 

Fig.  84. 


Lyinpliatic  Network  from  between  the  Muscle  Coats  of  the  Intestinal  Wall, 
with  fine  vessels  and  many  valves,  causing  the  walls  to  bulge.    (Cadiat.) 

of  masses  of  a  delicate  trellis-work  of  adenoid  tissue,  packed  with 
nucleated  protoplasmic  cells,  called  lymph  corpuscles,  the  com- 
bination making  what  is  known  as  lymphoid  tissue.  (Figs.  83,  84, 
and  85.)  These  masses  of  cells  and  their  delicate  supporting 
reticulum  are  inclosed  in  a  fibrous  case  or  capsule,  from  which 
branching  trabeculae  pass  into  the  gland  and  separate  the  masses 


192 


MANUAL   OF    PHYSIOLOGY. 


of  lymphoid  tissue  from  one  auother.  Through  the  convex  side 
of  the  capsule  the  lymph  channels  enter  and  pour  their  contents. 
The  lymph  then  flows  through  irregular  paths,  which  lie  between 
the  lymph  follicles  next  to  the  capsule  and  trabeculaj,  and  lead  to 
the  concavity  of  the  gland  from  which  the  efferent  vessel  escapes. 
These  lymph  glands  occur  in  groups  in  the  flexures  of  the 
limbs,  the  recesses  of  the  neck,  and  the  thoracic  and  abdominal 

Fig.  85. 


Section  through  the  central  or  luedulhiry  part  ol"  a  Lymphatic  Gland, 
showing  adenoid  tissue  (a)  containing  capillaries  (6)  and  a  fibrous  trabecula 
(c)  containing  an  artery.     (Cadiat.) 


cavities,  a  large  number  being  placed  in  the  mesentery,  in  the 
course  of  the  intestinal  lacteals. 

In  the  submucous  tissue  of  the  intestine  there  is  much  of  this 
lymphoid  tissue,  arranged  in  numerous  small  follicles,  which, 
doubtless,  have  a  similar  function  to  the  lymph  glands  found 
elsewhere. 

There  are  various  modes  of  origin  of  the  lymphatic  vessels 
which  are  more  or  less  characteristic  of  the  diflferent  parts  in 
which  they  occur. 


STRUCTURE   OF    I.YMPHATIC    GLANDS. 


193 


In  the  connective  and  allied  tissues  there  are  variously-formed 
fissures  or  splits,  which  can  be  filled  with  fluid  forced  into  the 
tissues  by  puncturing  the  skin  with  the  nozzle  of  a  fine  syringe, 
such  as  is  used  for  hypodermic  injection. 

These  fissures  contain  the  protoplasmic  units  of  the  tissue,  and 
transmit  the  ordinary  transudation  stream  for  nourishing  the  tis- 


CJefts  in  the  Corneal  Tissue  of  a  Frog  treated  with  nitrate  of  silver,  which 
leaves  the  spaces  clear  and  stains  the  intermediate  structure.  These  clefts 
(a)  and  their  processes  (6)  form  the  lymph  canalicular  system,  and  at  the  same 
time  are  the  spaces  in  which  the  corneal  corpuscles  reside.     (Klein.) 


sues.  They  freely  communicate  one  with  another,  and  lead  into 
the  beginnings  of  the  network  of  lymphatic  capillaries. 

The  lymph  capillaries  run  midway  between  the  blood  capilla- 
laries,  and  are  made  up  of  a  single  layer  of  nucleated  endothelial 
cells,  which  can  be  brought  to  light  with  silver  staining. 

In  some  tissues,  such  as  that  of  the  central  nervous  system,  the 
liver  and  bone,  the  lymph  vessels  commence  as  channels  encir- 
cling the  bloodvessels,  or  perivascular  lymph  spaces,  as  they  are 


194 


MANUAL   OF   PHYSIOLOGY. 


called.  Here  the  lymph  channels  form  a  kind  of  sheath  for  the 
minute  bloodvessels,  and  pass  along  to  the  connective  tissue  form- 
iug  the  adventitia. 

Fig.  87. 


Endotheliiiiu  from  serous  surface  wiihoul  siomata  (nitrate  of  silver). 

The  lymph  vessels  may  also  be  said  to  commence  on  the  surface 
of  serous  membranes  which  are  intimately  connected  with  the 
lymphatic  system,  and  may  indeed  be  regarded  as  nothing  more 

Fig.  88. 


Endothelium  from  serous  surface  with  stomata  surrounded  with  granular 
protoplasmic  cells. 


than  exaggerated  lymph  spaces.  In  most  parts  of  the  endothelial 
surface  of  serous  cavities  are  a  number  of  so-called  stomata,  or 
small  apertures  surrounded  by  a  few  cells,  which  differ  from  the 
ordinary  endothelial  cells  in  every  respect,  and  probably  have 


INTESTINAL   ABSORPTION. 


195 


to  control  the  passage  of  the  fluid  from  the  serous  cavity  into 
the  lymph  vessels.  These  stomata  may  be  regarded  as  the  com- 
mencement of  the  dense  network  of  lymph  capillaries,  which 
lie  in  the  subserous  tissue. 


II.  Intestinal  Absorption. 

The  intestinal  absorbents  are  merely  a  special  department  of 
the  lymphatic  system,  which,  on  account  of  the  white  chyle  they 
carry  being  seen  through  their  trans- 
parent walls,  have  been  called  lac-  Fig.  89. 
teals,  their  function  being  to  take  up 
the  nutrient  fluid  from  the  intestinal 
cavity,  as  well  as  to  drain  the  tissue 
in  which  they  lie.     In  order  to  fulfil 
their  function,  they  are  arranged  in 
a  particular  way,  especially  adapted 
to  the  peculiar  construction  of  the 
mucous  membrane  lining  this  part  of 
the  alimentary  tract,  which  must  be 
briefly  described  before  the  mechan- 
ism of  absorption  can  be  understood. 

The  most  striking  characteristic  of 
the  lining  membrane  of  the  small  in- 
testine is  the  existence  of  villi,  which 
are  only  found  in  this  part  of  the  ali- 
mentary tract.  They  consist  of  nip- 
ple-shaped processes  projecting  into 
the  intestinal  cavity,  so  closely  set  that 
they  have  the  appearance  of  the  pile 

of  velvet;  and  being  just  visible  to  the  naked  eye,  they  give  the 
mucous  membrane,  when  washed  and  held  under  water,  a  pecu- 
liar velvety  look.  On  account  of  these  villi,  and  also  of  the 
ring-like  folds  of  mucous  membrane  in  the  upper  part  of  the 
small  intestine,  the  extent  of  surface  over  which  the  chyme  has 
to  travel  is  greatly  increased. 

The  surface  of  the  villi  is  covered  over  with  a  simple  layer  of 
columnar  epithelial  cells  in  continuity  with  the  epithelium  lining 


Diagram  of  relation  of 
the  epithelium  to  the  lac- 
teal radical  in  villus.  The 
protoplasmic  epithelial 
cells  supposed  to  be  con- 
nected to  the  absorbent 
vessel  by  adenoid  tissue. 
(After  Funke.) 


196 


MANUAL   OF    PHYSIOLOGY. 


the  rest  of  the  intestinal  tract.  The  free  surface  of  these  cells  is 
marked  by  a  clear  margin  which  is  composed  of  a  row  of  minute 
rods  closely  packed  together,  while  the  deep-seated  end  of  the 
cells  is  branched,  and  appears  to  be  prolonged  into  the  substance 
of  the  villus  and  in  some  way  to  be  connected  with  the  support- 

FiG.  90, 


Section  of  Intestine  of  a  Dog  in  which  the  bloodvessels  (c)  and  the  lac. 
teals  (d)  have  been  injected.  The  blind  ending  or  simple  loop  of  the  black 
lacteal  is  seen  to  be  surrounded  by  the  capillary  network  of  the  bloodvessels. 
(Cadiat.) 


ing  retiforra  tissue.  Some  of  the  cells  are  seen  to  swell  upon  the 
addition  of  certain  reagents,  owing  to  their  containing  mucus, 
which  gives  them  a  peculiar  goblet  shape ;  hence  they  are  called 
goblet  cells.  These  cells  occur  at  intervals,  and  some  observers 
consider  that  they  form  a  distinct  variety  of  cells,  differing  from 


LYMPH    FOLLICLES   OF   SMALL    INTESTINE. 


197 


the  neighboring  cells  just  as  the  border  cells  of  the  stomach-glands 
differ  from  the  central  cells. 

The  body  of  the  villus  is  composed  of  a  very  delicate  kind  of 
connective  tissue,  forming  a  slender  frame  in  which  a  little  cage- 
like network  of  bloodvessels  surrounds  a  central  lacteal  radicle. 
The  interstices  of  this  connective  tissue  are  filled  with  pale  proto- 


Diagram  of  Section  of  the  Mucous  Membrane  of  the  Intestine,  showing  the 
position  of  tlie  lymph  follicles  (a).     (Cadiat.) 


plasmic  cells,  like  those  formed  in  the  lymph.  Under  the  base- 
ment membrane  forming  the  foundation  of  the  epithelium  are 
some  unstriated  muscle  cells  which  embrace  the  villus  and  are 
able  to  squeeze  it  and  empty  the  vessel  it  contains. 

The  lacteal  radicles  which  lie  in  the  villi  are  sometimes  double, 
and  have  a  communication  with  the  lymph  spaces  of  the  connec- 
tive tissue.  They  frequently  branch  as  they  pass  down  from  the 
villi  to  reach  the  dense  network  of  lacteal  vessels  which  lies  be- 
neath the  mucous  membrane.  At  irregular  intervals  throughout 
the  submucous  tissue  are  found  masses  of  lymphoid  tissue  similar 


198 


MANUAL   OF  PHYSIOLOGY. 


to  that  seen  in  packets  within  a  lymph  gland  or  in  other  lymph- 
follicles.  These  are  either  isolated  (solitary  glands)  or  collected 
into  groups  (agminated  or  Peyer's  glands;.  Though  called  glands 
by  anaton)ists,  it  should  be  borne  in  mind  that  they  are  in  no  way 

Fio.  92. 


r*\V»  'iy\j'y    0^L '*    .  .••W'"'  "•.'■V   " - 


Section  of  Single  Lj'niph-follicle  of  the  Small  Intestine,  showing  (a)  fol- 
licle covered  with  epithelium  (6),  which  has  fallen  from  the  villi  (c) ;  {d) 
Lieberkiihn's  follicles  ;  (c)  Muscularis  mucosae.     (Cadiat.) 

connected  with  the  secretion  of  any  of  the  intestinal  juices,  but 
belong  to  the  absorbing  arrangements  of  the  intestine.  Around 
these  solitary  and  grouped  lymph-follicles  are  spaces  and  net- 
works from  which  the  lacteal  vessels  arise  (Fig.  93). 


Mechanism  of  Absorption. 

Formerly  absorption  was  supposed  to  take  place  by  means  of 
the  bloodvessels  alone,  but  after  the  discovery  of  lymph  and  chyle 
vessels  by  Caspar  Asellius  the  belief  in  the  direct  absorption  by 
the  bloodvessels  was  completely  abandoned,  and  all  the  work  of 


METHODS   OF   ABSORPTION. 


199 


absorption  was  attributed  to  the  lymphatics.  However,  ample 
evidence  exists  to  show  that  substances  capable  of  absorption  can 
make  their  way  into  the  bloodvessels  of  any  part  not  protected 
by  an  impermeable  covering  like  the  horny  layer  of  the  skin, 


Fig.  93. 


Section  through  the  Intestinal  Wall  in  the  neighborhood  of  the  grouped 
lymph-follicles  (/)  (Peyer's  patch),  showing  the  upper  narrow  (b)  and  the 
deep  wide  (c)  lymphatic  plexuses. 


and  thus  be  carried  directly  to  the  general  circulation.  The  share 
taken  by  the  bloodvessels  in  interstitial  absorption  in  the  tissues 
is  not  defined,  and  when  no  impediment  to  the  lymph-flow  exists 
is  probably  very  small. 

In  the  absorption  from  the  alimentary  tract,  however,  the 
bloodvessels  appear  to  take  a  considerable  part. 

JIow  far  the  tissue  interspaces  and  the  local  lymph-channels, 


200  MANUAL  OF   PHYSIOLOGY. 

many  of  which  surround  the  bloodvessels,  aid  in  the  passage  of 
substances  into  the  blood-currents,  is  not  known  ;  but  most  likely 
they  have  some  such  effect,  for  the  experiments  showing  direct 
absorption  by  the  bloodvessels  leave  the  local  lymph-channels  in 
operation,  while  at  the  same  time  the  normal  flow  of  lymph 
towards  the  thoracic  duct  is  more  or  less  prevented. 

Any  part  which  has  only  the  protection  of  a  single  layer  of 
epithelial  cells  well  supplied  with  superficial  bloodvessels  has  also 
a  supply  of  absorbent  vessels,  and  therefore  is  capable  of  absorb- 
ing materials  in  solution  which  are  in  contact  with  the  surface, 
and  large  quantities  of  fluids  and  solutions  of  various  materials 
are  absorbed  from  the  stomach  and  large  intestine — partly,  no 
doubt,  by  means  of  the  lacteals  or  lymphatics,  and  partly  by  the 
minute  bloodvessels  themselves. 

However,  the  small  intestine  seems  to  be  the  part  of  the  ali- 
mentary tract  which  is  especially  adapted  for  taking  up  the  ma- 
terials elaborated  from  the  food. 

In  the  upper  part  of  the  small  intestine  the  valvulse  conni 
ventes  are  most  marked,  and  the  villi  are  long  and  closely  set 
together.  It  is  here  we  find  the  thickest  layer  of  creamy  chyme 
covering  the  mucous  membrane,  but  seldom  any  masses  of  par- 
tially digested  food.  All  these  points,  which  are  directly  related 
to  absorption,  tend  to  show  that  the  upper  part  of  the  intestine  is 
the  part  specially  adapted  for  this  purpose.  The  chyme  which 
clings  to  the  mucous  membrane  contains  all  the  substances  which 
are  destined  to  pass  into  the  economy.  Into  this  mixture  the 
villi  dip,  so  that  each  villus  is  completely  bathed  with  chyme. 
From  what  has  been  said  of  the  construction  of  the  villi  it  is  ob- 
vious that  such  an  arrangement  is  admirably  well  adapted  to  the 
absorption  of  the  nutrient  material,  which  has  every  facility  for 
making  its  way  into  the  lacteals  and  bloodvessels. 

The  principal  ingredients  in  the  chyme  may  now  be  examined 
in  detail  with  reference  to  their  powers  of  being  absorbed. 

Water  can  be  absorbed  from  the  intestinal  tract  in  almost  un- 
limited quantity,  but  not  so  with  solutions  of  salts.  The  amount 
of  the  solution  of  any  salt  capable  of  absorption  seems  to  depend 
on  its  endosmotic  equivalent.     The  lower  the  endosmotic  equiva- 


ABSORPTION   OF   SPECIAL    MATERIALS.  201 

lent  the  more  readily  the  solution  passes  into  the  bloodvessels. 
In  those  cases  where  the  equivalent  is  very  high,  such  as  mag- 
nesium sulphate,  there  is  a  tendency  of  the  fluid  to  pass  out  from 
the  bloodvessels  into  the  intestinal  cavity  ;  hence  the  watery  stools 
caused  by  this  and  such-like  saline  purgatives. 

Among  the  carbohydrates  we  need  only  take  into  account 
the  sugars,  for  starch  unchanged  is  but  little,  if  at  all,  absorbed. 
Ouly  a  certain  quantity  of  sugar  can  be  taken  up  by  the  intesti- 
nal absorbents,  since  some  is  found  in  the  fieces  when  the  amount 
taken  with  the  food  exceeds  a  certain  quantity.  Some  of  the 
sugar  in  the  intestine,  moreover,  undergoes  fermentation,  by  which 
it  is  converted  into  lactic  and  butyric  acid.  How  much  of  the 
sugar  is  absorbed  as  lactic  and  butyric  acid  has  not  been  deter- 
mined, but  the  amount  of  sugar  found  in  the  portal  vessels  or 
lacteals  does  not  at  all  correspond  with  the  amount  that  disap- 
pears from  the  cavity  of  the  intestine. 

Ordinary  proteids,  being  colloids,  can  only  pass  slowly  through 
an  animal  membrane,  hence  they  are  said  to  be  changed  into  pep- 
tones under  ordinary  circumstances  before  they  are  absorbed. 
Their  absorption  takes  place  chiefly  in  the  stomach,  and  is  com- 
pleted in  the  small  intestine,  as  only  a  small  quantity  of  albu- 
minous substances  is  found  in  the  large  intestine  even  after  an 
excessive  meat  diet.  The  more  concentrated  the  solutions  of  pep- 
tones are  the  more  rapidly  are  they  absorbed,  and  the  rate  of 
absorption  is  greatest  at  first  and  then  by  degrees  diminishes. 
The  presence  of  alkali  is  also  said  to  facilitate  the  absorption  of 
peptones.  It  is  a  curious  fact  that  neither  in  the  lacteals  nor  in 
the  portal  blood  can  any  quantity  of  peptone  be  found,  even  dur- 
ing active  proteid  digestion  ;  so  that  it  is  impossible  to  trace  out 
their  course  as  peptones,  or  to  say  by  which  set  of  channels  they 
reach  the  blood.  If  we  assume  that  all  proteids  must  be  absorbed 
as  diff'usible  peptone,  we  are  forced  to  conclude  that  during  their 
passage  from  the  intestinal  cavity  they  must  be  reconverted  into 
ordinary  proteids.  But  we  know  that  soluble  forms  of  albumin 
are  to  some  extent  diff'usible  (when  a  solution  of  salt  is  used) 
through  a  dead  animal  membrane.    But  even  were  this  quite  im- 

17 


202  MANUAL   OF   PHYSIOLOGY. 

possible  it  would  not  preclude  the  possibility  of  their  passing 
through  the  intestinal  wall,  which  presents  no  such  obstacle,  for  it 
is  a  living  structure  that  can  overcome  such  physical  difficulties 
as  the  non-diffusion  of  colloids.  When  we  know  that  solid  par- 
ticles of  fat  can  enter  the  lacteals  we  can  have  no  difficulty  in  be- 
lieving that  a  solution  of  albumin  is  admitted.  We  may  then 
conclude  that  it  is  not  only  possible,  but  even  probable,  that  a 
good  deal  of  proteid  is  absorbed  as  ordinary  soluble  albumin.  A 
certain  limit  to  proteid  absorption  exists,  so  that  any  albuminous 
materials  above  the  maximum  that  escape  conversion  into  leucin 
and  tyro«in  are  thrown  off  with  the  fieces. 

In  the  absorption  of  water,  watery  solutions  of  salts,  sugars, 
and  peptones,  there  are  no  physical  difficulties  to  be  got  over ; 
so  that  we  are  in  the  habit  of  sj)eakiug  confidently  about  the 
mechanism  of  their  absorption,  although  in  all  probability  many 
circumstances  of  which  we  are  ignorant  co-operate  in  bringing 
about  the  results  which  seem  to  us  so  simple. 

It  is  not  the  same,  however,  with  the  fatty  food-stuffs.  A  small 
quantity  of  these  may  no  doubt  be  split  up  into  soluble  glycerin 
and  fiitty  acids,  which  are  at  once  changed  into  soluble  soaps, 
and  in  this  condition  are  capable  of  simple  osmotic  transmission 
into  the  bloodvessels  or  lacteals.  Plowever,  the  greater  portion 
of  the  fat  enters  the  lacteals  as  such  in  a  condition  of  a  fine  emul- 
sion, i.e.,  composed  of  solid  particles.  This  process  is  difficult  to 
reconcile  Avith  our  physical  experiences ;  for,  however  finely 
divi(i  i  it  may  be,  fat  emulsified  does  not  pass  through  an  animal 
membrane  more  freely  than  ordinary  fluid  fat.  The  fat  emulsion 
is  chiefly  taken  up  by  the  villi  of  the  small  intestines,  as  in  the 
ston)ach  it  exists  only  in  large  fluid  masses  or  globules,  and  the 
amount  of  fat  found  in  the  large  intestine  is  small,  unless  used  as 
food  in  great  excess.  This  can  also  be  seen  in  examining  the  ab- 
sorbent vessels  after  a  fatty  meal  when  those  which  carry  mate- 
rials from  the  stomach  and  large  intestine  are  clear  and  trans- 
parent, while  those  coming  from  the  small  intestines  are  filled 
with  the  white  milky  fluid  which  gives  them  their  special  name 
of  lacteals.  There  is  a  limit  to  the  absorbent  capacity  of  the 
intestine  for  fatty  matters,  for  when  a  great  excess  of  fat  is  eaten 


ABSORPTION   OF  SPECIAL   MATERIALS.  203 

it  appears  with  the  excrement,  sometimes  giving  rise  to  adipose 
diarrhoea,  thus  showing  that  the  amount  has  exceeded  this  limit. 
The  important  question  remains,  how  does  the  fat  emulsion 
make  its  way  through  the  intestinal  mucous  membrane  ?  That 
it  really  does  so  there  can  be  no  shadow  of  doubt ;  for  it  disap- 
pears from  the  intestinal  cavity,  and  can  be  detected  in  the  chyle 
with  the  aid  of  the  microscope  more  easily  than  any  other  of  the 
intestinal  contents  that  are  absorbed. 

It  has  been  shown  that  while  a  membrane  moistened  with  water 
acts  as  a  complete  barrier  to  a  fat  emulsion,  and  only  after  pro- 
longed exposure  under  high  pressure  allows  traces  of  fats  to  pass, 
the  same  membrane  when  saturated  with  bile  will  without  pres- 
sure permit  the  passage  of  a  considerable  amount  of  oil.  It  has 
therefore  been  suggested  that  the  epithelial  cells  of  the  mucous 
membrane  are  more  or  less  moistened  with  bile,  and  the  particles 
of  fat  in  the  emulsion  are  also  coated  with  a  film  of  bile  or  soap. 
Thus  they  are  enabled  to  pass  into  the  epithelial  cells,  in  which 
they  can  be  detected  during  digestion.  The  bile  or  soapy  coating 
of  the  fat  particles  may  no  doubt  aid  in  their  transit  through  the 
various  obstacles  on  their  way  to  the  lacteal  radicles.  But  the 
course  taken  by  the  fat  particles  can  hardly  be  explained  in  this 
way,  and  many  circumstances  force  us  to  the  belief  that  the  ac- 
tivity of  the  protoplasm  of  the  epithelial  or  of  some  special  wan- 
dering cells  is  the  real  factor  in  the  case.  When  the  fat  is  once 
scattered  through  the  protoplasm  of  the  cells  and  their  prolonga- 
tion into  the  delicate  connective  tissue  of  the  villi,  then  in  all 
probability  other  forces,  such  as  the  contraction  of  the  villi,  may 
aid  in  their  further  movement  to  the  central  lacteal  space  of  the 
villus. 

The  exact  utility  of  the  marginal  bands  of  rods  or  pores  which 
characterize  the  surface  of  the  intestinal  epithelium  is  not  known, 
though  it  is  supposed  to  be  connected  with  the  absorption  of  fats. 

We  may  conclude  then  that  the  passage  through  the  intestinal 
wall  of  some  of  the  materials  taken  as  food  may  possibly  be  ac- 
complished by  mere  physical  processes,  but  it  is  probable  that  the 
vital  activity  of  the  epithelial  cells  modifies  or  controls  their 
absorption.    The  passage  of  the  fat  can  only  be  explained  by  the 


204  MANUAL   OF   PHYSIOLOGY. 

aid  of  the  direct  activity  of  cells  which  by  araoiboid  movement 
take  up  the  fine  particles  and  pass  them  on  to  the  interstices  of 
the  connective  tissue  of  the  villi. 

Lymph  and  Chyle. 

As  these  two  fluids  are  mixed  together  in  the  thoracic  duct, 
whence  the  lymph  is  commonly  obtained  for  examination,  they 
had  better  be  considered  at  the  same  time. 

As  we  should  expect,  the  fluids  coming  from  the  tissue  drain- 
age, from  the  lymphatic  glands,  and  from  the  lacteals  of  the 
alimentary  tract  form  an  opaque  mixture  which  holds  a  consider- 
able quantity  of  proteid  in  solution,  and  contains  a  number  of 
morphological  elements,  viz.:  (1),  protoplasmic  cells  similar  to 
those  found  in  the  lymph  follicles,  and  in  most  essential  points 
identical  with  the  pale  cells  found  in  the  blood  ;  (2),  some  red 
blood  corpuscles  which  give  the  fluid  in  the  thoracic  duct  a 
pinkish  color ;  (8),  a  quantity  of  very  finely  divided  fat,  which 
varies  in  proportion  to  the  amount  of  fat  recently  digested  ;  (4), 
other  minute  particles  of  unknown  function  and  origin. 

When  removed  from  the  body  and  allowed  to  stand,  the  lymph 
becomes  converted  into  a  soft  jelly.  This  coagulation  no  doubt 
depends  upon  chemical  changes  in  the  lymph  which  give  rise  to 
fibrin.  This  subject  will  be  discussed  more  fully  in  a  future 
chapter.  The  amount  of  fibrin  formed  in  the  lymph  is  very 
small,  and  therefore  the  clot  is  very  soft  and  shrinks  considerably. 
The  quantity  of  chyle  which  can  be  obtained  from  the  lacteals  is 
also  small,  and  therefore  the  thorough  investigation  of  it  is  diflft- 
cult.  The  fluid  from  the  tissues  diflfers  from  the  mixed  lymph  in 
appearance  and  constitution  only  during  digestion,  and  then 
chiefly  in  containing  a  greater  amount  of  fat  and  solids  derived 
from  the  intestinal  cavity.  The  lymph  of  the  thoracic  duct  con- 
tains three  forms  of  proteid  :  (1),  serum  albumin,  which  can  be 
coagulated  by  heat ;  (2),  alkali  albumin  precipitated  by  neutral- 
ization ;  and  (3),  globulin.  It  also  contains  in  solution  soap, 
cholesterin,  grape-sugar,  urea,  leucin,  and  some  salts,  particu- 
larly sodium  chloride,  and  the  sulphates  and  phosphates  of  the 
alkalies. 


CHARACTERS   OF    LYMPH    AND    CHYLE.  205 

On  their  way  to  enter  into  the  blood-curreut  both  the  lymph 
and  chyle  undergo  certain  changes.  Before  passing  through  the 
lymphatic  glands  the  fluid  contains  much  fewer  lymph  corpuscles 
than  after  it  has  traversed  the  glands  :  from  this  fact  and  from 
the  structure  of  the  lymph  glands  we  may  conclude  that  they  are 
the  chief  sources  of  these  white  cells.  The  chyle  of  the  lacteal 
vessel  of  the  mesentery  contains  particles  of  fat  which  greatly  ex- 
ceed in  size  those  found  in  the  thoracic  duct,  so  we  may  infer  that 
the  fat  emulsion  undergoes  a  further  subdivision  or  modification 
on  its  way  through  the  glands. 

The  lymph  which  has  been  collected  from  the  lymph  channels 
of  the  extremities  has  been  found  to  contain  less  albumin  than 
that  coming  from  the  main  trunk,  and  after  long  fasting  the 
lymph  from  the  thoracic  duct  has  the  same  characters. 

The  lymph  contains  a  considerable  quantity  of  carbonic  acid 
gas,  about  50  vol.  per  cent.,  some  of  which  is  readily  removed 
by  the  air-pump,  and  is  therefore  said  to  be  merely  absorbed  by 
the  fluid,  and  some  of  which  can  only  be  removed  by  the  addi- 
tion of  acids,  and  is  therefore  considered  to  be  in  chemical  com- 
bination. Only  mere  traces  of  oxygen  have  been  found  in  the 
lymph. 

The  quantity  of  chyle  and  lymph  poured  into  the  blood  varies 
so  much  that  any  estimation  of  the  amount  entering  in  a  given 
time  is  unreliable. 

The  following  circumstances  upon  which  the  variations  may 
depend  are  instructive : 

1.  The  ingestion  of  liquid  and  solid  food  causes  a  great  in- 

crease in  the  amount  of  chyle.  This  is  obvious  from 
the  change  in  the  state  of  the  lacteal  vessels,  which 
from  being  transparent  and  almost  empty  become 
widely  distended  and  white. 

2.  The  activity  of  any  organ  causes  an  increase  of  lymph  to 

flow  from  it. 

3.  Impediment  to  the  return  of  the  venous  blood  irom  any 

part  increases  the  irrigation,  i.e.,  the  lymph. 


206  MANUAL  OF   PHYSIOLOGY. 

4.  Increase  of  the  amount  or  the  pressure  of  the  blood  flow- 

ing through  any  part  augments  the  lymph  flow. 

5.  Theadministration  of  curare  increases  thearaouutof  lymph. 

The  history  of  the  structural  elements  or  lymph  corpuscles, 
which  exist  in  such  numbers  in  the  large  lymph  channels,  re- 
quires some  further  discussion,  as  these  cells  are  composed  of 
active  protoplasm  and  therefore  must  be  destined  for  some  im- 
portant function,  and  are  produced  by  some  vital  process. 

The  origin  of  the  lymph  corpuscle  is  not  restricted  to  any  one 
part  of  the  body  or  to  any  special  organ.  It  has  been  said  already 
that  the  lymphatic  glands  are  supposed  to  be  the  most  important 
source  of  these  cells,  because  the  follicular  tissue  is  filled  with 
them,  and  the  lymph  contains  a  much  larger  number  after  it  has 
passed  through  some  lymph  glands.  In  the  lymphoid  tissue  of 
the  spleen  they  are  also  very  numerous,  and  no  doubt  many  of 
them  have  their  origin  in  that  organ  as  well  as  in  the  intestinal 
follicular  tissue  and  in  the  red  marrow  of  the  bones.  Although 
their  number  is  relatively  small,  lymphatic  cells  occur  in  the 
lymph  channels  that  are  unconnected  with  a  lymphatic  gland, 
and  these  cells  no  doubt  come  from  the  blood,  which,  as  we  shall 
see,  contains  many  cell  elements,  which  are  actually  the  lymph  cells 
poured  into  it  from  the  lymphatic  duct.  These  cells,  when  they 
arrive  at  the  minute  bloodvessels,  sometimes  leave  the  vessels  and 
creep  by  amoeboid  movements  into  the  interstices  of  the  tissue,  along 
with  the  irrigation  stream.  They  may  permanently  abide  in  the  tis- 
sue, or  they  may  be  washed  back  into  the  larger  lymph  channels 
with  the  stream  of  surplus  lymph.  When  the  abnormal  increase 
of  activity  of  a  tissue  known  as  inflammation,  occurs,  this  escape  of 
the  white  cells  from  the  blood  takes  place  with  great  rapidity,  and 
the  stages  in  the  process  can  be  watched  under  the  microscope. 

Still  another  source  of  the  lymph  cells  may  be  from  prolifera- 
tion of  the  cells  which  lie  in  the  tissues.  The  fixed  tissue-cells 
are  said  to  be  capable  of  producing  cells  identical  with  lymph 
cells,  and,  by  division,  possibly  multiply  and  produce  their  like, 
which  may  be  carried  along  by  the  lymph  stream  as  lymph  cells. 

The  enormous  number  of  cells  which  accumulate  as  pus  when 
an  abscess  forms,  are  structurally  identical  with  lymph  cells,  and 


MOVEMENT  OF  THE  LYMPH.  207 

probably  arise  from  these  combined  sources,  viz.,  escape  from  the 
bloodvessels  and  proliferation  of  the  tissue  cells. 

The  lymph  cells,  therefore,  whether  they  have  their  origin  in 
a  lymph  gland,  the  spleen,  or  in  connective  tissue,  perform  a  kind 
of  circuit,  going  with  the  lymph  into  the  blood,  and  are  distrib- 
uted with  the  latter  to  the  tissues,  whence  they  may  be  once 
more  carried  into  the  lymph  stream. 

Movement  of  the  Lymph. 

In  some  of  the  lower  animals  small  muscular  sacs  occur  in  the 
course  of  the  main  lymph  channels  which  pump  the  lymph  into 
the  great  veins  by  contracting  rhythmically,  much  in  the  same 
way  as  the  heart. 

In  man  and  the  higher  animals,  no  such  lymph  hearts  have 
been  found  ;  the  onward  movement  of  the  fluid  depends  chiefly 
on  the  pressure  with  which  the  irrigation  stream  leaves  the  blood- 
vessels. The  fluid  in  the  bloodvessels,  as  we  shall  presently  see, 
is  under  considerable  pressure,  which  causes  the  plasma  to  leave 
the  capillaries.  Hence,  if  a  lymphatic  trunk  be  tied,  there  is 
intense  filling  of  all  its  tributaries  until  a  great  pressure  (8-10 
mm.,  soda  solution)  is  developed  in  the  vessels. 

While  the  pressure  exerted  on  the  small  tributaries  of  the 
lymph  channels  is  considerable,  that  in  the  thoracic  duct  is  ex- 
tremely small,  for  the  following  reasons :  The  blood  in  the  large 
veins,  into  which  the  duct  opens,  is  under  less  pressure  than  in 
any  other  part  of  the  vascular  system,  owing  to  the  thoracic  suc- 
tion, or  absence  of  pressure  in  the  thorax,  caused  by  the  elastic 
traction  of  the  lungs.  In  fact,  the  pressure  in  the  large  veins, 
e.g.,  brachial,  etc.,  varies  from  0  to  — 4  mm.  Hg,  and  that  in  the 
venae  cavje  is  always  negative,  except  in  forced  expiration,  and 
varies,  according  to  the  period  of  the  respiratory  rhythm,  from 
— 5  mm.,  in  inspiration,  to  — 2  mm.  in  expiration. 

The  fact  that  the  lymph  at  the  origin  of  the  small  channels  is 
at  a  pressure  of  8  to  10  ram.  of  water,  whilst  at  the  entrance  to 
the  vein  it  is  nil,  would  be  sufficient  to  explain  the  movement, 
even  if  there  were  no  other  force  aiding  its  movement. 

It  must  be  remembered  that  every  lymph  vessel  is  furnished 


208  MANUAL   OF   PHYSIOLOGY. 

with  closely  set  valves,  which  prevent  the  fluid  it  contains  from 

P^iG.  94. 


Diagram  showing  the  Course  of  the  Main  Trunks  of  the  Absorbent  Sys- 
tem; The  lymphatics  of  lower  extremities,  etc.,  meeting  the  lacteals  of  in- 
testines at  the  receptaculum  ohyli  (r.  c),  which  opens  into  the  thoracic  duct. 
The  superficial  vessels  are  shown  in  the  diagram  on  the  left  arm  and  leg 
(s.),  and  the  deeper  ones  on  the  arm  to  the  right  (d.).  The  glands  are  here 
and  there  shown  in  groups.  The  small  right  duct  opens  into  the  veins  on 
the  right  side.  The  thoracic  duct  discharges  into  the  union  of  the  great 
veins  of  the  left  side  of  the  neck  (t.). 

being  forced  backwards,  so  that  any  accidental  local  pressure  exer- 
cised on  the  exterior  of  a  lymph  channel  helps  the  fluid  onwards 


MOVEMENT  OF  THE  LYMPH.  209 

to  the  veins.  Along  their  entire  extent  these  vessels  are  subject 
to  certain  forces  which  must  materially  aid  the  flow  of  the  lymph 
stream.  The  first  of  these  is  the  pressure  exerted  on  the  small 
vessels  by  the  movement  of  the  muscles  in  the  neighborhood. 
The  second  is  the  unequal  distribution  of  atmospheric  pressure, 
which  has  full  force  on  the  peripheral  channels,  but  is  kept  off 
the  thoracic  duct  and  its  termination,  as  already  mentioned,  by 
the  rigidity  of  the  thoracic  wall,  which,  together  with  the  tendency 
of  the  elastic  lungs  to  shrink,  causes  a  permanent  negative  pressure 
in  the  thoracic  cavity  through  which  the  duct  passes.  And  lastly, 
the  thin-walled  lymphatics  are  everywhere  surrounded  with  very 
elastic  textures,  inclosed  in  an  elastic  skin,  which  exert  an  amount 
of  pressure,  suflScient  to  empty  and  press  together  the  walls  of  the 
vessels  after  death,  and  therefore  during  life  must  have  consider- 
able influence  upon  the  fluid  they  contain. 

The  movements  of  the  chyle  depend  on  the  same  forces,  with 
the  addition  of  the  power  used  in  the  contraction  of  the  villi, 
which  pump  the  chyle  from  the  lacteal  radicles  into  the  network 
of  valved  vessels  in  the  submucous  tissue. 

The  commencement  of  the  thoracic  duct  is,  moreover,  placed  in 
the  abdominal  cavity,  and  therefore  is  constantly  under  the  influ- 
ence of  the  positive  pressure  exerted  by  the  abdominal  wall  on 
the  contained  viscera.  The  rest  of  the  duct  is  in  the  thorax,  where 
the  pressure  is  habitually  negative,  but  where  certain  variations 
coincident  with  inspiration  and  expiration  take  place,  which  must 
aid  the  onward  flow  of  fluid  in  a  vessel  containing  valves  so  closely 
set. 


18 


CHAPTER  XIII. 

THE  CONSTITUTION  OF  THE  BLOOD  AND   BLOOD  PLASMA. 

In  all  animals,  except  those  which  form  the  lowest  class  (Pro- 
tozoa), the  distribution  of  the  nutritious  materials  to  the  various 
parts  of  the  body,  as  well  as  the  collection  of  the  effete  matters 
prior  to  their  expulsion,  is  carried  on  by  the  medium  of  a  fluid 
which  circulates  through  the  different  parts  of  the  body.  This 
fluid  is  the  blood. 

In  vertebrate  animals  the  blood  passes  through  a  closed  system 
of  elastic  pipes,  and  it  is  kept  in  constant  motion  by  the  action  of 
a  muscular  pump.  It  is  first  forced  through  strong- walled  branch- 
ing canals  called  arteries,  whose  walls  gradually  become  thinner 
as  the  branches  get  smaller,  and  these  end  in  a  network  of  delicate 
channels  (capillaries),  through  which  it  slowly  trickles  into  the 
wide  soft-walled  veins  by  means  of  which  it  flows  gently  back 
again  to  the  heart.  In  its  course  it  receives  the  nutritive  materials 
from  the  stomach  and  intestines  after  digestion,  the  specially  elab. 
orated  substances  from  the  liver,  spleen,  and  lymph  glands,  and 
the  oxygen  absorbed  from  the  air  in  the  lungs.  In  short,  it  con- 
tains and  bears  to  their  destination  all  the  materials  required  for 
the  chemical  changes  of  the  various  tissues.  While  passing 
through  the  capillary  networks  of  the  various  tissues,  it  takes  up 
the  waste  materials  resulting  from  the  tissue  changes  and  bears 
them  to  their  proper  point  of  exit  from  the  body  ;  at  the  same  time 
the  nutriment  is  allowed  to  ooze  through  the  delicate  vessel  walls 
and  be  diffused  in  the  tissues. 

General  Characteristics  of  the  Blood. 

The  blood  of  vertebrate  animals  is  of  a  bright  scarlet  color 
when  exposed  to  the  oxygen  of  the  air,  but  when  not  in  contact 
with  oxygen  it  is  a  dark  purplish  red. 

The  blood  is  remarkably  opaque,  as  may  be  seen  by  placing  a 


AMOUNT   OF    BLOOD.  211 

thin  layer  on  a  piece  of  glass  over  the  page  of  a  book.  This 
opacity  depends  on  the  fact  that  the  blood,  as  will  presently  be 
seen,  is  not  a  red  fiuicl,  but  owes  its  color  to  the  presence  of  solid 
red  particles  or  corpuscles  which  float  in  a  clear  pale  fluid.  The 
blood  has  a  peculiar  smell  (halitus)  distinct  in  different  animals 
and  man,  dependent  on  certain  volatile  fatty  acids.  Its  specific 
gravity  varies  from  1045  to  1075,  the  average  beiug  1055.  The 
solid  parts  (corpuscles)  are  heavier  (sp.  gr.  1105)  than  the  liquor 
sanguinis  (1027). 

When  first  shed  the  blood  has  a  slippery  feel,  which  it  soon 
loses,  becoming  sticky  as  it  passes  through  the  various  stages  of 
the  process  of  coagulation. 

Amount  of  Blood  in  the  Body. 

The  total  amount  of  blood  has  been  estimated  to  be  from  Jg  to  j\ 
of  the  body-weight  for  an  adult  man,  and  somewhat  less  for  a  new- 
born child. 

Much  difficulty  has  been  found  in  arriving  at  an  accurate  esti- 
mation of  the  amount  of  blood  in  the  body.  In  the  first  place,  all 
the  blood  cannot  be  made  to  flow  out  of  the  vessels  of  an  animal 
when  it  is  killed.  Secondly,  the  quantity  and  quality  of  blood 
are  constantly  varying  with  the  capacity  of  the  bloodvessels. 
Thirdly,  when  slowly  withdrawn  from  the  body  during  life  it  is 
rapidly  replaced  by  more  fluid  passing  into  the  bloodvessels. 
This  explains  the  enormous  quantity  of  blood  occasionally  reported 
to  be  shed  in  cases  of  bleeding  to  death.  In  these  cases,  as  quickly 
as  the  blood  is  lost,  fluid  is  absorbed  by  the  fine  vessels  to  replace 
it,  so  that  if  the  bleeding  be  gradual  the  standard  quantity  is  still 
kept  up  in  the  vessels.  Thus  the  very  sudden  loss  of  a  compara- 
tively small  quantity  of  blood  may  cause  death,  whereas,  if  the 
bleeding  go  on  sufficiently  slowly  and  gradually,  as  much  or  even 
more  in  quantity  than  normally  exists  in  the  entire  body  may  es- 
cape without  fatal  result,  but  of  course  much  of  this  is  fluid  which 
has  recently  entered  the  vessels  to  replace  the  blood  already  lost. 
Weber's  Method. — The  percentage  of  solid  matters  in  the  blood 
is  first  carefully  estimated.  The  absolute  quantity  of  solids  in 
the  drawn  blood  is  then  ascertained  and  added  to  the  solids  ob- 


212  MANUAL   OF   PHYSIOLOGY. 

tained  by  washing  out  the  bloodvessels.  Here  the  error  arises 
from  the  fact  that,  in  washing  out  the  bloodvessels,  nauch  solid 
matter  besides  .that  belonging  to  the  blood  is  taken  from  the 
tissues  and  thus  an  excess  is  found. 

Valentine's  Method. — A  small  quantity  of  blood  is  drawn  from 
a  vein  and  measured  and  its  percentage  of  solids  is  accurately  es- 
timated ;  thereupon  a  known  quantity  of  water  is  injected  into  the 
vessels.  After  some  time  being  allowed  for  proper  distribution  of 
the  water  a  sample  of  the  diluted  blood  is  taken  and  its  solids  es- 
timated. The  difference  in  solid  contents  of  the  two  samples 
shows  the  degree  of  dilution  caused  by  a  known  quantity  of  water 
introduced  into  blood  of  ascertained  strength,  and  thus  the  amount 
of  the  fluid  diluted  (the  blood)  may  be  calculated  and  added  to 
the  amount  of  the  first  sample  of  blood. 

This  method  cannot  give  accurate  results,  because  in  the  time 
necessary  for  the  distribution  and  mixture  of  the  water  with  the 
circulating  blood  much  of  the  former  is  excreted  by  the  kidneys 
and  skin,  and  the  second  sample  of  blood  is  more  concentrated 
than  should  result  from  such  dilution. 

Welcker's  Method  depends  upon  the  estimation  of  the  coloring 
matter  of  the  blood.  He  connected  the  carotid  with  a  small  T 
piece,  and  allowed  the  animal  to  bleed  into  a  bottle  in  which  the 
blood  could  be  defibrinated  by  shaking  with  pieces  of  glass.  One 
cubic  centimetre  of  this  defibrinated  blood  was  carefully  measured 
off  and  saturated  with  carbon  monoxide  (CO),  which  gives  a  per- 
manent and  equally  bright  red  color.  In  was  then  diluted  with 
500  c.c.  distilled  water  and  kept  as  a  standard  color-solution.  The 
bloodvessels  of  the  animal  were  then  washed  out  with  .6  per  cent, 
solution  of  sodium  chloride  until  the  solution  flowing  from  the 
jugular  vein  was  colorless.  The  tissues  of  the  animal  were  then 
chopped  up  and  steeped  in  water  and  pressed.  The  washings  of 
the  vessels  and  the  infusion  from  the  tissues  were  added  together 
and  diluted  until  they  had  the  same  color-intensity  as  a  layer  of 
the  standard  solution  of  the  same  thickness.  Every  500  c.c.  of 
these  diluted  washings  corresponds  to  1  c.c.  of  blood. 

By  this  method  the  following  estimates  have  been  made  of  the 
relation  of  the  blood  to  the  body  weight : 


PHYSICAL   CONSTRUCTION   OF   BLOOD.  213 

Mice, -^s-^ 

Guinea  pig, tV~tj 

Eabbit, •  jt—si 

Dog, TT-rV 

Cat,             •        •  2T 

Birds, iV-TJ 

Frog, tWV 

Fish W? 

Only  approximate  estimates  of  the  distribution  of  blood  in  the 
body  during  life  can  be  made,  since  there  can  be  no  accurate 
method  of  investigation,  and  the  amount  varies  enormously  ac- 
cording as  the  organ  or  part  is  in  a  state  of  rest  or  activity.  It 
is  supposed  that  a  quarter  of  the  entire  amount  is  habitually  flow- 
ing through  the  heart  and  great  vessels,  a  quarter  in  the  skeletal 
muscles,  another  quarter  in  the  liver,  and  in  all  the  other  parts 
only  a  fourth. 

Physical  Construction  of  the  Blood. 

As  already  stated  the  blood  is  not  a  red  fluid.  It  is  seen  with 
the  microscope  to  be  made  up  of  a  clear  fluid  called  plasma  or 


Fig.  95. 


Human  Blood  after  death  of  the  elements.  The  red  corpuscles  are  seen 
in  different  positions  showing  their  shape,  some  also  are  seen  in  rolls.  Only 
one  white  cell  (w)  is  seen,  misshapen  and  entangled  in  fibrin  threads. 

liquor  sanguinis,  which  contains  an  immense  number  of  little 
disk-shaped  bodies  called  red  corpuscles,  and  a  few  colorless  pro- 
toplasmic cells,  which  are  called  white  corpuscles;  so  that  the 
living  blood  may  be  physically  tabulated,  giving  approximately 
an  estimation  of  the  relative  amounts,  thus : 

Plasma  or  Liquor  Sanguinis f 

«'»»->  ^  Solid  or  Corpuscle  {K^S'l-t 


214 


MANUAL  OF   PHYSIOLOGY. 


Plasma. 

The  fluid  part  of  the  blood  is  of  a  pale  straw-color,  when  pure 
and  free  from  the  coloring  matter  of  the  blood-corpuscles,  and  of 
slightly  less  density  than  the  blood-corpuscles  (v.  p.  211).  Ex- 
cept special  precautions  are  taken  the  plasma  is  altered  when 
removed  from  the  bloodvessels  and  coagulation  of  the  blood  takes 
place ;  so  that,  under  ordinary  circumstances,  plasma  does  not 


Fig.  96. 


Reticulum  of  Fibrin  Threads  after  staining  has  made  them  visible.     The 
network  (6)  appeals  to  start  from  granular  centres  (a).     (Ranvier.) 

come  under  observation,  except  when  the  constitution  of  the  blood 
is  revealed  by  the  microscope.  It  was  first  separated  from  the  cor- 
puscles by  the  filtration  of  frog's  blood,  to  which  had  been  added 
strong  syrup,  which  checks  coagulation  and  spoils  the  flexibility 
of  the  corpuscles,  so  that  they  are  caught  in  the  meshes  of  the 
filter,  and  the  clear  plasma  passes  through. 

To  obtain  mammalian  plasma  free  from  corpuscles  it  is  neces- 
sary to  use  some  other  method,  as  the  small  elastic  corpuscles 
easily  run  through  the  meshes  of  the  thickest  filter-paper. 


PLASMA.  215 

The  blood  of  the  horse  is  chosen  because  it  coagulates  more 
slowly  than  that  of  most  mammals,  and  delay  in  the  coagulation 
or  postponement  of  the  change  in  the  plasma  is  the  chief  object 
to  be  obtained.  To  encourage  this  delay  the  blood  is  drawn  from 
a  vein  into  a  cylinder  surrounded  with  a  freezing  mixture.  The 
cold,  however,  must  not  be  so  intense  as  to  absolutely  freeze  the 
blood,  for  the  wished-for  subsidence  of  corpuscles  could  not  go 
on  if  the  blood  becomes  solid.  It  is  then  left  quite  motionless  for 
twenty-four  hours,  after  which  time  it  will  be  found  that  the 
heavy  corpuscles  have  fallen  and  left  a  clear  supernatant  fluid, 
which  is  plasma,  containing  some  white  cells.  This  can  be  re- 
moved with  a  cool  pipette  and  passed  through  an  ice-cold  filter 
to  remove  the  cells,  then  tolerably  pure  plasma  is  obtained  which 
soon  coagulates  at  the  ordinary  temperature. 

Another  method  of  checking  coagulation  consists  of  letting  the 
blood  flow  into  a  25  per  cent,  solution  of  magnesium  sulphate 
(about  three  volumes  of  blood  to  one  of  the  solution).  This,  if 
left  in  a  cool  place,  will  not  coagulate,  and  the  corpuscles  will 
separate  by  subsidence  from  the  plasma  and  salt  solution,  which 
form  an  upper  layer  of  clear  fluid.  If  the  salt  be  removed  by 
dialysis  or  weakened  by  dilution  with  water,  coagulation  com- 
mences. 

The  coagulation  of  plasma  can  be  seen  with  the  microscope  to 
depend  upon  the  appearance  of  a  close  feltwork  of  exquisitely 
delicate,  finely  granular  elastic  fibrils,  which  pervade  the  entire 
fluid,  and  cause  it  to  set  into  a  soft  jelly.  The  substance  forming 
the  meshes  is  caWed  fibrin. 

Some  time  after  the  plasma  has  gelatinized,  the  threads  of  fibrin 
break  away  from  their  attachment  to  the  vessel  in  which  the 
coagulura  is  contained,  and  owing  to  their  elasticity  the  general 
mass  of  fibrin  contracts,  squeezing  out  of  its  meshes  clear  drops 
of  fluid  termed  serum. 

The  fibrin  clot  gradually  shrinks  into  unappreciable  dimen- 
sions, and  floats  in  the  abundant  fluid  serum. 

The  separation  of  the  serum  is  accelerated  by  agitation  of  the 
soft  clot;  and  if  brisk  agitation,  such  as  whipping,  be  kept  up  for 
a  few  minutes,  the  plasma  does  not  form  a  jelly,  but  the  fibrin 


216  MANUAL   OF    PHYSIOLOGY. 

firmly  adheres  to  the  stirring  rods  and  at  once  contracts  around 
them. 

Chemical  Composition  of  Plasma. 

On  account  of  the  rapid  spontaneous  formation  of  fibrin  and 
serum  when  the  plasma  is  removed  from  the  body  and  allowed  to 
die,  the  exact  chemical  condition  of  the  liquor  sanguinis  during 
life  cannot  be  investigated,  the  separation  occurring  before  the 
simplest  chemical  method  can  be  carried  out. 

We  have  no  reason  to  suppose  that  fibrin  exists  normally  in 
the  blood,  but  it  would  appear  that  this  substance  is  only  formed 
at  the  moment  of  coagulation,  and  is  one  of  the  most  obvious  of 
many  changes  which  take  place  at  the  time  of  the  death  of  blood 
plasma. 

The  chemical  change  comprehended  under  the  term  coagula- 
tion occurring  when  plasma  is  deprived  of  its  means  of  vitality, 
and  ending  in  the  production  of  fibrin  and  serum,  is  naturally 
of  the  first  importance  in  studying  the  chemical  relationships  of 
living  plasma.  It  can  best  be  followed  out  in  the  coagulation  of 
plasma  when  separated  from  the  corpuscles,  for  (although  the 
stages  in  the  coagulation  of  blood  are  the  same,  the  appearance 
of  an  insoluble  albumin — fibrin — being  the  one  essential  in  either 
case)  the  corpuscles  complicate  the  process  and  modify  the  ap- 
pearance of  the  clot. 

Not  only  is  the  fibrin  not  present  as  such  in  the  living  plasma, 
but  it  requires  for  its  production  the  presence  of  other  substances 
which  either  do  not  exist  in  the  living  plasma,  or  are  there  so 
chemically  associated  as  not  to  bring  about  the  change  which 
occurs  when  the  plasma  is  dead. 

The  reasons  for  believing  this  are  the  following.  Fluids  which 
sometimes  collect  by  a  slow  process  in  the  serous  cavities  of  the 
body,  e.g.,  hydrocele  fluid,  pleural  effusion,  etc.,  if  kept  quite 
clean  do  not  generally  undergo  spontaneous  coagulation.  If  to 
one  of  these  some  serum  from  around  a  blood  clot  be  added, 
coagulation  takes  place  just  as  in  plasma  (Buchanan).  That  is 
to  say,  we  have  here  two  fluids,  neither  of  which  coagulates  when 
left  to  itself,  but  which  do  coagulate  when  mixed  together.   From 


CHEMICAL   COHrPOSITION   OF    PLASMA.  217 

each  of  these  fluids  a  substance  can  be  precipitated  by  passing  a 
stream  of  carbon  dioxide  (CO2)  through  the  fluids.  Both  pre- 
cipitates readily  re-dissolve  in  weak  saline  solutions. 

The  solution  prepared  from  the  hydrocele  fluid  causes  blood 
serum  to  coagulate ;  that  prepared  from  the  blood  serum  causes 
the  hydrocele  fluid  to  coagulate;  and  when  mixed  together  the 
mixture  of  the  two  solutions  coagulates;  while  the  serum  and 
hydrocele  fluid  from  which  the  substances  have  been  removed  no 
longer  have  the  power  of  exciting  coagulation  in  each  other  or 
in  like  fluids.  Here,  then,  are  brought  to  light  two  materials: 
one,  which  may  be  obtained  in  considerable  quantity  from  serum 
after  coagulation,  is  called  serum-globulin  or  paraglobulin,  the 
other  occurring  in  serous  fluids  is  named  fibrinogen.  Both  of 
these  substances  are  present  in  the  dying  plasma  of  the  blood 
prior  to  coagulation.  They  can  be  obtained  both  together 
from  the  plasma  (when  either  of  the  precautions  already  men- 
tioned, viz.,  the  application  of  cold,  or  the  addition  of  neutral 
salt,  has  been  taken  to  prevent  the  formation  of  fibrin)  if  the 
plasma  be  treated  with  sodium  chloride  to  saturation.  This  pre- 
cipitates a  substance  which  readily  dissolves  if  water  be  added  to 
weaken  the  salt  solution,  and  after  some  time  the  solution  under- 
goes spontaneous  coagulation,  while  the  plasma  from  which  it  has 
been  made  has  last  that  power.  This  plasmin  (Denis)  no  doubt 
is  made  of  different  globulins,  chiefly  serum-globulin  and  fibrin- 
ogen, and  contains  in  itself  all  the  necessary  "factors"  of  fibrin 
formation,  but  is  not  at  all  identical  with  fibrin,  since  it  readily 
dissolves  in  weak  saline  solutions,  like  the  class  of  proteids  called 
globulins,  while  fibrin  is  quite  insoluble  in  such  solutions. 

In  plasma  removed  from  its  normal  relationships,  then,  both 
serum-globulin  and  fibrinogen  exist;  but  the  former  in  far  greater 
quantity  than  the  latter,  since  the  serum,  after  the  blood-clot  is 
formed,  contains  no  more  fibrinogen,  while  the  serum-globulin  or 
paraglobulin  makes  up  nearly  half  the  entire  solids  of  the  remain- 
ing serum. 

In  preparing  fibrinogen  and  paraglobulin  (or,  as  he  called  the 
latter,  fibrinoplastin)  Schmidt  found  that  the  more  carefully  they 
were  made,  the  weaker  and  more  uncertain  their  action  as  fibrin 


218  MANUAL   OF   PHYSIOLOGY. 

factors  became;  and  finally  he  made  solutions  which,  when  added 
together,  did  not  produce  coagulation,  but  which,  when  added  to 
less  pure  solutions,  gave  good  firm  clots.  From  this  he  suspected 
that  a  third  agent  which  acted  as  a  ferment  was  necessary  to  put 
into  operation  the  fibrin-producing  properties  of  the  other  two 
factors.  He  moreover  succeeded  in  separating  the  third  agent, 
to  which  he  gave  the  name  of  fibrin-ferment.  By  treating  blood 
serum  with  twenty  times  its  volume  of  strong  alcohol  and  allow- 
ing it  to  stand  a  month  or  two,  the  proteids  are  precipitated  and 
rendered  quite  insoluble  in  water,  and  with  them  the  ferment  is 
carried  down.  From  the  dried  and  powdered  precipitate  the 
ferment  is  extracted  with  water.  This  solution  when  added  to 
the  mixture  of  the  pure  fibrin  factors  which  by  themselves  did 
not  coagulate  caused  rapid  coagulation,  but  not  when  added  to 
either  one  or  the  other  of  them  singly  (Schmidt). 

This  material  seemed  to  have  been  influenced  by  those  circum- 
stances which  affect  the  activity  of  ferments  in  general :  it  has  a 
minimum,  0°  C,  maximum,  80°  C,  and  optimum,  38°  C,  tempera- 
ture of  activity,  with  various  gradations  of  rapidity  of  action 
between  each,  and  it  is  destroyed  by  a  temperature  above  80°  C. 
The  amount  of  fibrin-ferment  only  seems  to  influence  the  rapidity 
with  which  the  fibrin  is  formed,  not  the  amount,  which  rather 
depends  on  the  quantity  of  serum-globulin  (paraglobulin). 

The  source  of  the  three  fibrin  generators  is  a  question  of  much 
difficulty,  and  may  be  discussed  with  more  profit,  together  with 
the  question  of  blood  coagulation,  within. and  without  the  vessels, 
after  the  morphological  elements  have  been  described. 

Preparation  and  Properties  of  Fibrin. 

Fibrin  may  be  procured  either  from  plasma  or  blood  by  whip- 
ping and  then  washing  the  insoluble  fibrin  with  water.  When 
fresh  it  has  a  pale  yellow  or  whitish  color,  a  filamentous  structure, 
and  is  singularly  elastic.  It  is  not  soluble  in  water,  weak  saline 
solution,  or  ether.  Alcohol  makes  it  shrink  by  removing  its 
water.  When  quite  dry  it  is  brittle  and  hard,  and  can  be  reduced 
to  a  powder.  It  swells  in  1  per  cent,  hydrochloric  acid,  and  if 
then  warmed  is  soon  converted  into  acid  albumin. 


SERUM.  219 

The  amount  formed  varies  very  much  even  in  the  blood  drawn 
from  the  same  animal  at  the  same  time,  but  is  always  very  small 
compared  with  the  size  of  the  blood-clot.  It  never  reaches  as 
much  as  1  per  cent.,  commonly  varying  from  0.1  per  cent,  to  0.3 
per  cent,  of  the  entire  mass  of  blood. 

Serum. 

This  name  is  given  to  the  clear  fluid  which  oozes  out  of  the 
clot  of  plasma.  It  only  differs  from  the  latter  in  its  chemical 
composition  in  so  far  that  fibrin  is  separated  from  it.  Though 
chemically  this  is  a  slight  difference,  it  signifies  the  change  from 
a  complex  living  body  (blood  plasma)  into  a  solution  of  dead 
albumins,  etc. 

Serum  is  a  clear  straw-colored  alkaline  fluid  of  1028-1030  sp. 
gr.,  holding  in  solution  different  organic  substances  and  some  in- 
organic salts.  After  a  full  meal  the  serum  is  said  to  be  more  or 
less  milky  from  the  presence  of  finely  divided  fat. 

It  contains  about  9  per  cent,  of  solid  matters,  of  which  a  large 
proportion,  7  per  cent.,  are  proteids.  Of  these,  the  most  abundant 
is  (1)  serum-albumin  (about  4  per  cent,  in  man),  a  solution  of 
which  becomes  opaque  at  60°  C,  and  coagulates  at  a  heat  of  73"- 
75°  C.  The  proteid  next  in  importance  is  (2)  serum-globulin  or 
paraglobulin  (about  3  per  cent,  in  man),  which  has  already  been 
mentioned.  It  may  be  precipitated  imperfectly  by  CO^,  or  com- 
pletely by  magnesium  sulphate.  (3)  Serum-casein  has  been  ob- 
tained from  serum  by  careful  neutralization  with  acetic  acid  after 
the  removal  of  the  paraglobulin  by  CO2.  This  is  said  to  be  para- 
globulin which  has  failed  to  come  down  with  the  COj.  (4)  Neu- 
tral fats  in  a  state  of  fine  subdivision  are  present  in  a  variable 
quantity  ;  also  (5)  lecithin  ;  (6)  traces  of  sugar ;  (7)  various 
products  of  tissue  change — kreatine,  urea,  etc. ;  and  (8)  inorganic 
salts,  viz.,  sodium  chloride,  about  5  per  cent.,  and  sodium  car- 
bonate, which  probably  existed  in  the  blood  as  sodium  hydric 
carbonate.  There  is  also  a  small  quantity  of  potassium  chloride. 
But  it  should  be  remembered  that  there  is  about  ten  times  more 
sodium  than  potassium  salts  in  the  serum,  and  probably  in  the 
blood  plasma. 


CHAPTER  XIV. 

BLOOD-CORPUSCLES. 

The  relative  number  of  red  disks  to  the  colorless  cells  is  said  to 
be,  on  the  average,  350  to  1.     This  is  true  of  the  blood  drawn 


Human  Blood  after  deatli  of  the  elements.  The  red  corpuscles  are  seen 
in  different  positions  showing  their  shape,  some  also  in  rolls.  Only  one 
white  cell  (w)  is  seen,  missliapen  and  entangled  in  fibrin  threads. 

from  the  fine  vessels  by  puncture.  While  in  the  vessels  the  blood 
must  contain  a  greater  proportion  of  the  colorless  cells,  for  by 
the  ordinary  method  of  obtaining  blood  for  examination,  they  do 
not  flow  out  of  the  punctured  capillaries  as  readily  as  the  red 
disks ;  and,  moreover,  many  of  them  become  disintegrated  very 
shortly  after  they  are  removed  from  the  circulation.  Although 
the  number  of  red  disks  normally  alters  but  little,  the  relative 
number  of  red  to  white  varies  very  much  on  account  of  the  con- 
stant changes  occurring  in  the  number  of  the  white  cells,  which 
has  been  found  to  differ  according  to  the  observer,  the  situation, 
and  other  circumstances,  as  shown  in  the  following  table,  which 
gives  the  number  of  red  corpuscles  to  one  colorless  cell. 
Observer's  estimate  of  normal  proportion  : 

Red.  White. 

Welcker, 335—1 

Moleschott, 357 — 1 

In  various  parts  of  the  circulation  : 

Splenic  vein, ,        .       60 — 1 

Splenic  artery, 2260—1 


THE    WHITE    BLOOD-CELLS.  221 

Red.  White. 

Hepatic  vein, 170 — 1 

Portal  vein,       ....  ...    740—1 

According  to  age  or  sex  : 

Girls, 405—1 

Boys, 226—1 

Adult, 334—1 

Old  age, 381—1 

According  to  general  conditions: 

When  fasting, 716—1 

After  meal 347—1 

During  pregnancy, 281 — 1 

In  a  disease  of  the  spleen  and  lymphatic  glands  called  Leuco- 
cythemia  there  may  appear  to  be  nearly  as  many  white  cells  as 
red  disks.  Here,  however,  the  red  disks  are  deficient,  while  the 
colorless  cells  are  multipled. 

The  White  Blood-cells. 

The  protoplasmic  cells  of  the  blood,  commonly  called  the  white 
corpuscles,  differ  in  no  essential  respect  from  the  pale  round  cells 
which  are  found  in  most  of  the  tissues  of  the  body.  They  exist 
in  great  numbers  in  the  fluid  which  drains  back  from  the  tissues 
into  the  blood,  namely,  the  lymph,  and  occupy  a  great  part 
of  the  lymphatic  glands  and  spleen.  They  are  often  spoken  of 
as  lymphoid  cells,  leucocytes,  or  indifferent  formative  cells,  on  ac- 
count of  their  being  so  widely  distributed  throughout  the  tissues. 

When  fresh  blood  is  examined  with  the  microscope  these  cells 
can  be  seen  generally  adhering  to  the  glass  slide  or  cover-glass 
and  lying  singly,  apart  from  the  groups  of  red  disks.  They  can 
be  recognized  by  their  faintly  bluish  hue  or  absence  of  marked 
color,  their  finely  granular  structure,  spherical  shape,  and  the 
nuclei  which  may  often  be  recognized  near  the  centre  of  the  cell. 
Though  not  always  visible  in  quite  fresh  preparations,  the  nuclei 
can  be  brought  to  light  by  the  action  of  many  reagents — e.g., 
acetic  acid.  If  looked  at  while  being  moved  by  the  blood-cur- 
rent in  the  capillary  vessels,  they  are  seen  to  pass  slowly  along 
in  contact  with  the  vessel  wall,  while  the  red  corpuscles  rush  rap- 


222 


MANUAL   OF   PHYSIOLOQY. 


idly  past  them  flown  the  centre  of  the  channel  (Fig.  98).  This 
may  partly  be  due  to  their  peculiar  adhesiveness,  which  also  causes 
thera  to  stick  to  the  glass  slide,  whilst  the  red  disks  are  washed 
away  when  a  little  stream  of  saline  solution  is  allowed  to  flow 
under  the  cover-glass.  These  cells  show  all  the  manifestations  of 
activity  characteristic  of  independent  living  beings.  If  kept  in 
a  medium  suitable  to  them,  and  at  the  temperature  of  the  body, 
they  will  be  seen  soon  to  alter  their  appearance;  their  outline 
becomes  faint,  they  are  no  longer  spherical,  but  very  irregular  in 

Fig.  98. 


Vessels  of  the  Frog's  Web. — (a)  Trunk  of  vein,  and  (6  b)  its  tributaries 
passing  across  the  capillary  network.     The  dark  spots  are  pigment  cells. 

shape,  and  constantly  change  their  form  by  sending  out  and  re- 
tracting delicate  processes,  by  means  of  which  they  change  their 
position,  so  that  they  may  be  said  to  perform  locomotion.  These 
movements  are  rendered  more  active  by  a  slight  increase  of  tem- 
perature, and  are  checked  by  cold.  For  continued  observation, 
about  38°  C.  is  the  best  temperature.  They  respond  to  many 
other  influences,  such  as  electricity,  etc.,  even  for  a  considerable 
time  after  removal  from  the  body. 


ORIGIN   OF   THE   COLORLESS    BLOOD-CELLS.  223 

No  doubt  they  absorb  fluid  uutriment  contiuually  from  the 
surrounding  medium,  as  is  shown  by  the  effect  of  poisons  on  them  ; 
and,  by  the  repeated  contractions  and  relaxations  of  parts  of  their 
substance  in  the  form  of  pseudopodia,  they  appear  to  take  into 
the  inner  parts  of  the  protoplasm  solid  particles,  which  after  some 
time  are  ejected  after  the  manner  of  the  small  unicellular  animals 
known  as  amcebte  (p.  85). 

While  in  motion  in  the  circulation  none  of  these  amceboid 
movements  appear  to  take  place,  but  when  an  arrest  of  the  flow 
of  blood  in  the  capillaries  occurs  they  not  only  change  their  form, 
but  also  their  position  ;  and  if  there  be  no  onward  flow  of  blood 
for  some  little  time,  they  creep  out  of  the  capillaries,  passing 
through  the  delicate  vessel-walls.  This  emigration  of  the  blood- 
cells  is  possibly  a  common  event  when  a  tissue  is  in  need  of  textu- 
ral  repair.  When  excessive  it  forms  one  of  the  most  striking 
items  of  the  series  of  events  occurring  in  inflammation. 

The  cells  differ  much  in  size  ;  generally  they  are  somewhat 
larger  than  the  red  disks.  Nothing  like  a  cell-wall  can  be  seen 
to  surround  them,  and  from  the  movements  above  described  it 
would  appear  certain  that  they  are  free  masses  of  active  proto- 
plasm. 

The  number  of  white  cells  that  can  be  collected  is  too  small  to 
allow  of  accurate  chemical  analysis,  but  there  is  no  reason  to 
suppose  that  they  differ  from  other  forms  of  protoplasm. 

Origin  of  the  Colorless  Blood-cells. 

Since  such  an  unimportant  circumstance  as  a  hearty  meal  can 
materially  influence  the  numbers  of  the  white  corpuscles,  it  would 
appear  that  they  must  be  usually  undergoing  rapid  variations  in 
their  number — probably  by  their  being  constantly  used  up  and 
periodically  replaced  by  new  ones.  The  places  in  which  they 
occur  in  greatest  number  are  the  lymphatic  glands,  the  spleen, 
and  the  lymph  follicular  tissue  in  the  intestinal  tract. 

There  is  no  doubt  that  the  lymph  contains  a  much  larger  pro- 
portion of  these  cells  after  it  has  passed  through  the  lymph 
glands,  and  the  blood  coming  from  the  spleen  contains  an  exces- 
sive proportion  of  them. 


224  MANUAL   OF   PHYSIOLOGY. 

It  is  then  not  unreasonable  to  suppose  that  many  of  the  white 
cells  found  in  the  blood  have  their  origin  in  these  organs. 

They  may  also  be  developed  from  similar  cells  in  any  tissue, 
but  their  multiplication  by  division,  other  than  that  which  prob- 
ably occurs  in  the  lymph  follicles  where  it  cannot  be  seen,  is  a 
circumstance  of  the  greatest  rarity,  and  few  observers  have  been 
fortunate  enough  to  witness  the  phenomenon. 

The  destiny  of  the  white  blood-cells  is  probably  manifold. 
From  the  readiness  with  which  they  escape  from  the  capillaries 
and  wander  by  their  amoeboid  movement  through  the  neighboring 
tissues  to  reach  any  point  of  injury,  it  would  appear  that  they 
take  an  active  part  in  the  repair  of  any  tissue  whose  vitality  has 
in  any  way  suffered.  During  the  growth  of  all  tissues  the  cells 
seem  to  contribute  active  agents  in  their  formation  ;  thus  in  the 
formation  of  bone  it  has  been  stated  that  escaped  blood-cells  or 
their  immediate  offspring  help  to  lay  down  the  calcareous  mate- 
rial, and  some  even  settle  themselves  as  permanent  inhabitants 
of  the  lacunae. 

Further,  they  are  in  all  probability  the  means  of  renewing  the 
red  disks.  Their  protoplasm  either  takes  up  the  coloring  matter 
from  its  surroundings,  or  forms  it  within  itself  from  suitable  ingre- 
dients. Certain  it  is  that  cells  are  found  which  are  recognizable 
as  white  blood-cells  which  have  more  or  less  of  the  red  coloring 
matter  imbedded  in  their  substance.  As  this  increases  the  cell 
gradually  loses  its  distinctive  characters  and  assumes  those  of  a 
red  corpuscle.  Such  elements,  it  will  be  seen,  are  common  in  the 
spleen  and  the  blood  leading  from  it. 

The  Red  Corpuscles. 

The  red  disks  were  discovered  in  the  human  blood  by  Leuwen- 
hoek,  about  1673.  They  alone  give  the  red  color  which  charac- 
terizes the  blood  of  all  vertebrated  animals  (except  the  amphi- 
oxus),  but  are  not  found  in  the  blood  of  the  invertebrata,  which 
only  contains  colorless  cells.  When  the  blood  of  the  inverte- 
brates has  a  color,  it  owes  it  to  the  fluid,  not  to  the  corpuscles. 
The  individual  disks  when  viewed  singly  under  the  microscope 


THE   RED   CORPUSCLES. 


225 


appear  to  be  pale  orauge,  but  when  in  masses  the  red  becomes 
apparent. 

The  shape  of  the  corpuscles  differs  in  different  classes  of  animals. 
In  man  and  all  mammalia  they  are  disks  which  are  concave  on 
each  side,  and  rounded  off  at  the  margin.  The  only  class  of 
mammals  which  form  an  exception  to  this  rule  are  the  camalidse, 

Fig.  99. 


Diagram  of  the  relative  sizes  of  red  corpuscles  of  different  animals.  The 
measurements  below  are  in  fractions  of  an  inch  :  1.  Amphiuraa,  ^  X  jV* 
2.  Proteus,  ^\  X  ^\.  3.  Frog,  ^V  X  ^V-  4.  Pigeon,  ^^-g  X  jh-  5.  Elephant, 
tJj.  6  Man,  tI^.  7  Dog,  j}^.  8.  Horse,  j^t-  9.  Goat,  ^^^.  10.  Musk- 
deer,  :f\j. 

whose  red  corpuscles  are  elliptical  in  shape,  like  those  of  all  non- 
mammalian  vertebrates. 

The  corpuscles  of  birds,  amphibia  and  fish  are  flattened  ellip- 

19 


226  MANUAL   OF   PHYSIOLOGY. 

tical  plates,  which  are  slightly  convex  on  each  side,  and  contain 
a  distinct  oval  nucleus  in  their  centre. 

The  size  of  the  corpuscles  varies  greatly  in  different  classes  of 
animals,  but  is  strikingly  constant  in  the  same  class.  A  glance 
at  the  diagram,  Fig.  99,  in  which  the  corpuscles  are  drawn  to 
scale,  will  give  an  idea  of  their  relative  sizes  in  examples  of  the 
different  classes  of  animals,  and  will  make  the  following  points 
more  rapidly  obvious  than  any  description. 

The  size  of  the  animal  has  no  general  relation  to  the  size  of  the 
corpuscles.  The  human  red  disks  are  of  a  fair  average  size  when 
compared  with  those  of  other  mammals,  and  therefore  man's  hlood- 
cannot  be  distinguished  from  that  of  the  other  mammalia. 

The  maiiiiualiau  corpuscles  are,  on  the  whole,  small  when  com- 
pared with  those  of  the  other  vertebrates.  The  batrachians  are 
distinguished  by  the  great  size  of  the  corpuscles.  Those  of  the 
Araphiuma  Tridactylum  are  visible  to  the  naked  eye. 

The  following  measurements  are  given  by  \yelcker  for  the  hu- 
man disks : 

Diameter,  .  .     .     0.0,077  of  a  niilliiuetre  =  55\55th  of  an  inch. 

Thickness,  .  .     .     0.0,019  of  a  millimetre  ^  yj^joth  of  an  inch. 

Volume,    .  .  0.000,000,077  of  a  cubic  millimetre. 

Surface,     .  .  .      0.000,128  of  a  square  millimetre. 

The  last  measurement  would  give  about  2816  square  metres  for 
the  entire  blood  of  an  adult.  A  surface  of  11  square  metres  is 
exposed  every  second  in  the  lungs  for  the  absorption  of  oxygen. 

When  circulating  in  the  vessels,  or  immediately  after  removal, 
the  red  corpuscles  are  very  soft  and  elastic,  being  bent  and  altered 
in  shape  by  the  slightest  pressure,  and  easily  stretched  to  twice 
their  diameter.  But  the  moment  pressure  or  traction  is  removed, 
they  return  to  their  normal  biconcave,  disk-shape  if  the  medium 
in  which  they  lie  continue  of  the  normal  density.  (See  Fig.  97, 
p.  220.) 

Changes  take  place  in  the  blood  shortly  after  it  is  removed 
from  the  body,  which  seem  to  be  associated  with  the  loss  of  func- 
tion (death)  of  the  red  disks,  as  shown  by  their  rapid  destruction 
if  reintroduced  into  the  circulation. 


ACTION   OF    REAGENTS    ON    RED    CORPUSCLES. 


227 


These  chauges  are  checked  by  cold,  and  facilitated  by  heat,  a 
temperature  above  that  of  the  body  causing  them  to  take  place 
almost  immediately.  Associated  with  the  loss  of  function  of  the 
disks  is  observed  a  change  accompanied  by  an  apparent  increase 
of  adhesiveness,  which  causes  them  to  stick  together,  commonly 
adhering  by  their  flat  surfaces,  so  as  to  form  into  rolls,  like  so 
many  coins  placed  side  by  side.  That  this  adhesion  is  not  a  mere 
physical  process,  independent  of  the  chemical  properties  of  the 
corpuscles  themselves,  seems  proved  by  the  following  facts:  (1) 
It  does  not  occur  immediately  when  the  blood  is  drawn,  and  it 
disappears  after  a  certain  time  without  the  addition  of  reagents; 
(2)  while  the  blood  is  in  the  living  vessels  under  normal  condi- 
tions there  is  no  adhesion,  but  it  soon  appears  when  any  stand- 
still in  the  circulation  takes  place — as  in   inflammation;  (3)  it 


Fig.  100. 


Fig.  101. 


Fig.  100. — Microscopic  appearance  of  the  blood  after  the  addition  of 
distilled  water.  Ked  Corpuscles  become  colorless  or  pale,  separate,  and 
spherical.  The  white  are  seen  to  be  swollen  round  and  granular,  with  clear 
nuclei. 

Fig.  1,01. — Showing  effect  of  evaporation.  Six  Ked  Corpuscles  crenated. 
(w)  White  cell  changing  shape. 


does  not  occur  when  saline  solutions  are  added  to  the  blood.  It 
seems  then  to  be  dependent  upon  a  peculiar  property  of  the  disks, 
which  only  exists  for  a  time  coincident  with  the  changes  that 
accompany  the  appearance  of  fibrin. 

The  shape  of  the  disks  changes  when  the  density  of  the  medium 
in  which  they  are  suspended  is  altered.  When  the  density  is 
reduced,  as  by  the  addition  of  water,  they  swell  and  become 
spherical,  and  break  up  the  rouleaux,  the  coloring-matter  at  the 
same  time  becoming  dissolved  in  the  medium.  (Fig.  100.)    When 


228  MANUAL   OF    PHYSIOLOGY. 

the  density  is  increased  by  slight  evaporation,  or  the  addition  of 
salt  solution  about  1  per  cent.,  they  cease  to  be  concave,  and  be- 
come crenated  or  spiked  like  the  green  fruit  of  the  horse-chestnut. 
(Fig.  101.)  The  addition  of  strong  syrup  causes  the  corpuscles 
to  shrivel  and  assume  a  great  variety  of  peculiar  bent  or  con- 
torted forms.  (Fig.  102.)  Elevation  of  temperature  or  repeated 
electric  shocks  causes  peculiar  changes  in  shape,  but  since  the 
change  is  associated  with  the  death  of  the  element,  it  cannot  be 
attributed  to  vital  activity  comparable  with  that  which  is  seen  in 
the  white  cells. 

The  disks  show  no  signs  of  structure  under  the  microscope ; 
they  look  perfectly  homogeneous  transparent  bodies  of  a  pale 
orange  color,  all  efforts  to  demonstrate  the  limiting  membranes, 

Fig.  102.  Fig.  103. 

Fig.  102. — Red  Ckjrpuscles,  shrivelled  by  the  addition  of  strong  syrup. — 
(w)  White  Corpuscle. 

Fig.  103. — Blood-Corpuscles  after  the  addition  of  tannic  acid. 

formerly  supposed  to  surround  them,  having  failed.  Their  be- 
havior when  certain  reagents  are  added  to  the  blood  shows  that 
the  corpuscles  have  two  constituents:  (1)  the  coloring-matter, 
Oxyhcemoghbin ;  and  (2)  the  Stroma.  The  coloring-matter  may 
be  removed — as  above  stated,  by  water — from  the  corpuscles,  and 
then  leaves  a  perfectly  colorless  transparent  foundation  or  ground- 
work, which  appears  to  be  in  some  way  porous,  so  as  to  hold  the 
coloring  matter  in  its  interstices.  The  effect  on  the  naked-eye 
appearance  of  the  blood  produced  by  the  removal  of  the  coloring- 
matter  from  the  stroma,  is  to  alter  the  color  and  increase  the 
transparency  of  the  fluid.  The  oxyhsemoglobin  now  forms  a 
transparent  dark-red  laketj  solution,  and  the  corpuscles,  being 
quite  colorless,  are  practically  invisible.     This  transparency  of 


ACTION   OF    REAGENTS    ON    RED    CORPUSCLES, 


229 


the  fluid  does  not  depend  on  any  change  in  the.oxyhsemoglobin, 
but  merely  on  its  being  dissolved  out  of  the  disks.  This  process, 
which  is  commonly  spoken  of  as  rendering  the  blood  "  lakey," 
may  be  brought  about  by  the  following  means.  (1)  The  addi- 
tion of  about  4  its  bulk  of  distilled  water,  to  dissolve  the  coloring- 
matter  out  of  the  stroma,  which  may  then  be  rendered  obvious 
by  a  weak  solution  of  iodine.  (2)  By  the  addition  of  chloroform, 
ether,  neutral  alkaline  salts,  or  alkalies.  (3)  By  passing  repeated 
strong  induction  shocks  through  the  blood.  *(4)  By  rapidly  freez- 
ing and  thawing  the  blood  several  times. 

Fig.  104. 


Malassez's  Apparatus  for  the  Enumeration  of  Blood-Corpuscles. — A.  Mea- 
suring and  mixing  pipette,     b.  Flattened  and  calibrated  capillary  tube. 


All  of  these  processes  produce  the  same  effect,  viz.,  the  red 
matter  leaves  the  stroma  intact.  Solutions  of  urea,  bile  acids, 
and  heat  of  about  60°  C.  seem  to  destroy  the  disks,  and  thus  re- 
move the  coloring-matter.  Carbolic,  boracic,  and  tannic  acids 
cause  the  coloring-matter  to  coagulate  and  localize  itself  either 
at  the  centre  or  margin  of  the  corpuscles.  (Fig.  103.) 

The  number  of  disks  in  the  blood  of  man  is  enormous,  namely, 
in  a  cubic  millimetre  of  blood,  about  5  millions  for  males  and 


230 


MANUAL   OF   PHYSIOLOGY. 


4J  millions  for  females,  or  about  250,000  millions  for  one  pound 
of  blood.  The  number  varies  much,  not  only  in  disease,  but  also 
as  a  result  of  the  many  physiological  processes,  such  as  changes 
in  the  amount  of  plasma,  brought  about  by  pressure-differences, 
etc. 

In  order  to  count  the  corpuscles  the  following  method  is  em- 
ployed.    The  blood  is  diluted  with  artificial  plasma  to  100  or 

Fjg.  lUO. 


The  appearance  presented  by  the  Capillary  Tube  of  Malassez's  Apparatus 
when  filled  with  diluted  blood  and  examined  under  a  microscope  magnify- 
ing 100  diameters  provided  with  an  eye-piece  micrometer. 

1000  times  its  volume,  and  the  corpuscles  in  a  portion  of  the  mix- 
ture carefully  measured  off  by  a  capillary  tube,  and  counted. 
This  operation  requires  great  care  and  delicate  apparatus.  One 
of  the  best-known  methods  is  that  of  Malassez,  the  details  of 
which  are  as  follows  : 

Blood  is  drawn  into  the  capillary  tube  of  a  specially  prepared 
delicate  pipette  (Fig.  104,  a)  up  to  a  mark  which  indicates  y^ 


OXYHEMOGLOBIN.  231 

part  of  the  capactity  of  the  pipette.  This  kuovvn  quantity  of 
blood  is  then  washed  into  the  bulb  of  the  pipette  by  drawing  up 
artificial  serum  to  fill  the  bulb,  where  the  fluids  are  mixed  by 
shaking  about  the  fine  bead  contained  in  the  bulb.  Some  of  this 
mixture  is  then  allowed  to  pass  into  a  flattened  capillary  tube  of 
known  capacity  fixed  on  a  slide,  and  the  number  of  corpuscles  in 
a  given  length  of  this  tube  at  two  or  three  places  is  carefully 
counted.  The  important  question,  how  much  oxyhtemoglobin 
exists  in  a  given  sample  of  blood,  can  be  determined  by  diluting 
a  drop  until  the  color  equals  that  of  a  standard  solution  of  known 
strength. 

Chemistry  of  the  Coloring  Matter  of  the  Blood. 

Of  the  chemical  constituents  found  in  the  red  blood-corpuscles, 
the  red  coloring  matter  is  by  far  the  must  important.  To  it  alone 
the  blood  owes  one  of  its  most  important  functions — the  respira- 
tory. 

Oxyhcemoglobin  is  a  chemical  compound  of  great  complexity, 
and  of  which  the  percentage  composition  is  given  as: 

Carbon, 53.85 

Hydrogen, 7.32 

Nitrogen, 16.17 

Oxygen,         .         • 21.84 

Sulphur, 39 

Iron, 43 

Its  rational  formula  is  unknown,  but  the  following  has  been 
propo.sed  as  approximate,  Cgg(,H3g„N,5^FeS30j,g.  It  is  commonly 
regarded  as  a  form  of  globulin,  associated  with  a  colored  mate- 
rial containing  iron,  called  hiematin.  Its  chief  peculiarities  are 
(1)  that,  although  it  contains  a  colloid  substance,  it  crystallizes 
more  or  less  readily  in  all  vertebrates  when  removed  from  the 
stroma  of  the  corpuscles  ;  (2)  the  considerable  amount  of  iron  it 
contains  (0.4  per  cent.)  ;  (3)  the  remarkable  manner  in  which  it 
is  combined  with  oxygen  to  form  an  unstable  compound  ;  and  (4) 
the  ease  with  which  it  yields  its  oxygen  to  the  tissues  and  takes 
it  from  the  air. 


232  MANUAL   OF   PHYSIOLOGY. 

The  readiness  with  which  the  oxyhoemoglobin  crystals  are  formed 
varies  much  in  different  animals  and  under  different  circum- 
stances, as  may  be  seen  from  the  following  list: 

Most  readily — guinea  pig,  rat,  mouse. 
Readily — cat,  dog,  horse,  man,  ape,  rabbit. 
With  difficulty — sheep,  cow,  pig. 
Not  at  all — frog. 

The  presence  of  oxygen  causes  the  crystals  to  form  more  rapidly, 
so  that  a  stream  of  oxygen  passed  through  a  strong  solution  of 
hsemoglobin  causes  small  crystals  of  oxyhteraoglobin  to  form. 

The  crystals  always  belong  to  the  rhombic  system,  being  most 
commonly  plates  (man,  etc.)  and  prisms  feat),  and  rarely  tetra- 
hedra  (guinea  pig)  and  hexagonal  plates  (squirrel). 

Fig.  106. 


Crystals  of  Htemoglobin  from  difierent  aniiimls,  sliowiiig  tlie  variety  in  form 
of  crystals — 1,  guinea  pig;  2,  man;  3,  squirrel. 

The  color  of  the  crystals  and  their  solution  vary  according  to 
the  light  by  which  they  are  looked  at.  By  reflected  light  they 
are  bluish-red  or  greenish  in  color,  and  by  direct  light  scarlet. 

The  preparation  of  oxyhcemoglohin  crystals  is  accomplished  by 
first  separating  the  coloring  matter  from  the  corpuscles  by  freezing, 
or  the  addition  of  water  or  ether,  and  rendering  it  less  soluble  by 
evaporation,  cold,  and  the  addition  of  alcohol. 

For  microscopic  observation  it  generally  suffices  to  kill  a  rat 
with  ether,  and  expose  a  drop  of  the  blood  diluted  with  distilled 
water  on  a  slide  until  half  dried,  and  then  cover.  Crystals  ap- 
pear in  the  ffuid  as  it  becomes  more  concentrated. 


OXYHEMOGLOBIN.  233 

The  combinations  which  haemoglobin  enters  into  are  numerous, 
and  throw  much  light  upon  the  function  of  the  corpuscles. 

As  already  stated,  the  coloring  matter,  when  exposed  to  the 
air,  combines  with  oxygen  to  form  a  loose  chemical  compound 
called  oxyhtemoglobin.  This  is  the  condition  in  which  the  color- 
ing matter  of  the  blood  is  commonly  met  with.  Although  so 
prone  to  combine  with  oxygen,  the  oxyhgeraoglobin  very  readily 
parts  with  some  of  it.  lu  the  circulation  it  is  always  united  with 
oxygen,  normally  leaving  the  lungs  in  a  state  of  saturation.  On 
its  way  through  the  capillaries  of  the  tissues  it  parts  with  some  of 
its  oxygen,  becoming  more  or  less  reduced  (haemoglobin),  but  even 
the  most  venous  blood  always  contains  some  oxyhsemoglobiu. 

The  oxygen  can  be  removed  by  reducing  the  pressure  under  an 
air-pump,  or  by  exposing  the  solution  to  a  mixture  of  nitrogen 
and  hydrogen.  Various  reducing  agents  rob  the  oxyhtemoglobin 
of  its  ox3'gen  ;  and  if  blood  or  a  solution  of  oxyhiemoglobin  be 
sealed  in  a  glass  tube  so  as  to  exclude  the  air,  the  loose  oxygen 
is  taken  up  by  sonie  of  the  other  constituents  of  the  blood,  and 
the  oxyhaemoglobin  becomes  gradually  reduced  to  haemoglobin. 
This  depends  on  the  putrefactive  changes  in  the  proteids,  and 
may  be  prevented  by  careful  aseptic  precautions.  If  the  reduced 
haemoglobin  be  shaken  for  a  few  moments  with  air,  the  bright 
color  characteristic  of  oxyhaemoglobin  soon  reappears,  and  if  the 
reducing  agent  be  not  injurious  to  the  blood,  the  reduction  and 
reoxidation  may  be  repeated  several  times,  the  haemoglobin  going 
through  the  changes  which  take  place  in  it  during  normal  respi- 
ration. 

The  union  of  oxygen  with  haemoglobin  solutions  is  not  mere 
absorption  of  the  oxygen  by  the  liquid,  but  a  definite  chemical 
combination.  This  is  seen  from  the  following  facts :  (1.)  When 
the  pressure  is  removed  the  oxygen  does  not  come  away  from  the 
liquid  in  accordance  with  the  law  which  governs  the  escape  of 
absorbed  gas  (vide  p.  239).  (2.)  The  two  substances  give  a  dif- 
ferent result  when  examined  with  the  spectroscope.  The  reduced 
haemoglobin  gives  one  wide  diffuse  band,  which  lies  between  the 
D  and  E  lines  of  the  solar  spectrum,  and  much  of  the  violet  end 

20 


234 


MANUAL  OP  PHYSIOLOGY. 

Fig.  107. 


'  -^  W  M  rr*  "3  CD" 

The  Spectra  of  Oxyhjeraoglobin,  reduced  haemoglobin,  and  CO-haemo- 
globin.  (Ganigee.) — 1,  2,  3,  and  4.  Oxy haemoglobin  increasing  in  strength 
or  thickness  of  solution.     5.  Reduced  haemoglobin.     6.  CO-ha;moglobin. 


DECOMPOSITION  OF  HEMOGLOBIN — H.EMATJN,  ETC.     235 

is  cut  off.  This  spectrum,  which  is  characteristic  of  reduced  hoerao- 
globiu,  is  replaced  by  two  bauds  when  the  haemoglobin  combines 
with  oxygen — one  broad  baud  in  the  green  near  E,  and  a  narrow 
one,  more  clearly  defined,  in  the  yellow  close  to  the  D  line  ;  both 
bands  lie  between  D  and  E.  With  stroug  solutions  the  spectrum 
is  darkened  at  either  extremity,  and  the  two  bands  become  wider 
and  tend  to  fuse  into  one.  (3.)  Further,  the  oxygen  may  be  re- 
placed by  other  substances  which  unite  with  the  haemoglobin. 
One  of  the  most  important  of  these  is  carbonic  oxide,  which  forms 
a  much  more  stable  compound  with  haemoglobin  than  oxygen. 
It  is  of  a  bright  cherry-red  color,  aud  has  two  absorption  bands  in 
the  spectrum  very  like  those  of  oxyhsemoglobiu  ;  that  in  the  yellow 
i?,  however,  removed  a  greater  distance  from  the  D  line  towards 
the  violet  end. 

It  is  this  compound  which  is  formed  in  poisoning  with  carbonic 
oxide.  The  CO  occupying  the  place  of  the  oxygeu,  thus  destroys 
the  function  of  the  blood  corpuscles.  CO-hsemoglobin  may  be  dis- 
tinguished from  0-hfemoglobin  by  not  being  reduced  by  reagents 
greedy  of  oxygen,  aud  by  the  bright  red  color  which  appears  when 
10  per  cent,  solution  of  caustic  soda  is  added,  and  the  mixture 
heated.  O-hseraoglobiu  gives  a  muddy  brown  color  under  the 
same  treatment. 

Decomposition  of  Hemoglobin 

Haemoglobin  may  easily  be  broken  up  into  two  constituents — 
namely,  (a)  a  colorless  substance  which  is  nearly  related  to  the 
class  of  proteids  called  globulin,  and  (b)  a  blackish  red  amorphous 
material  called  Hcematin,  which  contains  all  the  iron  of  the  haemo- 
globin. 

This  change  is  brought-about  by  whatever  causes  the  coagula- 
tion of  albumin,  such  as  the  addition  of  acids,  strong  alkalies,  and 
heat  to  70^  C. 

HCEMATIN,  ETC. 

Hsematin  is  a  secondary  product,  being  the  result  of  the  oxida- 
tion of  a  substance  called  hajmochroraogen,  which  is  the  first  out- 
come of  the  decomposition  of  the  haemoglobin.     Haemochromogen 


236  MANUAL   OF   PHYSIOLOGY. 

can  only  be  obtained  in  an  atmosphere  of  hydrogen  or  nitrogen, 
as  it  immediately  takes  up  oxygen  to  form  hsematin.  The  form- 
ula CjgH,gN^Fo,,OiQ  has  been  given  for  hjematin.  It  dissolves  in 
weak  alkaline  and  acid  solutions,  but  not  in  water  or  in  alcohol. 
H£ematin  is  readily  prepared  by  mixing  acetic  acid  with  a  strong 
solution  of  hremoglobin,  which  becomes  a  dark  brown  color.  The 
dark  ha;matin  can  be  removed  by  ether.  But  if  the  acid  used  be 
strong,  the  solution  of  ha;matiu  is  found  to  be  free  from  iroD. 
This  iron-free  hrematiu  Preyer  calls  luematoin.  If  now  the  acid 
hsematin  solution  be  saturated  with  ammonia,  the  iron  again  be- 
comes united  with  the  hseraatoin,  forming  alkali-hsematin. 

HiEMIN. 

Hsematin  unites  with  hydrochloric  acid  to  form  a  crystallizable 
body  called  hcemin  or  hydrochlorate  of  hrematin  (Teichmann's 
crystals). 

If  blood  or  dry  hsematin  be  mixed  with  a  small  quantity  of 
common  salfr,  a  drop  of  glacial  acetic  acid  added,  and  the  mixture 

Fig.  108. 


Haemin  Crystals. 

boiled,  small  characteristic  crystals  appear  which  have  been 
found  to  be  produced  by  the  union  of  two  atoms  of  hydrochloric 
acid  with  the  hsematin. 

The  formation  of  these  crystals  is  very  easily  accomplished  with 
a  small  quantity  of  old  dried  blood ;  therefore  this  substance 
becomes,  in  medico-legal  inquiries,  an  important  test  for  blood 
stains. 

Crystals  of  a  substance  called  Hsematoidin  are  formed  in  old 
blood-clots  retained  in  the  body.  It  does  not  contain  any  iron, 
and  has  the  chemical  formula  Cj.^HjgN^Og.  It  is  probably  iden- 
tical with  bilirubin,  one  of  the  coloring  matters  found  in  bile. 


DEVELOPMENT   OF   THE   RED    DISKS.  237 


Globin. 

This  name  has  been  given  by  Preyer  to  the  proteid  part  of 
the  haemoglobin,  on  account  of  its  slightly  differing  from  globulin, 
though  it  resembles  it  in  being  precipitated  by  the  weakest  acids, 
even  carbon  dioxide. 

Chemistry  of  the  Stroma. 
The  stroma  forms  only  about  10  per  cent,  of  the  solid  parts  of 
the  corpuscles,  the  rest  being  bsemoglobin.  The  proteid  basis  of 
the  stroma  is  probably  mostly  made  up  of  a  globulin,  also  con- 
taining lecithin,  cholesterin,  and  fats  in  minute  proportions.  There 
is  little  more  than  one-half  per  cent,  of  inorganic  salts  in  the 
red  blood  corpuscles,  of  which  more  than  half  consists  of  potassium 
phosphate  and  chloride. 

Development  of  the  Red  Disks. 

In  the  early  days  of  the  embryo  the  bloodvessels  and  corpuscles 
appear  to  be  formed  at  the  same  time  from  the  middle  layer  of 
tbe  blastoderm  (mesoblast).  They  first  consist  of  round,  nucleated, 
colorless  cells,  which  subsequently  become  colored, gradually  lose 
their  nucleus,  and  assume  the  characteristic  shape  of  the  red  cor- 
puscles, the  rest  of  the  original  mass  of  protoplasm  remaining  as 
a  rudimentary  bloodvessel. 

In  the  later  stages  of  embryonic  life  the  red  corpuscles  are  said 
to  be  formed  in  the  liver,  possibly  out  of  protoplasmic  elements 
which  are  made  in  the  spleen  and  thence  carried  to  the  liver  by 
the  portal  circulation. 

In  the  connective  tissue  of  rapidly  growing  animals — tadpole 
(Kolliker),  rabbit  (Ranvier),  rat  (Schiifer) — certain  cells  can  be 
seen  to  be  connected  in  the  form  of  a  capillary  network,  and  within 
the  protoplasm  of  these  cells  red  coloring  matter  is  developed,  and 
the  particles  of  color  can  soon  be  recognized  as  characteristic 
blood  corpuscles,  arranged  in  rows  within  the  newly- formed  net- 
works. Thus  isolated  small  networks  of  capillaries,  consisting  of 
a  few  meshes  filled  with  blood  corpuscles,  are  formed  independently 
of  the  general  circulation. 


238  MANUAL  OF   PHYSIOLOGY. 

These  corpuscles  and  their  haemoglobin  are  manufactured  by 
isolated  protoplasmic  elements  in  the  connective  tissue,  and 
subsequently  added  to  the  general  mass  of  blood  by  the  growth 
of  the  network  bringing  it  into  continuity  with  the  neighboring 
vessels. 

In  the  adult  the  formation  of  red  blood  corpuscles  is  of  course 
much  less  active,  but  certainly  never  ceases  to  take  place  in 
health,  for  the  corpuscles  must  be  renewed  as  they  become  worn 
out,  and  incapable  of  performing  their  function.  This  reproduc- 
tion can  go  on  with  considerable  rapidity,  as  we  see  after  severe 
haemorrhage,  when  the  normal  richness  in  haemoglobin  and  cor- 
puscles is  soon  arrived  at.  Their  formation  is,  however,  probably 
confined  to  a  few  special  organs — spleen,  liver,  red  medulla  of 
bones — where  transitional  forms  are  found  in  such  numbers  as  to 
point  to  the  probability  of  the  red  corpuscles  being  the  offspring 
of  the  colorless  cells,  whose  protoplasm  either  manufactures  anew 
or  collects  the  necessary  haemoglobin,  and  then  loses  its  nucleus 
and  ordinary  cellular  characters. 

We  can  only  guess  at  the  fate  of  the  disks,  but  there  are  many 
things  which  point  to  the  spleen  as  the  organ  in  which  they  are 
destroyed.  In  the  spleen  an  enormous  number  of  protoplasmic 
elements  are  produced,  and  the  blood  comes  into  relationship  with 
the  nascent  cells  in  a  way  unknown  in  any  other  part  of  the  body. 
Further,  various  unusual  elements,  some  like  altered  red  cor- 
puscles, others  like  white  cells  enveloping  haemoglobin,  are  found 
in  this  organ. 

The  blood  corpuscles,  on  coming  to  the  spleen,  are  possibly  sub- 
mitted to  a  kind  of  preliminary  test  of  general  fitness,  some 
elements  of  the  spleen  pulp  having  the  faculty  of  examining 
their  condition  and  deciding  upon  their  fate.  Many,  no  doubt, 
pass  the  trial  without  any  change,  being  found  in  good  working 
order.  Others  that  are  found  totally  unfit  are  broken  up,  and 
their  effete  haemoglobin  carried  to  the  liver  to  be  eliminated  as 
bile  pigment.  Some  possibly  undergo  a  form  of  repair,  a  white 
cell  taking  charge  of  a  weakly  disk  renews  its  stroma,  adds  to  its 
haemoglobin,  and  carries  it  through  the  final  proof  in  the  liver, 
where  it  is  chemically  refreshed  before  going  to  the  lungs  for  the 
load  of  oxygen  which  it  has  to  carry  to  the  systemic  capillaries. 


the  gases  of  the  blood.  239 

The  Gases  of  the  Blood. 

These  are  present  in  two  conditions  :  i.e.,  (1)  dissolved  in  it 
in  accordance  with  well-established  physical  laws,*  and  (2)  chemi- 
cally combined.  But,  since  the  latter  are  but  loosely  combined, 
they  may  be  separated  by  the  same  means  as  the  former,  and  thus 
the  oxygen,  carbon  dioxide,  and  nitrogen,  can  all  be  removed  by 
reducing  the  pressure  with  the  air-pump.  For  this  purpose  a 
mercurial  pump  must  be  used,  by  means  of  which  a  practically 
perfect  vacuum  can  be  formed,  and  all  the  gases  obtained  in  a 
manner  which  facilitates  further  analysis.  Together  they  are 
found  to  measure  about  60  volumes  for  every  100  volumes  of 
blood. 

Cxygen. — The  amount  of  oxygen  in  the  blood  is  found  to  vary 
much  with  circumstances.  In  arterial  blood  the  quantity  is  much 
more  constant,  and  always  exceeds  that  in  venous  blood.  It  is 
estimated  (at  0°  C.  and  760  mm.  pressure)  that  every  100  volumes 
of  arterial  blood  yield  20  volumes  of  oxygen,  whilst  the  volume 
of  oxygen  in  venous  blood  varies  from  8  to  12  per  cent. 

The  oxygen  which  comes  off  in  the  Torricellian  vacuum  exists 
in  the  blood  in  two  distinct  states:  (1)  a  very  small  quantity 
simply  absorbed — about  as  much  as  water  absorbs  under  atmos- 
pheric pressure;  (2)  chemically  combined,  in  which  state  nearly 
all  the  oxygen  exists,  and  forms  with  the  haemoglobin  a  loose 
combination  called  oxyhsemoglobin.  This  oxygen,  therefore,  does 
not  follow  the  laws  of  absorption  by  leaving  the  blood  in  propor- 
tion as  the  pressure  is  reduced,  but  at  a  certain  point  of  reduction 

*  I.  A  given  liquid  absorbs  the  same  volume  of  a  given  gas  independent  of 
the  pressure  exercised  by  that  gas. 

II.  At  the  same  temperature  the  volume  of  a  gas  varies  inversely  with  the 
pressure,  so  that  with  twice  the  pressure  a  given  volume  of  gas  is  twice  the 
weight. 

III.  Therefore,  the  amount  by  ueight  of  gas  absorbed  by  a  liquid  depends 
directly  on  the  pressure,  being  nil  in  vacuo. 

Tlie  weight  of  a  given  volume  of  gas  decreases  as  the  temperature  in- 
creases ;  therefore,  the  amount  of  gas  absorbed  is  in  inverse  proportion  to  the 
temperature,  being  practically  niY  at  boiling  point. 


240  MANUAL   OF   PHYSIOLOGY. 

of  pressure  (20  inm.  mercury)  the  oxygen  comes  off'  almost  com- 
pletely. 

Carbon  Dioxide  (COJ. — The  amount  of  carbon  dioxide  also 
varies  more  in  venous  than  in  arterial  blood,  for  under  certain 
circumstances  (suffocation)  it  may  rise  to  over  CO  volumes  per 
cent.,  although  ordinary  venous  blood  on  an  average  contains 
only  46  volumes  in  every  100.  On  the  other  hand,  the  amount 
of  this  gas  in  arterial  blood  varies  little  from  39  volumes  per 
cent. 

The  larger  proportion  of  carbon  dioxide  exists  in  the  plasma, 
where  it  appears  to  be  chemically  combined  with  soda  salts. 

Nitrogen. — The  amount  of  nitrogen  does  not  vary  much,  being 
in  both  venous  and  arterial  blood  about  1.5  volume  per  cent., 
and  it  would  appear  to  be  simply  absorbed. 

For  further  details  about  arterial  and  venous  blood,  see  Respira- 
tion. 


CHAPTER  XY. 

COAGULATION  OF  THE  BLOOD. 

In  speaking  of  the  chemical  relationship  of  the  plasma  (see  p. 
216),  the  formation  of  fibrin  has  been  mentioned  as  the  essential 
item  in  coagulation,  and  the  relation  of  fibrin  to  its  probable  pre- 
cursors has  been  discussed.  If  the  points  there  explained  be  borne 
in  mind,  and  the  presence  of  the  corpuscles  be  taken  into  account, 
the  various  characteristics  of  the  clot  which  forms  when  blood  is 
shed  into  a  vessel  can  be  easily  understood,  and  should  require  no 
further  description.  The  great  importance  of  the  coagulation  of 
the  blood  in  pathological  processes  makes  it  expedient,  however, 
to  consider  more  closely  the  steps  of  the  process  as  well  as  the 
various  circumstances  under  which  it  occurs  after  its  removal,  as 
well  as  in  the  living  vessels.. 

Before  the  formation  of  a  perfect  clot,  blood  may  be  seen  after 
it  is  shed  to  pass  through  three  stages :  1,  viscous  ;  2,  gelatinous  ; 
3,  contraction  of  clot  and  separation  of  serum. 

The  first  stage  is  very  short  and  in  thin  layei's  of  blood  passes 
immediately  into  the  second.  With  considerable  quantities  of 
blood,  contained  in  deep  vessels,  the  central  parts  take  some  little 
time  to  turn  into  a  firm  jelly,  so  that  the  completion  of  the  second 
stage  may  occupy  from  one  to  thirty  minutes. 

After  from  ten  to  fifteen  hours  the  third  stage  begins ;  clear 
drops  of  serum  appear  about  the  clot,  which  contracts  until  it 
forms  but  a  comparatively  small  mass  floating  in  the  serum.  If 
the  jelly-like  clot  be  disturbed,  the  serous  fluid  makes  its  appear- 
ance much  sooner  than  the  time  just  stated. 

During  the  formation  of  the  clot  under  ordinary  circumstances 
the  corpuscles  are  entangled  in  the  meshwork  of  fibrin,  so  that 
the  gelatinous  mass  has  throughout  a  dark  red  color. 

If  the  coagulation  takes  place  slowly — as  it  does  in  very  cold 
weather,  in  horses'  blood,  or  in  human  blood  if  removed  from  a 


242 


MANUAL   OF    PHYSIOLOGY. 


person  during  fever — then  the  heavier  red  corpuscles  have  time 
to  subside  to  the  lower  layers  of  the  clotting  plasma,  while  the 
white  cells  are  caught  in  the  meshes  of  the  fibrin  and  remain  in 
the  upper  layer  of  the  clot,  which  then  has  the  pale  color  familiar 
to  the  physician  in  the  old  days  of  bleeding  as  the  "bufTy  coat," 
or  crusta  phlogistica.  This  huffy  coat  contains  a  greater  proportion 
of  the  elastic  fibrin  and  soft  white  cells  than  the  rest  of  the  clot, 
and  incloses  but  few  red  corpuscles,  therefore  the  fibrin  can  con- 
tract more  completely  in  this  upper  layer  than  in  the  deeper  part 
of  the  clot,  which  includes  the  red  corpuscles.     The  effect  of  this 

Fig.  109. 


Reticulum  of  Fibrin  Threads  after  staining  has  made  them  visible.     The 
network  (6)  appears  to  start  from  granular  centres  (a).     (Ranvier.) 

is,  that  the  upper  surface  becomes  concave,  and  a  "cupped"  clot 
is  formed.  The  contraction  of  the  clot  proceeds  for  days,  and  in 
order  to  see  the  characters  described  above,  the  blood  should  be 
kept  in  a  cool  place  and  perfectly  motionless. 

The  contraction  of  the  fibrin  and  separation  of  the  serum  can 
be  made  to  take  place  much  more  quickly  by  gentle  agitation 
causing  the  ends  of  the  fibrin  threads  to  separate  from  the  sides  of 
the  vessel,  but  by  thus  disturbing  the  clot,  during  its  formation, 


COAGULATION  OF  THE  BLOOD,  243 

the  corpuscles  are  displaced  and  escape  iuto  the  serum,  which  is 
thus  stained  and  cannot  be  seen  in  its  clear  transparent  state. 

If  brisk  agitation  with  a  glass  rod — or  better,  a  bundle  of 
twigs — be  commenced  the  moment  the  blood  is  drawn,  the  fibrin 
is  formed  more  rapidly,  but  the  corpuscles  are  not  entangled  in 
its  meshes,  for  as  quickly  as  the  elastic  threads  are  formed  they 
adhere  firmly  to  the  rod  or  twigs.  Thus  the  fibrin  is  formed  very 
rapidly,  and  the  ordinary  blood-clot,  consisting  of  fibrin  and  the 
corpuscles,  does  not  appear,  for  the  fibrin  is  separated  from  the 
latter  during  the  coagulation.  We  then  have  what  is  commonly 
spoken  of  as  "defibrinated  blood,"  which  does  not  give  a  clot. 
Not  that  the  clotting  has  been  prevented,  but  the  material  essen- 
tial for  the  formation  of  a  clot  has  been  removed  as  quickly  as 
formed,  and  instead  of  catching  the  corpuscles  in  the  meshes  of  its 
delicate  fibrils  to  form  the  clot  in  the  ordinary  way,  the  stringy 
shreds  of  fibrin  cling  around  the  beating-rod  as  a  jagged  mass. 
The  following  tables  show  the  relation  of  the  different  constituents 
of  coagulated  and  defibrinated  blood  respectively  : 

f  Serum  (appearing  as  clear 
T  •   .       -Di      1         f  Plasma       ]  I       fluid). 

Living  Blood  =  {corpuscles  j  =  1  Fibrin 


I  ^  ,      ^  =  Blood  clot. 

l^  Corpuscles 


( Fibrin    (removed    on    the 
(-  -pi  -)  I       rod). 

Living  Blood  =  \  ^'^^'"^        '         ^  )   _  Defibri- 

^  I  Corpuscles  j  j  Serum  f   —      ^^^^^ 


j^  Corpuscles  j  ^^^^^ 

Many  circumstances  influence  the  rapidity  with  which  a  blood 
clot  is  formed.  Speaking  generally,  the  removal  of  the  blood  from 
its  normal  supply  of  nutrition  and  from  the  opportunity  of  pre- 
serving the  necessary  equilibrium  of  chemical  interchange  be- 
tween the  corpuscles,  the  plasma,  and  the  tissues — in  short,  cir- 
cumstances which  tend  to  injure  the  corpuscles  or  the  plasma, 
and  promote  the  changes  resulting  in  their  death — must  hasten 
coagulation  ;  while,  on  the  other  hand,  the  conditions  which  pro- 
tect the  corpuscles  and  impede  the  stages  in  fibrin  formation  must 
retard  coagulation. 


244  MANUAL   OF   PHYSIOLOGY. 

These  may  be  arranged  categorically,  viz. 

A.  Circumstances  promoting  coagulation  : 

1.  Contact  uiih  foreign  bodies  is  of  the  first  importance  in 

hasteuing  coagulation.  The  greater  the  surface  of  con- 
tact with  the  vessel  or  the  air,  the  more  the  corpuscles 
are  exposed  to  injury,  and  the  more  rapid  are  the 
destructive  chemical  changes  inducing  fibrin  formation. 
Thus  a  drop  or  two  of  blood  falling  on  any  surface  so  as 
to  spread  out  in  a  thin  layer  clots  almost  instantly. 

2.  Motion,  by  renewing  the  points  of  contact  between  the 

blood  and  the  moving  agent,  hastens  coagulation.  Thus, 
by  whipping  fresh  blood,  all  the  fibrin  can  be  removed 
in  a  few  minutes,  and  the  defibrinated  blood  left  without 
a  clot. 

3.  Moderate  heat.  The  formation  of  the  fibrin  generators  and 

the  action  of  the  ferment  seem  to  go  on  more  rapidly  at 
38°-40°  C.  than  any  other  temperature. 

4.  A  tvatery  condition  of  the  blood  causes  rapid  coagulation 

but  a  soft  clot.  This  is  seen  in  repeated  bleedings  or 
hseraorrhages ;  the  blood  which  flows  last  clots  first. 

5.  The  addition  of  a  small  quantity  of  water  by  setting  up 

rapid  changes  in  the  corpuscles  accelerates  coagulation. 

6.  A  supply  of  oxygen. — Oxygen  is  used  up  in  the  chemical 

changes  attendant  upon  the  death  of  the  blood,  and  its 
])resence  aids  the  formation  of  firm  clots,  such  as  are 
produced  in  arterial  blood.  Exposure  to  the  air  in  a 
shallow  vessel  facilitates  coagulation,  partly  by  exten- 
sive contact  and  partly  by  a  free  supply  of  oxygen.  But 
exposure  to  air  is  not  necessary,  for  blood  collected  in 
mercury,  without  ever  coming  in  contact  with  the  air, 
coagulates  very  rapidly. 

B.  Circumstances  which  retard  coagulation  : 

1.  Constantly  renewed  and  close  inter-relationship  with  the 
lining  of  healthy  bloodvessels  alone  aflfords  the  require- 
ments essential  for  the  preservation  of  the  living  cor- 
puscles and  plasma  in  their  normal  condition. 


CIRCUMSTANCES    INFLUENCING   COAGULATION.         245 

2.  When  the  blood  is  surrounded  by  healthy  living  tissues, 

interchanges  may  occur  between  them,  and  if  the  oxygen 
supply  is  deficient,  coagulation  is  much  delayed.  Thus 
considerable  quantities  of  blood  effused  into  the  tissues 
may  be  liquid  and  black  for  many  days  after  its  escape 
from  the  vessels.  This  dark  blood  clots  on  removal  and 
exposure  to  the  air. 

3.  Low  temperature. — The  rate  of  coagulation  decreases  with 

a  temperature  below  38°  C,  and  the  process  is  checked 
at  0°  C. 

4.  A  great  quantity  of  water  seems  to  render  the  action  of  the 

fibrin  factors  weak. 

5.  The  addition  of  egg  albumin,  syrup  or  glycerin,  retards  the 

process. 

6.  The  addition  of  concentrated  solutions  of  neutral  salts 

(about  three  volumes  of  30  per  cent,  solution  of  magne- 
sium sulphate)  quite  prevents  coagulation. 

7.  The  addition  of  small  quantities  of  alkalies. 

8.  The  addition  of  acetic  acid  until  very  slight  acid  reaction 

is  obtained. 

9.  Increase  in  the  amount  of  carbon  dioxide.    This,  together 

with  the  want  of  oxygen,  explains  why  venous  blood 
clots  more  slowly  and  loosely  than  arterial,  and  why  the 
blood  in  the  distended  right  side  of  the  heart  is  commonly 
liquid  after  death  from  suffocation. 

10.  The  blood  of  persons  suffering  from  inflammatory  dis- 

ease coagulates  slowly,  but  forms  a  very  firm  clot. 

Since  the  blood  coagulates  spontaneously  when  removed  from 
the  body,  the  question  now  arises,  how  does  it  remain  fluid  in  the 
bloodvessels? 

Though  this  question  has  long  occupied  much  attention,  it  is 
still  difficult  to  formulate  a  definite  answer.  Nor  can  we  expect 
to  find  any  adequate  explanation  until  we  are  better  acquainted 
with  the  exact  details  of  the  origin  of  the  fibrin  generators.  It 
must  be  remembered  that  the  blood  should  be  regarded  as  a  tissue, 
made  up  of  living  constituents  requiring  constant  assimilation  and 
elimination  for  the  maintenance  of  its  perfectly  normal  conditions 


246  MANUAL   OF   PHYSIOLOGY. 

and  life.  One  can  confidently  say  that  coagulation  is  the  out- 
come of  certain  chemical  changes  concomitant  with  the  death  of 
this  tissue,  and  that  while  the  tissue  lives  no  such  changes  take 
place.  But  such  an  answer  adds  little  to  our  knowledge  of  the 
matter. 

Since  constant  chemical  intercourse  must  be  kept  up  between 
the  blood  and  its  surroundings  in  order  to  sustain  the  complex 
clii'inical  integrity  essential  for  its  life,  we  cannot  be  surprised 
that  its  waste  materials  accumulate,  and  that  it  soon  dies  when 
shed,  just  as  other  tissues  do  when  deprived  of  their  means  of  sup- 
port. The  formation  of  a  solid  and  the  separation  of  a  liquid 
form  of  proteid  is  in  no  way  unusual  as  a  first  step  in  the  decline 
from  exalted  chemical  construction,  for  similar  changes  occur  in 
other  tissues,  and  in  protoplasm  itself  The  soft  contractile  sub- 
stance of  muscle,  probably  during  its  contraction,  and  certainly 
at  its  death,  tends  to  undergo  almost  exactly  the  same  kind  of 
change  as  the  blood  in  coagulation. 

If  we  knew  accurately  the  nutritive  process  taking  place  in  the 
blood  itself,  and  with  which  of  its  surroundings  it  keeps  up  chemi- 
cal interchange,  the  answer  would  be  much  simplified.  But  we 
have  in  the  blood  three  elements  that  probably  have  different 
modes  of  assimilation  and  elimination,  viz.,  plasma,  white  cells, 
and  red  disks.  But  we  practically  know  nothing  of  the  changes 
they  undergo  during  their  nutrition  ;  or  whether  their  tissue- 
changes  have  any  necessary  relation  to  those  of  the  neighboring 
tissues.  We  do  know,  however,  that  there  exists  some  very  inti- 
mate relation  between  the  membrane  lining  the  vessel  walls  and 
the  contained  blood.  They  seem  to  require  frequently-repeated 
contact  one  with  the  other  in  order  that  the  normal  condition  of 
both  may  be  maintained  in  perfect,  vital  integrity.  Thus  fresh 
supplies  of  blood  are  required  by  the  vessel  wall,  for,  when  de- 
prived of  its  nutriment  by  a  stoppage  of  the  blood-flow,  it  soon 
loses  its  power  of  retaining  the  blood,  and  admits  of  extravasa- 
tion ;  and  renewed  contact  with  the  vessel  wall  is  equally  neces- 
sary for  the  blood,  for  the  cells  congregate,  and  the  plasma,  when 
the  stasis  becomes  injurious  to  the  intima,  coagulates.  Probably 
the  chemical  changes  going  on  in  the  one  are  useful  for  the  nutri- 


CAUSE   OP   COAGULATION.  247 

tion  of  the  other,  and  that  they  mutually  supply  one  another  with 
some  material  essential  for  their  life.  This  is  apparent  in  those 
cases  where  coagulation  takes  place  during  life  iu  the  vessels.  It 
never  occurs  so  long  as  the  intima  of  the  vessel  is  perfect,  and  the 
blood-flow  constant,  but  it  follows  lesion  of  this  delicate  mem- 
brane whether  caused  by  injury  or  by  mal-nutrition. 

The  gradual  occurrence  of  this  impairment  of  function  of  the 
intima  can  be  watched  under  the  microscope  iu  the  small  vessels 
of  a  transparent  part  during  the  initial  stages  of  inflammation. 
Owing  to  the  arrest  of  the  flow  of  blood,  the  walls  of  the  small 
vessels  suffer  from  defective  nutrition,  and  may  be  seen  to  allow 
some  elements  to  escape,  while  the  disks  adhere  together  and  the 
blood  coagulates. 

In  the  larger  vessels  the  same  thing  occurs  when  inflammation 
of  their  lining  membrane  destroys  its  capability  of  keeping  up 
the  necessary  nutritive  equilibrium.  Thus  clots  form  on  the  inner 
lining  to  the  walls  of  an  inflamed  vein,  often  growing  so  as  to 
fill  the  entire  vessel,  and  give  rise  to  a  condition  called  thrombosis. 

On  the  left  valves  of  the  heart  and  in  the  arteries,  where  the 
delicate  intima  is  subjected  to  great  mechanical  strain,  it  is  com- 
mon enough  to  find  slight  injuries  of  it  covered  over  with  ihin 
clots.  To  the  surgeon  this  mutual  nutrition  of  intima  and  blood 
is  of  the  utmost  importance  in  studying  the  occlusion  of  vessels, 
for  it  is  upon  this  fact  he  has  mainly  to  depend  for  the  stoppage 
of  haemorrhage  from  a  wounded  artery.  A  tightly-tied  ligature 
either  injures  the  inner  coats  mechanically,  or  starves  the  intima 
by  checking  the  flow  of  blood  through  the  vessel  up  to  the  next 
branch,  and  that  portion  of  the  vessel  is  filled  with  stationary 
blood,  which  soon  clots  and  forms  an  adherent  plug.  But  if  the 
ligature  be  applied  too  loosely,  a  slight  blood-current  passes 
through  the  point  where  the  vessel  is  tied,  and  this  suffices  for  the 
nutrition  of  the  intima  by  the  renewal  of  the  blood's  contact,  so 
that  no  clot  is  formed,  the  vessel  is  not  closed,  and  most  probably 
when  the  ligature  has  cut  through  the  outer  coat  secondary  haem- 
orrhage occurs. 

It  has  also  been  shown  that  if  any  foreign  substance,  such  as  a 
thread,  be  introduced  into  the  blood  while  circulating,  a  coagulura 


248  MANUAL   OF   PHYSIOLOGY. 

will  form  arouud  it.  From  this  it  would  appear  that  the  presence 
of  a  substauce  which  cannot  carry  on  the  necessary  chemical  in- 
tercourse with  the  blood  will  excite  irritation  in  its  elements,  and 
so  effect  slight  local  death  of  the  plasma  and  the  production  of 
fibrin. 

The  time  required  for  the  production  of  intravascular  coagula- 
tion as  a  result  of  mere  stasis  is  happily  long,  for  it  has  been  found 
that  the  blood  current  may  be  stopped  in  a  limb,  by  pressure  or 
otherwise,  for  many  hours  without  coagulation  occurring.  In- 
deed, cases  have  occurred  where  a  tight  bandage  has  stopped  the 
circulation  for  an  entire  day  without  coagulation  taking  place. 
This  is  explained  by  the  fact  that  so  long  as  the  intinia  lives,  the 
blood  remains  fluid  ;  in  short,  the  tissues  die  before  the  blood 
clots  in  the  vessels. 

The  tissues  continue  to  live  for  some  time  after  an  animal  is 
dead,  and  so  we  see  the  blood  remains  fluid  in  the  vessels  a  long 
time  after  death  ;  in  fact,  as  long  as  the  vessel  wall  can  nourish 
itself  and  live.  Thus  it  has  been  shown  that  blood  in  a  horse's 
jugular  vein  separated  by  ligature  from  the  circulation,  and 
removed  from  the  animal,  will  remain  fluid  for  fully  twenty-four 
hours. 

In  cold-blooded  animals  the  tissues  live  for  even  a  longer 
time.  The  heart  of  the  tortoise,  if  kept  under  suitable  conditions, 
will  beat  for  two  days  when  removed  from  the  body,  and  as 
Briicke  has  shown,  blood  contained  in  it  will  remain  fluid  until 
after  the  heart  is  dead. 

If  the  details  of  fibrin  formation  be  followed  within  the  blood- 
vessels, it  is  found  that  the  injured  spot  or  foreign  body  first  be- 
comes covered  over  with  white  corpuscles,  around  which  threads 
of  fibrin  appear  attached  to  the  rough  surface.  As  more  fibrin  is 
formed  and  the  layer  thickens,  only  a  few  cells  can  be  seen  in  its 
meshes,  but  a  great  number  always  exist  on  the  surface  of  the  new 
fibrin,  forming  a  layer  between  it  and  the  blood.  It  is,  moreover, 
remarked  that  coagulation  has  some  relation  to  the  abundance  of 
white  cells  in  all  spontaneously  coagulating  fluids.  The  more 
cells,  the  firmer  the  clot.  In  pathological  exudations,  also,  and 
those  acute  serous  collections  which  coagulate  on  removal  from 


FIBRIN   FORMATION.  249 

the  body,  fine  granular  threads  of  fibrin  seem  to  start  from  the 
white  cells,  and  radiate  from  them  in  a  stellate  manner. 

Alex.  Schmidt  believes  that  a  great  number  of  white  blood 
cells  undergo  chemical  disintegration  the  instant  the  blood  is 
shed,  and  he  considers  that  the  fibrin  ferment,  and  probably  other 
fibrin  generators,  are  the  result  of  the  destruction  of  these  weak 
cells;  and  he  excludes  the  red  corpuscles  from  taking  any  share 
in  the  process. 

However,  there  is  good  evidence  that  the  plasma  and  the  disks 
can  give  rise  to  all  the  fibrin  factors,  and  we  know  that  in  the 
circulation  white  cells  must  be  destroyed  and  yet  cause  no  coag- 
ulation. 

Moreover,  if  some  blood  be  allowed  to  flow  into  a  fine  capillary 
tube,  the  white  cells  can  be  seen  to  move  away  from  the  red  disks, 
and  the  formation  of  the  clot — a  delicate  fibrin  network  inclosing 
the  disks — may  be  watched.  Here  the  white  cells  exhibit  mani- 
festations of  life  for  a  considerable  time  after  the  clot  has  been 
formed,  and  tbeir  death  could  not  have  been  the  source  of  the 
fibrin  factors. 

In  conclusion,  then,  we  can  only  suppose  that  as  in  other  tissues 
some  chemical  changes  must  go  on  in  the  elements  of  the  blood. 
These  changes  give  rise  to  new  products  which  may  produce 
fibrin,  and  hence  cause  coagulation.  But  so  long  as  the  elements 
of  the  blood  are  frequently  brought  into  close  relationship  with  a 
healthy  vessel  wall,  the  fibrin  factors  are  either  produced  in 
such  small  quantity  as  to  be  ineffectual,  or  they  are  altered, 
destroyed,  or  taken  up  by  the  intima  and  possibly  utilized  for  its 
nutrition.  When  the  blood  is  removed  from  the  vessels,  the  pro- 
duction of  the  fibrin  factors  proceeds  effectually,  either  on  account 
of  the  blood  elements  undergoing  destructive  changes,  and  accu- 
mulating the  products — fibrin  generators;  or  owing  to  the  im- 
possibility of  re-integration,  the  fibrin  factors  suddenly  appear  as 
a  product  of  lethal  chemical  change  or  decomposition. 

In  accepting  the  first  view,  we  only  adopt  the  theory  of  John 
Hunter,  who  thought  coagulation  was  an  act  of  life.  If  we  adopt 
the  other  view,  we  must  needs  say  it  is  an  act  of  death.     But, 

21 


i60  MANUAL  OF   PHYSIOLOGY. 

after  all,  this  is  a  mere  difference  in  degree,  for  how  can  we  dis- 
tinguish between  the  failure  of  a  tissue  to  re-integrate  or  repair 
its  normal  chemical  changes  upon  which  its  life  depends,  and  the 
inevitable  result  of  this  failure  (if  prolonged  beyond  a  certain 
point)  namely,  its  death  ? 

When  white  cells  congregate  at  a  point  from  which  the  intima 
is  stripped  from  a  vessel,  their  more  active  exertion  possibly  pro- 
duces more  ferment,  etc.,  and  at  the  same  time  they  remain  at  the 
injured  part  of  the  vessel  wall,  and  the  removal  of  the  fibrin  factors 
cannot  occur,  since  the  intima  is  destroyed  ;  hence  local  clots  are 
formed  which  extend  over  the  injured  surface,  and  by  a  process 
of  organization,  probably  the  repair  of  the  denuded  patch  can  be 
accomplished. 


CHAPTER  XVI. 


THE  HEART. 


Fig.  110. 


The  course  taken  by  the  blood  on  its  way  to  the  various  parts 
of  the  body  is  called  the  circulation,  on  account  of  its  having  to 
make  repeatedly  the  circuit  of  vessels  leading  from  and  to  the 
heart.  The  heart  is  the  great  motor  power  which  drives  the  blood 
through  all  the  vessels,  of  which  there  is  one  set  leading  from 
and  to  the  organs  of  the  system  generally,  and  another  set  lead- 
ing to  and  from  the  lungs. 

Anatomists  speak  of  two  circulations — the  greater  or  systemic, 
and  the  lesser  or  pulmonary.  How- 
ever, if  we  follow  the  course  of  the 
blood,  we  see  that  both  these  sets 
of  vessels  really  belong  to  the  one 
circulation,  and  in  fact  form  but 
one  circuit.  The  blood  passing 
through  the  lungs  and  systemic 
vessels  consecutively  visits  the 
heart  twice  on  its  way,  in  order  to 
acquire  the  force  necessary  to 
overcome  the  resistance  of  the  two 
sets  of  capillaries.  Although  in 
all  the  higher  animals  the  heart 
forms  but  a  single  organ,  it  prac- 
tically is  composed  of  two  muscular  pumps  which  are  anatomically 
united  but  distinct  in  function.  In  its  course  round  the  circula- 
tion the  blood  visits  each  of  these  functionally  distinct  hearts  at 
quite  different  parts  of  its  circuit.  The  right  heart  is  placed 
before  the  pulmonary  vessels  and  pumps  the  blood  through  the 
lungs.  The  left  heart  is  placed  before  the  systemic  vessels  and 
pumps  the  blood  through  the  body  generally.  Thus  anatomically 
there  appear  to  be  two  circulations  and  but  one  heart;  physio- 


L.H. 


Diagram  of  Circulation,  show- 
ing right  (r.h.)  and  left  (l.h.) 
hearts,  and  the  pulmonary  (p)  and 
systemic  (s)  sets  of  capillaries. 


252 


MANUAL  OF   PHYSIOLOGY. 


logically  there  is  one  circulation  and  two  hearts  ;  or  two  points  of 
resistance  and  two  distinct  pumping  organs  to  drive  the  blood 
through  the  obstacles. 

The  circulation  might  then  be  represented  by  a  simple  diagram 
(Fig.  110)  in  which  the  direction  of  the  current  is  indicated  by 

the  arrows,    L.  H.  shows  the 
Fig.  111.  position  of  the  left  or  syste- 

mic pump,  and  S.  the  resist- 
ance iu  the  systemic  vessels. 
R.  H.  represents  the  pul- 
monary pump,  and  P.  the 
second  obstacle  in  the  cir- 
cuit, viz.,  the  vessels  of  the 
lungs. 

However,  it  must  be  re- 
membered  that    the    right 
and  left  pumping  organs  are 
fused  into  one  viscus,  which 
has  two  distinct  and  separate 
channels  for  the  pa.ssage  of 
the   blood   through  it.     In 
each  system  of  bloodvessels 
we   have  the  same  general 
arrangement  for  the  distri- 
bution and  re-collection  of 
the  blood. 
In  passing  from  either  the  right  or  left  side  of  the  heart  the 
blood  flows  into  tubes  called  arteries,  which  divide  and  subdivide 
until  the  branches  become  microscopical  in  size.     From  the  very 
minute  arteries  the  blood  passes  into  the  capillaries,  which  can- 
not be  said  to  branch  but  are  arranged  so  as  to  form  a  network  of 
delicate  tubes  with  more  or  less  close  meshes,  according  to  the  part. 
Connected  with  the  meshes  of  the  capillaries  are  other  small 
vessels  which  collect  the  blood   from  the  networks  (Fig.  111). 
These  unite  one  with  another  to  form  larger  vessels  which  again 
are  but  the  tributaries  of  the  larger  veins  which  bear  the  blood 
back  to  the  heart. 


Capillary  Network  of  the  Choroid  of  a 
Child  of  a  few  months  old.  (Cadiat.)— 
(a)  Artery.  (6)  Vein,  and  capillary  net- 
work intervening. 


CIRCULATION    OF   THE    BLOOD. 


253 


Fig.  112. 


About  three  hundred  years  ago  the  true  course  of  the  blood 
current  through  the  systemic  and  pulmonary  heart,  arteries,  and 
veins,  so  as  to  form  one  circle,  was  demonstrated  by  Harvey. 
Before  his  time  only  the  so-called  "  lesser  "  or  pulmonary  circuit 
was  known.  But  the  magnifying  glasses  at  his  disposal  did  not 
enable  him  to  see  the  capillaries 
which  were  first  described  by  Mal- 
pighi  some  fifty  years  later. 

Jn  the  hope  of  making  the  gen- 
eral differences  of  functions  more 
striking,  the  various  parts  of  the 
circulatory  apparatus  may  be  enu- 
merated according  to  their  several 
duties  and  roughly  illustrated  by 
a  diagram  (Fig.  112)  : 

1.  The  left  (systemic)  heart  (L. 
H.)  pumps  the  blood  into  the  sys- 
temic arteries,  and  thus  keeps  these 
vessels  over-filled. 

2.  The  larger  systemic  arteries 
(A.),  by  their  elasticity  exert  con- 
tinuous pressure  on  the  blood  with 
which  they  are  distended. 

3.  The  smaller  systemic  arterioles 
(A-.),  by  their  vital  contractility, 
check  and  regulate  the  amount  of 
blood  flowing  out  of  the  larger 
arteries  into  the  capillaries,  and 
thus  keep  up  the  tension  of  the 
larger  arteries. 

4.  The  systemic  capillaries  (S.  C),  where  the  essential  opera- 
tion of  the  blood  is  carried  out,  viz.,  the  interchange  between  it 
and  the  tissues. 

5.  The  wide  systemic  veins  (V.)  are  the  passive  channels  con- 
veying the  impure  blood  to  the  pulmonary  heart. 

6.  The  right  (pulmonary)  heart  (R.  H.),  pumps  the  blood  into 
the  pulmonary  arteries  and  over-fills  them. 


Diagram  of  the  Circulation  of 
the  Blood  and  the  absorbent  ves- 
sels.    For  details  see  text. 


254 


MANUAL   OF   PHYSIOLOGY. 


7.  The   pulmonary  arteries  (P.  A.)  press  steadily  upon  the 
blood  and  force  it  through  the  following,  viz. : 

8,  Small  pulmonary  arterioles  (P.  a.),  which  regulate  the  flow 
into  the  capillaries. 


Fig.  113. 


Interior  of  Eight  Auricle  and  Ventricle  exposed  by  the  removal  of  a  part 
of  their  walls.  (Allen  Thomson.) — 1.  Superior  vena  cava.  2.  Inferior  vena 
cava.  1'.  Hepatic  veins.  3,  3',  Z" .  Inner  wall  of  right  auricle.  4,  4. 
Cavity  of  right  ventricle.  4'.  Papillary  muscle.  5,  h\  h" .  Flaps  of  tri- 
cuspid valve.  6.  Pulmonary  artery,  in  the  wall  of  which  a  window  has 
been  cut.     7.  Placed  on  aorta  is  near  the  ductus  arteriosus. 

9.  The  pulmonary  capillaries  (P.  C),  where  the  blood  is  ex- 
posed to  the  air,  and  undergoes  active  gas-interchange. 

10.  The  pulmonary  veins  (P.  V.),  carrying  the  blood  to  the 
left  heart,  and  thus  completing  the  circuit. 


ANATOMY   OF   THE    HEART. 


255 


LA.  indicates  the  lymphatics,  which  drain  the  tissues,  and  Lc. 
the  lacteals,  which  absorb  from  the  stomach  and  intestines  (I). 


Fig.  114. 


The  Left  Auricle  and  Ventricle  opened  and  part  of  their  walls  removed 
to  show  their  cavities.  (Allen  Thomson.) — 1.  Eight  pulmonary  vein  cut 
short.  1''.  Cavity  of  left  auricle.  3.  Thick  wall  of  left  ventricle.  4.  Por- 
tion of  the  same  with  papillary  muscle  attached.  5,  5'.  The  other  papil- 
lary muscles.  6.  One  segment  of  the  mitral  valve.  7.  In  aorta  is  placed 
over  the  semilunar  valves. 

Although  the  blood  enters  the  arteries  by  jerks,  the  motion 
through  the  capillaries  is  constant.     The  reason  of  this  is,  that 


256  MANUAL   OF   PHYSIOLOGY. 

the  arteries  are  constantly  over-full,  their  elastic  walls  being  dis- 
tended by  the  pumping  of  the  heart,  which  fills  the  aorta  and 
arteries  more  quickly  than  they  can  empty  themselves,  until  the 
adequate  pressure  is  attained  through  the  contracting  arterioles. 
The  arterioles  are  the  chief  agents  in  resisting  the  outflow,  and 
keeping  up  the  arterial  pressure. 

The  Heart. 

The  heart  of  man  and  other  warm-blooded  animals  may  be 
said  to  be  made  up  of  two  muscular  sacs,  the  pulmonary  and 
systemic  hearts,  or,  as  they  are  commonly  termed,  the  right  and 
left  sides  of  the  heart,  between  which  no  communication  exists  in 
the  adult.  Each  of  these  sacs  may  be  divided  into  two  portions: 
the  one,  a  kind  of  antechamber,  which  receives  the  blood  from 
the  veins,  is  called  the  auricle,  and  has  very  thin  walls  ;  the  other, 
the  ventricle,  is  the  powerful  muscular  chamber  which  pumps  the 
blood  into  the  aorta  and  distends  the  arteries.  (See  Figs.  113 
and  114.) 

In  the  empty  heart  the  great  mass  of  the  organ,  which  forms 
a  blunted  cone,  is  made  up  of  the  ventricles,  while  the  flaccid 
auricles  are  found  retracted  to  an  insignificant  size  at  its  base. 
The  four  cavities  have  about  the  same  capacity,  namely,  about 
six  ounces  or  eight  cubic  inches  when  distended. 

The  walls  of  both  the  auricles  are  about  the  same  thickness, 
while  the  amount  of  muscle  in  the  wall  of  the  ventricle  differs 
materially  on  the  two  sides.  The  wall  of  the  left  ventricle,  in- 
cluding that  part  which  forms  the  interventricular  septem,  is 
nearly  three  times  as  thick  as  that  of  the  right  or  pulmonary 
ventricle. 

Arrangement  of  Muscle  Fibres. 

At  the  attachment  of  each  auricle  to  its  corresponding  ventricle 
there  is  situated  a  dense  ring  of  tough  connective  tissue,  which 
surrounds  the  openings  leading  from  the  auricles  to  the  ventricles. 
Similar  tendinous  rings  (zona  tendinosa)  exist  around  the  orifice 
of  the  aorta  and  pulmonary  arteries.    These  tendinous  rings  form 


CONSTRUCTION    OF   THE    HEART. 


257 


the  basis  of  attachment  for  the  muscle  bundles  of  the  walls  of 
both  the  ventricles  and  auricles. 

In  the  ventricles  many  layers  of  muscles  can  be  made  out.  The 
outer  fibres  pass  in  a  twisted  manner  from  the  base  towards  the 
apex,  where  they  are  tucked  in  so  as  to  reach  the  inner  surface 
of  the  ventricular  cavity.  They  then  pass  back  to  be  attached 
at  the  base ;  some  passing  into  the  papillary  muscles  are  con- 
nected with  the  cardiac  valves  through  the  medium  of  the  chordse 

Fig.  115. 


Striated  Muscle  Tissue  of  the  Heart,  showing  tlie  trellis-work  formed  by  the 
short  branching  cells,  with  central  nuclei. 

tendinese ;  and  the  others,  forming  irregular  masses  of  muscle  on 
the  inner  .surface  of  the  cavity,  pass  in  various  directions  towards 
the  base,  to  be  fused  with  the  tendinous  rings  around  the  arterial 
orifices.  Another  set  of  layers  passes  transversely  around  the 
ventricle,  lying  between  the  inner  and  outer  sets,  and  passing 
nearly  at  right  angles  to  them. 

The  muscle  fibres  forming  the  thin  auricular  walls  have  their 
origin  from  the  zones  of  the  auriculo-ventricular  orifices,  and  pass 
very  irregularly  around  the  cavities.     The  outer  set  of  fibres 

22 


258  MANUAL   OF   PHYSIOLOGY. 

have  a  transverse,  the  inner  a  longitudinal  direction.  Bands  of 
fibres  encircle  the  orifices  of  the  great  veins,  and  extend  for  some 
little  distance  along  the  vessels,  particularly  on  the  pulmonary 
veins,  which  have  thick  circular  muscular  coats  after  they  leave 
the  lungs. 

The  fibres  of  the  auricles  are  not  directly  continuous  with  those 
of  the  ventricles,  the  auricular  and  ventricular  fibres  being  only 
related  to  each  other  by  their  points  of  origin,  viz.,  the  auricuio- 
ventricular  fibrous  zones. 

Minute  Structure. 

The  muscle  tissue  of  the  heart  differs  both  in  structure  and  mode 
of  action  from  the  other  forms  of  contractile  tissues  of  the  body. 
The  elements  are  firmly  united  with  one  another  to  form  an  irreg- 
ular close  network,  which,  however,  can  be  broken  up  into  masses 
easily  recognizable  as  peculiar  cells.  These  cells  are  irregular 
prismoidal  blocks  with  blunt  ends,  often  split  into  two  to  allow  of 
connection  with  the  two  contiguous  cells.  They  contain  a  distinct 
nucleus,  situated  in  the  central  axis  of  the  cell.  The  cells  are 
not  surrounded  by  a  distinct  sheath  of  sarcolemma. 

Though  striated,  like  the  skeletal  muscles,  the  action  of  the 
heart  muscle  is  peculiarly  independent  of  the  great  nervous  centres, 
being  quite  involuntary,  and  characterized  by  a  definite  perio- 
dicity. Its  contraction  is  very  slow  when  compared  with  the 
skeletal  muscles,  but  it  is  much  more  rapid  than  that  of  the  con- 
tracting tissues  of  most  of  the  hollow  viscera. 

Valves, 

The  orifices  which  lead  into  and  out  of  the  ventricles  have 
peculiar  arrangements  of  their  lining  texture,  forming  valves 
which  allow  the  blood  to  pass  in  a  certain  direction  only.  These 
valves,  which  form  a  most  interesting  and  important  part  of  the 
economy  of  the  heart,  are  of  two  kinds,  diflTering  completely  in 
their  mode  of  action.  One  kind  directs  the  passage  of  the  blood 
from  the  auricles  to  the  ventricles,  the  other  guards  the  openings 
into  the  great  arteries.     The  auriculo-ventricular  openings  aie 


VALVES   OF   THE    HEART. 


259 


protected  by  valves  with  a  sail-like  action.  These  are  made  up 
of  delicate  curtains  formed  of  thiu  sheets  of  couuective  tissue 
arising  from  the  margins  of  the  auriculo-veutricular  openings, 
which  form  the  fixed  attachment  of  each  of  the  curtains  of  the 


Portion  of  the  Wall  of  Ventricle  {d  df)  and  Aorta  (a  h  c),  showing  attach- 
ments of  one  flap  of  mitral  and  the  aortic  valves;  {h  and  (j)  papillary  mus- 
cles; (e,  e',  and/)  attachment  of  the  tendinous  chords.     (Allen  Thomson.) 

valves.  The  free  edges  and  ventricular  surfaces  of  the  curtains 
are  blended  with  the  tendinous  cords  coming  from  the  papillary 
muscles,  and  thus  give  points  of  tendinous  attachment  to  some  of 
the  bundles  of  muscle  fibres  in  the  wall  of  the  ventricle.  At  the 
right  auriculo-ventricular  opening  there  are  three  chief  curtains  ; 


260 


MANUAL   O'?   PHYSIOLOGY. 


hence  it  is  called  the  "tricuspid"  valve  (Fig.  117,  kav).  The 
opening  from  the  left  auricle  to  the  left  ventricle,  which  is  about 
one-third  smaller,  is  guarded  by  two  large  valvular  flaps,  and  is 
hence  called  the  "bicuspid,"  or  more  commonly  "mitral"  valve 
(Fig.  116).  The  arterial  valves  are  made  up  of  three  deep  pockets 
with  free  semi-lunar  margins  looking  towards  the  vessel.     The 

Fig.  117. 


The  Orifices  of  the  Heart  seen  from  below,  the  whole  of  the  ventricles 
being  cut  away,  and  the  curtains  of  the  auriculo-ventricular  valves  drawn 
down  by  threads  attached  to  the  chorda  tendineoe.  ( Huxley.) — RA  V.  Right 
auriculo-ventricular  opening  surrounded  by  the  flaps  of  tricuspid.  LAV. 
Left  auriculo-ventricular  opening  and  attached  mitral  valve.  PA.  Pul- 
monary valves  when  closed.     AO.  Aortic  valves  closed. 

curved  base  of  each  pocket  is  attached  to  the  arterial  orifice  of 
the  ventricle,  with  the  lining  membrane  of  which  it  is  continuous. 

Action  of  the  Valves. 

The  mode  of  action  of  the  flaps  of  the  tricuspid  and  mitral 
valves  is  like  that  of  a  lateen  sail  of  a  boat,  if  we  substitute  the 


ACTION   OF  THE   VALVES. 


261 


bloodstream  for  the  air  current;  the  tendinous  cords  acting  as 
the  "  sheet "  or  rope  which  restrains  the  sail  when  filled  with  wind. 
The  blood  driven  in  from  the  auricle  at  first  pushes  the  cur- 
tains of  the  valves  against  the  ventricular  wall,  and  immediately 
fills  the  ventricle.  As  the  ventricle  becomes  distended  the  tendi- 
nous cords  coming  from  the  elastic  papillary  muscles  are  put  on 
the  stretch,  and  draw  the  valve  curtains  away  from  the  wall  of 
the  ventricle  into  the  midst  of  the  fluid.     When  the  ventricle 


Fig.  118. 


FA 


The  Orifices  of  tlie  Heart  seen  from  above,  both  the  auricles  and  the 
great  vessels  being  removed.  (Huxley.) — PA.  Pulmonary  artery  and  its 
semilunar  valves.  Ao.  Aorta  and  its  valves.  EAV.  Tricuspid,  and  LAV. 
BiscuspiJ  valves. 


begins  to  contract  upon  its  contained  blood,  the  pressure  of  the 
fluid  bellies  out  the  sail-like  valves  towards  the  auricles,  so  that 
their  convex  sides  come  into  close  apposition  with  one  another. 
Their  free  margins  are  held  firmly  by  the  papillary  muscles  con- 
tracting and  tightening  the  cords.  The  flaps  are  kept  at  much 
the  same  tension  by  the  papillary  muscles  shortening  in  propor- 


262  MANUAL  OF   PHYSIOLOGY. 

tion  as  the  ventricle  empties  itself  and  the  cavity  diminishes  in 
size.  By  this  mechanism  the  valves  are  prevented  from  bulging 
too  much  into  the  auricles,  or  allowing  the  blood  to  pass  back 
into  them. 

The  semilunar  valves  are  mere  membranous  pockets,  and  have 
no  tendinous  cords  attached  to  them,  but  on  account  of  the  ex- 
tent of  their  curved  attachment,  when  their  limited  free  margin 
is  made  tense  by  the  pocket  being  filled  from  the  artery,  the  valves 
can  only  pass  a  given  distance  from  the  wall  of  the  vessel,  and 
are  thus  held  firmly  in  position.  When  the  force  of  the  blood 
leaving  the  ventricle  begins  to  diminish,  the  semilunar  flaps  are 
raised  from  the  distending  wall  of  the  artery;  and  the  moment 
the  current  from  the  ventricle  has  ceased  to  flow,  the  pockets  are 
forcibly  distended  by  the  aortic  blood  pressure,  and  bulge  into 
the  lumen  of  the  vessel,  so  that  the  convex  surface  of  the  lunated 
portions  of  each  valve  is  pressed  against  corresponding  parts  of 
its  neighbor.  Their  union  then,  which  is  accomplished  by  their 
overlapping  to  some  extent,  forms  three  straight  radiating  lines, 
and  is  a  perfectly  impervious  barrier  to  any  backward  flow  of 
blood  (Fig.  118,  PA.  and  Ac). 

]\IOVEMENTS  OF  THE   HeART, 

It  is  only  by  means  of  these  valvular  arrangements  that  the 
heart  is  enabled  to  perform  its  function,  namely,  to  pump  the  blood 
in  a  constant  direction  onwards,  emptying  the  veins  and  filling  the 
arteries  against  great  opposition  on  the  part  of  the  latter  vessels. 

This  pumping  is  carried  on  by  the  successive  contractions  and 
relaxations  of  the  muscular  walls  of  the  various  cavities. 

The  blood,  flowing  from  the  systemic  and  pulmonary  veins, 
passes  unopposed  into  the  right  and  left  auricles  respectively.  As 
soon  as  the  auricles  are  full  their  walls  suddenly  contract  and  press 
the  blood  into  the  right  and  left  ventricles;  immediately  the  ven- 
tricles contract,  and,  pressing  upon  the  blood,  force  it  into  the 
great  arteries. 

The  contraction  of  each  pair  of  cavities  is  followed  by  their 
relaxation. 


MOVEMENTS   OF   THE    HEART.  263 

The  blood  cannot  pass  back  into  the  veins  from  the  auricles  when 
they  contract,  because  the  auricular  contraction  commences  in  the 
bundles  of  muscle  which  surround  the  orifices  of  the  great  venous 
trunks ;  and  it  cannot  flow  back  to  the  auricles,  because,  as  has  been 
seen,  the  force  of  the  blood  current  on  its  entry  into  the  ventricles, 
by  making  tense  the  cords,  closes  the  valves ;  while  a  backward 
flow  from  the  large  arteries  is  at  once  prevented  by  the  current  dis- 
tending the  semilunar  pockets,  and  thus  firmly  closing  the  valves. 

When  viewed  for  the  first  time,  the  beat  of  the  heart  appears 
to  be  a  single  act,  so  rapidly  does  the  ventricular  follow  the  auricu- 
lar beat.  More  careful  examination  shows  that  this  single  action 
is  composed  of  different  phases  of  activity  and  repose,  which  to- 
gether make  up  the  cycle  of  the  heart  beat.  The  time  occupied 
by  the  contraction  of  the  cavities  of  the  heart  is  called  their  systole, 
their  period  of  rest  is  called  diastole. 

The  systole  of  the  corresponding  cavities  of  both  sides  of  the 
heart  is  exactly  synchronous ;  that  is  to  say,  the  two  auricles  con- 
tract simultaneously,  and  immediately  the  contraction  of  the  two 
ventricles  follows  like  that  of  the  two  auricles  as  a  single  act. 

The  auricular  and  ventricular  contractions  are  separated  by  so 
short  a  space  of  time  that  it  is  not  easily  appreciable.  The  rapidly 
succeeding  acts  of  auricular  and  ventricular  systole  are  followed 
by  a  period  during  which  both  auricles  and  ventricles  are  in  dias- 
tole, which  is  commonly  spoken  of  as  the  passive  interval  or  pause. 

While  the  auricles  are  contracting  the  ventricles  are  relaxed, 
and  the  relaxation  of  the  auricles  commences  immediately  the 
ventricular  contraction  begins,  so  that  only  for  a  very  short  time 
both  auricles  and  ventricles  are  contracted. 

The  entire  cycle  or  revolution  of  the  heart-beat,  occupying  nearly 
a  second  in  the  healthy  adult,  may  be  divided  into  three  stages : 
Auricular  systole. 
Ventricular  systole. 
General  diastole. 

The  exact  time  occupied  by  each  phase  of  the  cycle  can  only 
be  calculated  approximately.  This  may  be  done  either  by  register- 
ing graphically  the  motions  of  the  auricles  and  ventricles  directly 


264 


MANUAL,  OF   PHYSIOLOGY. 


communicated  to  levers  brought  into  contact  with  their  surface, 
or  by  recording  graphically  the  pressure  changes  which  occur 
within  the  cavities  by  introducing  into  them  little  elastic  sacks 
filled  with  air,  whence  the  pressure  changes  are  communicated  to 
an  ordinary  "tambour,"  and  registered  on  a  smoked  surface. 

Of  the  whole  period  of  the  cycle  the  passive  interval  or  pause 
is  the  longest  and  the  most  variable,  for  in  ordinary  changes  in 

Fio.  119. 


BMBSBaaSSBBSlKB 


BiBSBaaSS! 

■■■■■■■■■! 


Ili 


S^3  iBiBiiiiiiii 

mmmmmwmmmmr'^ 

■■■■OfilHaHM 

■■■■«■■■■■! 

■■■■■■■iin 

QisainniiB!!!!! 
■B&aBHHnMi 

BBBBBBBBBBBBlil 

Bbbbbbbbbbbbbi 


Curves  drawn  on  a  moving  surface  by  three  levers,  wliich  are  connected 
with  tlie  interior  of  tlie  heart,  viz. :  Upper  line  shows  the  changes  of  pres- 
sure occurring  in  the  right  auricle ;  centre  line  shows  the  pressure  changes 
within  the  right  ventricle ;  lower  line  shows  the  changes  of  pressure  oc- 
curring in  the  left  ventricle.  (The  smoked  surface  is  moved  from  right  to 
left.)     (After  Chauvian.) 

the  heart's  rhythm  it  is  the  pause  that  varies.  Next  in  duration 
is  the  ventricular  systole,  while  the  shortest  is  the  auricular  systole. 
The  following  figures  give  approximately  the  proportion  of  time 
occupied  by  each  part  of  the  cycle  in  the  case  of  a  horse,  whose 
intra-cardiac  tension  was  registered  in  the  manner  just  referred 
to,  while  his  heart  beat  about  fifty  times  in  the  minute : 


CYCLE   OF   THE    HEART-BEAT.  265 


Proportion 
of  cycle. 

Duration 
in  seconds. 

Auricular  systole, 

•          .         i 

0.2'^ 

Ventricular  systole,     . 

2                   

0.4^' 

Passive  interval, 

.    .    1 

0.6^^ 

Or  if  we  assume  the  human  heart  to  beat  some  seventy  times 
a  minute,  each  cycle  would  occupy  about  ts  of  a  second,  which 
would  be  made  up  as  follows : 

Auricular  systole, =  yV  of  a  second. 

Ventricular  systole,  .....     ^  x^o         '     ' 
Pause, ^  A 

The  duration  of  the  auricular  and  ventricular  systole  varies 
but  little  except  under  abnormal  circumstances,  but  the  pause. is 
constantly  undergoing  slight  changes.  In  fact,  the  duration  of 
the  general  diastole  depends  upon  the  rate  of  the  heart  beat,  being 
less  in  proportion  as  the  heart  beats  more  quickly. 

If  the  thorax  of  a  recently  killed  frog  be  opened  the  heart  can 
be  observed  beating  in  sihc,  and  the  different  acts  in  the  cycle 
studied. 

In  mammalians,  in  order  to  see  the  heart  in  operation,  it  is 
necessary  to  keep  up  artificial  respiration,  during  which  the  heart 
continues  to  beat  regularly,  though  the  thorax  be  opened.  A 
careful  inspection  of  the  beating  heart  shows  that  during  its  cycle 
of  action  certain  changes  take  place  in  the  shape  and  relative 
position  of  its  cavities.  This  is  owing  partly  to  the  change  in 
the  amount  of  their  blood  contents  and  partly  to  the  form  as- 
sumed by  the  muscular  wall  when  contracting. 

During  the  passive  interval  the  auricles  are  seen  to  swell  grad- 
ually on  account  of  the  blood  flowing  into  them  from  the  veins ; 
when  the  auricular  cavities  are  nearly  full,  a  contraction,  com- 
mencing in  the  great  venous  trunks  near  the  heart,  passes  with 
increasing  force  over  the  auricles,  and  gives  rise  to  their  rapid 
systolic  spasm.  The  auricles  appear  suddenly  to  diminish  in  size, 
become  pale,  and  empty  themselves  into  the  ventricles. 

As  the  blood  is  shot  through  the  auriculo- ventricular  openings, 


2G6  MANUAL   OF   PHYSIOLOGY. 

and  the  ventricles  become  distended,  their  flaccid  walls  appear  to 
be  drawn  over  the  liquid  mass  by  the  contracting  auricles,  just  as 
a  stocking  is  drawn  over  the  foot  by  the  hands,  and  their  walls 
seem  to  approach  towards  the  base  of  the  heart.  The  moment 
the  ventricles  have  received  their  full  charge  of  blood  from  the 
auricles,  they  contract,  becoming  shorter  by  the  movement  of  the 
base  towards  the  apex,  and  thicker  by  the  elongated  ventricular 
cone  becoming  rounder.  The  great  arteries  are  at  the  same  time 
distended  with  blood  and  elongated,  their  elastic  walls  being 
drawn  down  over  the  liquid  wedge  on  its  exit  from  the  ventricle. 
The  soft  elastic  tissues  are  thus  in  turn  made  to  slide,  as  it  were, 
over  the  incompressible  fluid  blood  that  forms  the  fulcrum,  which 
the  power  of  the  muscular  walls  uses  as  a  firm  purchase.  During 
the  systole,  when  the  thorax  is  open,  the  ventricles  rotate  slightly 
on  their  long  axis,  so  that  the  left  comes  a  little  forwards,  and 
the  apex  also  forwards  and  towards  the  right.  On  the  ventricular 
systole  ceasing  the  gradual  refilling  of  the  auricles  begins ;  the 
ventricles  become  flaccid  and  flattened  ;  the  semilunar  valves 
being  closed,  the  large  arteries  grasp  firmly  the  blood,  and  by  their 
steady  resilient  pressure  force  it  onwards  towards  the  distal  ves- 
sels. During  this  pause  the  arteries  seem  to  become  shorter, 
drawing  the  base  of  the  heart  up  again  and  lengthening  the  flac- 
cid ventricles. 

The  part  of  the  heart  which  changes  its  position  most  is  the 
line  between  the  auricles  and  ventricles,  while  the  apex  remains 
fixed  in  the  one  position,  only  making  a  very  slight  lateral  and 
forward  motion,  which  probably  does  not  take  place  during  life. 
If  a  needle  with  a  light  lever  attached  be  made  to  enter  the  apex 
through  the  wall  of  the  chest,  the  lever  does  not  move  in  any 
definite  direction  during  the  systole,  but  simply  shakes.  If,  on 
the  other  hand,  the  needle  be  placed  in  the  base  of  the  ventricles, 
the  lever  moves  up  and  down  with  each  systole  and  diastole. 

Heart's  Impulse. 

If  the  ventricles  be  gently  held  between  the  fingers  during  their 
systole,  a  most  striking  sensation  is  given  by  the  sudden  harden- 


heart's  impulse. 


267 


ing  of  the  muscle.  The  mass  of  the  ventricles,  from  being 
quite  soft  and  compressible  during  diastole,  suddenly  acquires  a 
wooden  hardness,  owing  to  the  tightness  with  which  the  mus- 
cle grasps  the  fluid,  and  the  greater  firmness  of  the  contracting 
tissue. 

This  hardening  gives  the  sensation  of  a  sudden  enlargement  in 
all  directions.  No  matter  on  what  surface  the  finger  be  placed, 
the  heart  seems  to  move  in  that  direction,  so  as  to  give  a  slight 
knock  or  impulse.  Thus,  when  grasped  between  the  forefinger 
placed  below  the  diaphragm  and  the  thumb  on  the  antero-supe- 
rior  aspect,  the  impulse  is  equally  felt  by  each  digit. 

The  heart-beat  communicates  its  motion  to  the  chest,  and  this 
impulse  can  be  seen  over  a  limited  area,  which  varies  with  the 

Fig.  120. 


Cardiac  Tambour,  wliicli  can  be  strapped  on  to  chest-wall,  so  that  the  cen- 
tral button  lies  over  the  heart-beat,  and  the  pressure  may  be  regulated  by 
the  screws  at  the  side.  To  the  tube  bent  at  right  angles  is  attached  the  rub- 
ber tube  which  connects  the  air  cavity  with  that  of  the  writing  tambour 
shown  in  Fig.  119.  . 

thinness  of  the  individual.  This  cardiac  impulse,  as  the  stroke 
is  called,  can  be  best  felt  in  the  fifth  intercostal  space,  a  little  to 
the  median  side  of  the  left  nipple.  It  is  found  to  be  synchronous 
with  the  ventricular  systole.  The  more  important  item  in  causing 
the  impulse  is  the  hardening  of  the  ventricles,  while  their  simul- 
taneous change  in  shape,  from  a  flattened  to  a  rounded  cone,  no 


268 


MANUAL   OF   PHYSIOLOGY. 


Fio. 


doubt  helps  to  make  the  sucklcn  tenseuess 
more  distinctly  felt  through  the  wall  of  the 
chest. 

The  point  at  which  the  impulse  is  best 
felt  coresponds  to  the  anterior  surface  of 
the  ventricles  some  distance  above  the  apex; 
it  is  therefore  erroneous  to  call  it  the  "apex 
beat." 

Moreover,  the  motion  of  the  apex  is  so 
slight  when  the  wall  of  the  chest  is  removed, 
that  its  "tilting  forwards"  can  have  no 
share  in  causing  the  impulse :  the  thoracic 
wall  being  always  in  contact  with  the  apex, 
it  can  only  move  laterally,  and  cannot  ham- 
mer against  it  so  as  to  cause  a  shock.  The 
"  recoil  of  the  ventricles "  caused  by  the 
blood  leaving  them,  which  some  think  aids 
in  producing  the  impulse,  obviously  owes  its 
supposed  existence  to  the  confusion  of  cause 
and  effect. 

The  cardiac  impulse  is  a  valuable  measure 
of  the  strength  of  the  systole,  and  hence  is 
of  great  importance  to  the  clinical  physician. 
It  may  be  registered  by  means  of  an  instru- 
ment called  the  Caliograph.  Many  such 
instruments  have  been  devised,  most  of 
which  work  on  the  same  principle,  and  write 
a  record  on  a  moving  surface  with  a  lever 
attached  to  a  tambour,  to  which  the  move- 
ments of  the  chest  wall  are  transmitted  from 
a  somewhat  similar  drum  by  means  of  air 
tubes.  In  using  this  plan,  so  generally  em- 
ployed by  Marey,  one  air-tambour  (fig.  120) 
is  applied  over  the  heart,  the  motions  of 
which  cause  a  variation  in  the  tension  of 
the  air  it  contains ;  these  variations  are 
transmitted  by  a  tube  (/,  Fig.  121)  to  the 
other  tambour  (b),  where  they  give  rise  to 
a  motion  in  its  flexible  surface  to  which  a 
delicate  lever  is  attached  at  (a). 


Writing  Lever  and 
Tambour. — (a)  Joint 
of  the  lever;  (b)  Air 
chamber;  (/)  Rubber 
tubing  connecting  it 
with  cardiac  tambour. 


HEART  SOUNDS.  269 


Heart  Sounds. 


The  heart's  action  is  accompanied  by  two  distinct  sounds,  which 
can  be  heard  by  bringing  the  ear  into  firm  direct  contact  with 
the  precordial  region,  or  indirectly  by  the  use  of  the  stethoscope, 
an  instrument  of  which  there  are  many  varieties  to  suit  the  taste 
of  clinical  observers.* 

One  sound  follows  the  other  quickly,  and  then  comes  a  short 
pause;  consequently,  they  are  spoken  of  as  the  first  and  second 
sounds. 

The  first  sound  occurs  at  the  same  time  as  the  ventricular  sys- 
tole. It  is  a  low,  soft,  prolonged  tone,  and  is  most  distinctly  heard 
over  the  fifth  intercostal  space. 

The  second  sound  is  produced  when  the  two  sets  of  semilunar 
valves  are  closed,  that  is,  at  the  moment  when  the  blood  ceases 
to  escape  from  the  ventricles.  It  is  a  sharp,  short  sound,  and  is 
best  heard  at  the  second  costal  cartilage  on  the  right  side. 

The  cause  of  the  first  sound  is  as  yet  involved  in  doubt.  Pos- 
sibly there  are  several  factors  in  its  production.  The  principal 
events  occurring  at  this  time  may  be  enumerated  thus : 

1.  The  heart's  impulse. 

2.  The  contraction  of  the  heart-muscle. 

3.  The  rush  of  blood  into  the  arteries. 

4.  The  sudden  tension  of  the  ventricular  chambers  and  the 

auriculo-ventricular  valves. 

It  has  already  been  seen  that  the  heart's  impulse  is  caused  by 
a  sudden  change  in  density  of  the  muscle  rather  than  by  a  knock 
against  the  chest.  Moreover,  the  first  sound  is  heard  quite  readily 
when  the  chest  wall  is  removed,  so  that  the  apex  beating  against 
the  thorax  cannot  even  help  to  cause  the  sound. 

The  sound  is  not  unlike  the  tone  which  accompanies  the  con- 

*  A  flexible  stethoscope  to  listen  to  one's  own  heart  sounds  can  easily  be 
made  by  fitting  to  one  end  of  a  piece  of  rubber  tubing  about  18  inches  long 
the  mouth-piece,  and  to  the  other  end  the  bowl  of  a  wooden  pipe.  The 
bowl  is  applied  over  the  difl'erent  regions  of  the  heart,  and  the  mouth-piece 
firmly  fitted  in  the  ear. 


270  MANUAL   OF   PHYSIOLOGY. 

tiuuous  (tetauic)  contraction  of  the  skeletal  muscles.  It  corre- 
sponds in  time  and  duration  with  the  contraction  of  the  cardiac 
muscle.  In  disease  where  the  heart  muscle  is  weak,  the  sound 
becomes  faint  or  inaudible,  although  the  valves  are  made  tense 
by  an  intraventricular  force  sufficient  to  overcome  the  pressure  in 
the  arteries;  for  otherwise  the  circulation  would  cease.  A  prre- 
systolic  sound,  like  in  character  to  the  systolic  sound,  is  now  rec- 
ognized by  physicians  as  being  produced  by  the  auricular  systole, 
but  this  cannot  depend  on  the  vibrations  of  valves.  According 
to  the  most  recent  and  careful  observers,  the  first  sound  may  be 
heard  when  the  heart  is  empty. 

All  this  evidence  tends  to  show  that  the  sound  is  produced  by 
the  contraction  of  the  muscle  tissue  of  the  heart,  or,  in  short,  thf^t 
it  is  the  cardiac  muscle  tone. 

Against  the  view  that  the  muscular  tone  is  the  cause  of  the  first 
sound,  the  undoubted  fact  is  properly  urged  that  only  tetanus 
(i.e.,  a  rapidly  repeated  series  of  contractions  fused  into  a  con- 
tinued state  of  shortening  allowing  variations  of  tension)  can 
cause  a  muscle  sound,  and  a  single  contraction  is  not  accom- 
panied by  any  tone.  Though  in  many  ways  it  diflfers  from  the 
single  contraction  of  other  muscles,  yet  the  heart  beat  is,  no  doubt, 
a  single  contraction.  And  no  good  reason  exists  for  believing 
it  can  cause  vibrations  comparable  with  that  of  the  tetanus  of 
skeletal  muscles,  or  that  it  is  capable  of  giving  rise  to  a  definite 
sound. 

The  closure  of  the  auriculo-ventricular  valve  is  synchronous 
with  the  beginning  of  the  sound,  and  injury  or  disease  of  these 
valves  is  associated  with  a  weak  or  altered  first  sound  ;  this  is 
often  observed  in  disease  of  the  mitral  valve.  The  blood  is  said, 
by  some,  to  be  necessary  for  the  production  of  the  sound,  so  that 
the  act  of  closure  and  subsequent  tension  of  these  valves  would 
seem  to  have  a  share  in  causing  the  sound  ;  but,  on  the  other 
hand,  the  character  of  the  sound  is  not  like  that  which  would  be 
caused  by  the  sudden  closure  or  tension  of  the  membranes,  or  of 
the  delicate  tendinous  cords. 

As  before  remarked,  this  would  not  account  for  prsesystolic 
sounds,  and  the  first  sound  can  be  heard  in  an  empty  heart  re- 


INNERVATION    OF   THE    HEART,  271 

moved  from  the  animal,  in  which  the  valves  could  not  become 
tense  (Ludwig). 

The  sound  has  been  analyzed  with  suitable  resonators,  and  two 
distinct  tones  made  out :  one,  high  and  short,  corresponding  to  the 
closure  of  the  valves ;  the  other,  long  and  low,  corresponding  in 
duration  with  the  muscle  contraction.  From  this  it  would  ap- 
pear that  both  the  tension  of  the  valves  and  the  muscle  tone  have 
something  to  do  with  the  production  of  the  sound. 

The  good  reasons  given  for  thinking  that  the  heart  muscle  can- 
not produce  a  tone,  suggest  that  the  sudden  state  of  tension  of 
the  ventricular  wall  when  tightened  over  the  blood  may  give  rise 
to  vibrations,  and  be  an  important  item  in  causing  the  first  sound. 
This  would  explain  the  faiutness  of  the  sound,  both  when  the 
valves  were  injured  and  the  muscle  weak.  It  would  also  explain 
the  prsesystolic  sound,  which  requires  a  certain  auricular  tension 
for  its  production. 

The  production  of  the  second  sound  is  more  easily  explained. 
Occurring  just  after  the  ventricle  is  emptied,  it  is  synchronous 
with  the  closure  of  the  semilunar  valves  at  the  aorta  and  pulmo- 
nary orifices.  The  blood  in  the  aorta  forcibly  closes  the  valves 
as  soon  as  the  ventricular  pressure  begins  to  wane.  This  sudden 
motion  causes  a  vibration  of  the  valves,  which  is  immediately 
checked  by  the  continuous  pressure  of  the  column  of  blood. 

Innervation  op  the  Heart. 

The  most  interesting  phenomenon  in  the  heart's  action,  and 
that  most  difficult  to  explain,  is  the  wonderful  regularity  of  its 
rhythmical  contractions  under  normal  circumstances,  and  the  ex- 
treme delicacy  of  the  nervous  mechanism  by  which  it  is  regulated. 

The  vast  majority  of  the  active  contractile  tissues  of  the  higher 
animals  are  under  the  immediate  and  exclusive  direction  of  the 
central  nervous  system.  All  the  great  muscular  organs  are  con- 
nected with  the  cerebro-spinal  axis  by  means  of  nerves,  along 
which  impulses  pass  stimulating  the  contractile  tissue  to  action. 
Thus  the  skeletal  muscles  are  brought  under  the  control  of  the 
will,  and  the  nerves  coming  from  the  brain  carry  stimuli  to  cer- 
tain sets  of  muscles  when  we  wish  to  perform  any  simple  action. 


272  MANUAL   OF   PHYSIOLOGY. 

Other  muscles,  as  has  been  seen  in  the  pharynx,  oesophagus, 
etc.,  though  not  under  the  control  of  the  will,  are  yet  governed 
altogether  by  the  cerebrospinal  axis;  while  others,  of  which  the 
most  striking  example  is  the  heart,  have  nerve  elements  in  imme- 
diate relation  to  the  contractile  tissue,  capable  of  exciting  them 
to  contraction. 

It  will  materially  help  us  in  comprehending  the  nervous  mech- 
anism of  the  heart's  rhythm,  if  we  bear  in  mind  what  now  seems 
to  be  proved  beyond  doubt,  namely,  that  the  muscle  tissue  of  the 
heart  has — quite  independently  of  any  nervous  influences — an  in- 
herent tendency  to  rhythmical  contraction.  This  is  shown  by  the 
following  facts.  The  heart  cannot  continue  contracted  like  a 
skeletal  muscle  (in  tetanus)  under  any  circumstances,  or  like  an 
unstriated  muscle  (in  tonus)  except  when  the  tissue  is  spoiled  by 
deficient  nutrition,  etc.  The  hearts  of  many  of  the  lower  animals 
contract  rhythmically  without  any  nerve  elements  being  found 
by  the  most  careful  microscopic  examination.  A  strip  cut  from 
the  ventricle  of  the  tortoise  can,  by  judicious  excitations,  be  towg'Af 
to  beat  rhythmically  without  the  help  of  any  known  nerve  mech- 
anism. The  lower  part  of  the  frog's  ventricle — which  is  com- 
monly admitted  not  to  contain  any  nerves — beats  quite  rhythmi- 
cally if  fed  with  a  gentle  stream  of  serum  and  weak  salt  solu- 
tion, and  there  is  no  reason  to  assume  that  there  is  any  greater 
difficulty  in  conceding  to  muscle  tissue,  than  to  nerve  cells,  the 
property  of  acting  with  a  regular  rhythm. 

In  cold-blooded  animals,  such  as  a  frog  or  tortoise,  the  heart 
will  beat  even  for  days  after  its  removal  from  the  animal,  if  it  be 
protected  from  injury  and  prevented  from  drying.  In  warm- 
blooded animals  the  tissues  lose  their  vitality  almost  immediately 
after  they  are  deprived  of  their  blood  supply ;  however,  sponta- 
neously rhythmical  movements  can  be  seen  in  the  mammalian 
heart  if  removed  rapidly  after  death.  The  hearts  of  oxen,  skil- 
fully slaughtered,  commonly  give  a  few  beats  after  their  removal 
from  the  thorax.  If  a  blood  current  be  kept  up  through  the 
vessels  of  the  heart  tissue,  this  spontaneous  contraction  will  go  on 
for  some  time,  or  even  will  recommence  after  having  ceased. 

The  hearts  of  two  criminals  who  were  recently  hanged  were 


INNERVATION    OF   THE    HEART.  273 

found  to  continue  to  beat  for  four  and  seven  minutes  respectively 
after  the  spinal  cord  and  the  medulla  had  been  separated. 

These  facts  prove  conclusively  that  the  stimulus  which  causes 
the  heart  to  beat  rhythmically  arises  in  the  muscle  tissue  of  the 
organ  or  in  close  relation  to  it.  Upon  physiological  grounds 
alone  we  might  conclude  that  in  the  heart  tissue  of  the  vertebrata 
there  exist  the  nerve  elements  with  which  we  are  familiar  ana- 
tomically. These  nerve  cells  only  require  their  nutrition  to  be 
kept  up  by  a  continued  blood  supply  in  order  to  develop  the 
energy  necessary  for  their  function. 

Such  collections  of  nerve  elements  are  called  automatic  centres, 
and  are  made  up,  like  all  other  origins  of  nerve  force,  of  gangli- 
onic cells. 

The  heart  of  mammalian  animals  so  soon  ceases  to  beat,  that 
it  forms  an  unsatisfactory  subject  for  experimental  inquiry.  The 
heart's  innervation — which  will  be  seen  to  be  a  complicated  pro- 
cess— may,  therefore,  with  profit  be  studied  in  a  cold-blooded 
animal,  where  the  mechanisms  can  be  more  readily  observed,  and 
are  probably  more  simple  in  arrangement. 

The  frog,  being  readily  obtainable,  is  commonly  chosen. 

If  the  apex  of  the  ventricle  of  the  frog's  heart  be  separated, 
it  remains  motionless,  while  the  auricles  continue  to  beat.  But  it 
responds  to  short  direct  stimulus  by  an  ordinary  single  contraction, 
and  if  the  stimulus  be  kept  up  it  beats  rhythmically.  If  the 
auricles  be  removed  from  the  ventricles  so  as  to  leave  the  line  of 
union  attached  to  the  ventricle,  both  continue  to  beat.  But  each 
part  beats  with  a  different  rhythm,  and  under  like  conditions  the 
auricles  continue  to  beat  longer  than  the  ventricles. 

The  auricles  beat  even  when  subdivided  ;  and  the  dilated  ter- 
mination of  the  great  vein,  called  the  sinus  venosus,  opening  into 
the  right  auricle,  when  quite  separated  from  the  rest  of  the  heart, 
continues  to  beat  longer  and  more  regularly  than  any  other  part. 
When  the  entire  heart  is  intact  this  sinus  seems  to  be  the  starting- 
point  of  the  heart-beat. 

This  experimental  evidence  of  the  presence  of  nerve  centres  in 
the  heart-muscle  is  supported  by  the  results  of  anatomical  inves- 

23 


274 


MANUAL  OF   PHYSIOLOGY. 


tigations,  for  the  microscope  shows  that  there  are  many  ganglion 
cells  distributed  throughout  the  heart-tissue,  and  that  they  are 
located  just  where  we  should  expect  from  the  above  facts.     That 


Fig.  122. 


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Diagrammatic  Plan  of  the  Cardiac  Nerve  Mechanism.  The  direction  of 
the  impulses  is  indicated  by  the  arrows.  Both  right  and  left  sides  of  the 
cut  are  used  to  show  one  complete  lateral  half  of  the  fibres. 

is  to  say,  there  are  none  in  the  substance  of  the  ventricles,  while 
there  are  several  groups  of  cells  scattered  around  its  base  in  the 
auriculo-ventricular  groove  (Bidder).     There  are  others  in  the 


CARDIAC  NERVE  CENTRES.  275 

walls  of  the  auricles,  particularly  in  the  septum,  aud  the  greatest 
number  are  found  in  the  walls  of  the  sinus  venosus  (Remak). 

The  ganglia  in  the  sinus  venosus  are  most  easily  stimulated, 
and  are  probably  the  only  ones  which  habitually  act  as  automatic 
centres.  They  certainly  take  the  initiative  in  the  ordinary  heart- 
beat, and  regulate  the  rhythm  of  the  contraction  of  the  auricles 
and  ventricles. 

This  seems  more  than  probable  from  the  following  facts :  1. 
The  ordinary  contraction  wave  starts  from  the  sinus  venosus.  2. 
This  part  beats  longer  and  more  steadily  than  the  others  when 
separated  from  the  animal.  3.  When  cut  off  from  the  sinus  the 
beat  of  the  rest  of  the  heart  becomes  weak,  uncertain,  and 
changes  its  rhythm.  4.  When  the  sinus  venosus  is  suddenly 
separated  by  ligature  from  the  auricles  and  ventricle,  both  the 
latter  cease  to  beat,  while  the  motions  of  the  sinus  continue. 
However,  if  a  slight  stimulus,  such  as  the  touch  of  a  needle,  be 
now  applied  to  the  auriculo-ventricular  margin,  it  suffices  to  give 
rise  to  a  series  of  rhythmical  contractions.  Or  if  the  ventricle 
be  now  separated  from  the  auricles  by  incision,  the  former  com- 
mences to  beat  rhythmically,  while  the  auricles  commonly  remain 
motionless.  These  latter  observations  (which  are  known  as  the 
experiments  of  Stannius)  have  been  explained  in  various  ways, 
the  most  probable  of  which  seems  to  be  the  following.  When 
cut  off  from  the  influence  of  the  sinus  venosus,  the  heart  fails  to 
contract  spontaneously,  because  it  has  lost  the  initiatory  stimulus 
which  habitually  arrived  from  the  sinus.  When  the  ventricle  is 
cut  away  from  the  auricles,  the  act  of  incision  is  sufficient  stimulus 
to  set  going  its  rhythmical  contractions. 

Although  we  cannot  adequately  explain  the  relationships  borne 
by  the  different  sets  of  ganglia  in  the  frog's  heart  to  one  another, 
there  seems  no  doubt  that  the  following  conclusions  may  be  ac- 
cepted as  proven,  and  are,  moreover,  in  all  probability,  applicable 
to  the  hearts  of  mammals  :  That  nerve  centres  exist  in  the  muscle 
tissue  of  the  heart,  some  of  which  are  capable  of  originating 
stimuli  for  this  rhythmically  contracting  muscle.  That  there 
exist  other  ganglionic  groups  which  help  to  regulate  and  distribute 
the  stimuli  in  a  sequence  throughout  the  several  cavities. 


276  MANUAL  OF   PHYSIOLOGY. 

Extrinsic  Cardiac  Nerves. 

The  intrinsic  nerve  mechanism  of  the  heart  just  described  is 
under  the  immediate  control  of  the  great  nervous  centres  through 
the  medium  of  fibres  passing  along  the  vagus  and  sympathetic 
nerves  from  the  medulla  oblongata. 

Some  of  these  fibres  check  the  action  of  the  intrinsic  ganglia, 
and  cause  the  heart  to  beat  more  slowly  ;  hence  they  are  called 
inliihitory.  Others  cause  it  to  beat  more  quickly,  and  are  called 
acceleratory. 

Inhibitory  Nerves  of  the  Heart. 

It  was  observed  by  Weber  that  electric  stimulation  of  the  vagus 
nerve  caused  a  slowing  of  the  heart's  rhythm,  and  if  increased 

Fio.  123. 


Tracing  showing  the  effect  of  Stimuhition  of  Vagus  Nerve.  Stimulus 
applied  between  vertical  lines.  (Recording  surface  moved  from  left  to 
right.) 

gave  rise  to  a  standstill  of  the  heart  in  diastole ;  the  heart  beat 
gradually  recommencing  some  time  after  the  stimulus  had  been 
removed.  On  the  other  hand,  the  section  of  both  vagi  produced 
an  increase  in  the  rapidity  of  the  heart  beat  varying  according 
to  the  kind  of  animal  experimented  upon.  ' 

Section  of  only  one  vagus,  however,  has  not  this  effect.  From 
these  experiments  it  would  appear — 1.  That  some  fibres  of  the 


NERVE   MECHANISM    OF   HEART.  277 

vagus  bear  impulses  of  a  checking  or  inhibitory  nature  to  the 
intrinsic  nerves  of  the  heart.  2.  That  these  influences  are  con- 
stantly in  operation,  or  in  other  words,  the  vagi  exert  a  tonic 
inhibitory  influence  on  the  rapidity  of  the  heart-beat.  3.  The 
tonic  action  of  one  vagus  bears  inhibitory  influence  sufiicient  to 
regulate  the  heart's  action.  This  tonicity  of  the  vagus  inhibition 
is  moreover  more  marked  in  man  than  in  dogs  and  rabbits,  and 
is  reduced  to  a  minimum  in  frogs,  where  section  of  the  vagi  pro- 
duces very  little  effect  on  the  rate  of  the  beat. 

Vagus  inhibition  is  increased  by  the  following  circumstances: 
(a)  certain  psychical  phenomena,  such  as  terror,  which  is  said 
to  produce  a  temporary  standstill ;  (6)  deficiency  of  arterial  blood 
in  the  medulla  oblongata ;  (e)  increase  of  the  blood  pressure 
within  the  cranium;  and  (d)  reflexly  by  the  stimulation  of  many 
afferent  nerves,  particularly  the  sympathetic  and  those  bearing 
impulses  from  the  abdominal  viscera  to  the  medulla,  as  well  as 
the  ordinary  sensory  nerves,  or  by  the  afferent  fibres  of  the  oppo- 
site vagus,  (e)  Muscarine  produces  disastolic  standstill  of  the 
heart  by  exciting  the  local  inhibitory  ganglia  or  vagus  terminals. 

Atropin  causes  quickening  of  the  heart's  action  by  paralyzing 
the  endings  of  the  vagus,  and  also  those  intrinsic  mechanisms 
which  are  supposed  to  have  an  inhibitory  effect.  Nicotine  pro- 
duces at  first  a  slowing  of  the  heart  by  stimulating  the  inhibitory 
tone  of  the  vagus.  This  is  soon  followed  by  exhaustion  of  the 
terminal  fibres  and  a  consequent  quickening  of  the  heart  beat. 
Large  doses  of  curare  paralyze  the  inhibitoi'y  fibres.  Digitalis 
excites  the  vagus  centre  in  the  medulla,  and  thereby  reduces  the 
rapidity  of  the  heart's  beat. 

The  Accelerator  Nerves. 

After  the  possibility  of  increase  of  blood  pressure  has  been  re- 
moved by  section  of  the  splanchnic  nerves,  and  the  tonic  inhibi- 
tion of  the  vagi  has  been  cut  off,  it  has  been  found  that  stimula- 
tion of  the  cervical  portion  of  the  spinal  cord  causes  quickening 
of  the  heart  beat.  In  the  cervical  portion  of  the  spinal  cord  nerve 
channels  must  then  exist  which  are  capable  of  stimulating  the 
muscle  fibres  of  the  heart,  so  as  to  cause  it  to  beat  more  quickly. 


278  MANUAL  OF   PHYSIOLOGY. 

These  accelerator  fibres  pass  through  the  communicating  branches 
from  the  cord  to  the  last  cervical  or  first  dorsal  sympathetic  gan- 
glion, and  thence  to  the  heart.  Stimulation  of  the  ganglia  or  the 
branches  passing  thence  to  the  heart  quickens  its  beat.  The  effect 
of  stimulus  applied  to  these  nerves  does  not  begin  to  show  itself 
until  a  comparatively  long  time  after  it  has  been  applied,  and  the 
acceleratory  effort  continues  for  a  considerable  time  after  the 
stimulus  is  removed.  Stimulation  of  the  accelerator  fibres  has 
little  effect  on  the  tonic  inhibition  of  the  vagus,  which  takes  place 
equally  well  whether  the  accelerators  are  stimulated  or  not,  while 
the  action  of  the  accelerators  is  totally  suspended  so  long  as  the 
vagus  is  being  stimulated. 

Afferent  Cardiac  Nerves. 

Besides  these  nerve  channels  bearing  impulses  to  the  heart, 
others  pass  from  the  heart  to  the  medulla,  probably  having  tlieir 
origin  in  the  inner  lining  of  the  heart,  which  is  known  to  be  the 
part  most  sensitive  to  stimulus. 

These  fibres  appear  to  be  of  two  kinds,  one  of  which  affects  the 
cardio-iuhihitory  centre  and  diminishes  the  pulse  rate;  the  other 
affects  the  vaso-inhibitory  centre  and  lowers  the  blood  pressure. 
Increase  of  the  intra-ventricular  pressure  stimulates  both  these 
sets  of  fibres,  and  thus  we  see  that  over-filling  of  the  heart  from 
increase  of  blood  pressure,  etc.,  causes  retardation  of  its  beat,  and 
an  equilibrium  is  thus  established  between  the  general  blood 
pressure  and  the  force  of  the  heart  beat. 


CHAPTER  XVII. 

THE  BLOODVESSELS. 

The  channels  which  carry  the  blood  throughout  the  body  form 
a  closed  system  of  elastic  tubes,  which  may  be  divided  into  three 
varieties : 

1.  Arteries. 

2.  Capillaries. 

3.  Veins. 

The  arteries  and  veins  serve  merely  to  conduct  the  blood  to  and 
from  the  capillaries,  where  the  essential  function  of  the  blood,  viz., 
its  chemical  interchange  with  the  tissues,  is  carried  on. 

Fig.  124. 


Transverse  Section  of  part  of  the  Wall  of  the  Posterior  Tibial  Arterv 
(man).  (Schafer.)— (o)  Endothelium  lining  the  vessel,  appearing  thicker 
than  normal  from  the  contraction  of  {h)  the  elastic  layer  of  the  intiina;  (c) 
middle  coat,  composed  of  muscle  fibres  and  elastic  tissue ;  (d)  outer  coat  con- 
sisting chiefly  of  white  fibrous  tissue. 

The  arteries  are  those  vessels  which  carry  the  blood  from  the 
heart  to  the  capillaries.  The  great  trunk  of  the  aorta,  which 
springs  from  the  left  ventricle,  gives  off  a  series  of  branches, 
which  in  turn  subdivide  more  and  more  freely  in  proportion  to 
their  distance  from  the  heart.  Their  mode  of  division  is  com- 
monly dichotomous,  but,  from  the  larger  trunk,  branches  of  un- 
equal and  irregular  size  are  frequently  given  off. 


280 


MANUAL  OF   PHYSIOLOGY. 


Fig.  125. 


Arterial  twigs  of  considerable  size  here  and  there  form  con- 
nections with  those  of  a  neighboring  trunk  (anadomoses),  but 
these  unions  are  simple  junctions  of  single  branches,  never  being 
worthy  of  the  name  of  a  network  or  plexus,  such  as  those  seen  in 
the  capillaries  or  in  the  veins.  The  walls  of  the  arteries  are  made 
up  of  three  coats  :  1.  An  external  tougli  fibrous  layer  which  gives 
strength  to  the  vessels, — like  the  webbing  in  the  wall  of  rubber 

water-hose — and  acts  as  a  bond 
of  union  between  it  and  the  neigh- 
boring tissues.  This  coat  {tunica 
adventitia)  carries  the  minute 
vessels  necessary  for  the  nutri- 
tion of  the  vessel  wall,  and  nerves. 
2.  The  middle  coat  (tunica  media) 
forms  the  more  characteristic  i)art 
of  the  arterial  structure,  being  a 
mixture  of  pure  elastic  tissue  and 
unstriated  muscle.  It  is  much 
thicker  in  the  arteries  than  in  the 
veins,  where  its  special  functions 
are  not  required.  It  differs  essen- 
tially in  character  in  the  larger 
and  smaller  arteries,  the  change 
occurring  gradually  on  passing 
along  the  diminishing  branches. 
In  the  large  arteries  and  the  ar- 
terioles the  middle  coat  diflTers 
much  in  structure,  and  in  both  it 
forms  the  most  important  part 
for  the  due  performance  of  their  respective  functions.  In  the 
large  vessels  it  is  made  up  of  elastic  fibres  of  various  shapes, 
and  sheets  of  elastic  tissue  woven  into  a  dense  feltwork,  inter- 
spersed with  a  few  muscle  cells.  In  the  small  arteries  (ar- 
terioles) the  great  mass  of  the  middle  coat  is  made  up  of 
muscle  cells,  the  elastic  tissue  being  but  sparsely  represented. 
Between  the  large  arteries  and  the  capillaries  every  grade  of 
transition  between  these  two  extremes  may  be  found ;  the  elastic 


Portion  of  Small  Artery  from 
Submucous  Tissue  of  Mouse's  Stom- 
ach, stained  with  gold-chloride, 
showing  the  nuclei  of  the  muscle 
cells  (m)  passing  transversely 
around  the  vessel  to  form  the  mid- 
dle coat,  outside  which  is  the 
fibrous  tissue  of  the  outer  coat(F). 
Around  the  vessel  several  fine 
nerve  fibrils  form  a  network  (n). 


CAPILLARIES.  281 

tissue  gradually  becoming  less  abundant  and  the  muscle  elements 
more  important  in  proportion  as  the  capillaries  are  approached. 
3.  The  internal  lining  {tunica  intima)  of  the  arteries  is  composed 
of  a  delicate  elastic  homogeneous  membrane  lined  with  a  single 
layer  of  endothelial  cells.  The  intima  may  be  said  to  be  contin- 
uous throughout  all  the  vessels  and  the  heart  cavities. 

It  is  thus  seen  that  the  large  arteries  have  extremely  elastic 
and  firm  walls,  capable  of  sustaining  considerable  pressure.    The 

Fig.  126. 


Capillary  network  of  a  Lobule  of  the  Liver. 

smaller  the  arteries  become  in  calibre,  the  more  the  general  prop- 
erty of  elasticity  and  resiliency  is  reinforced  by  that  of  vital  con- 
tractility, due  to  the  greater  relative  number  of  muscle  cells  con- 
tained in  the  middle  coat. 

The  frequently-branching  arterioles  finally  terminate  in  the 
capillaries,  in  which  distinct  branches  can  no  longer  be  recognized, 
but  these  thin  canals  are  united  and  interwoven  into  a  network 
of  blood  channels,  the  meshes  of  which  are  all  made  up  of  ves- 
sels having  about  the  same  calibre.     They  communicate  indefi- 

24 


282  MANUAL  OF   PHYSIOLOGY. 

nitely  with  the  capillary  meshworksof  the  neighboring  arterioles, 
so  that  any  given  capillary  area  appears  to  be  one  continuous 
network  of  tubules,  connected  here  and  there  with  distinct  ar- 
terioles, and  thus  is  fed  with  blood  from  several  different  sources. 
The  walls  of  the  capillaries  are  soft  and  elastic,  and  permeable 
not  only  to  the  fluid  portion  of  the  blood,  but  also,  under  certain 
circumstances,  to  the  solids. 

It  is,  in  fact,  in  this  part  of  the  circulation  that  its  essential 
function  is  carried  on,  viz.,  the  establishment  of  a  free  interchange 
between  the  tissues  and  the  blood. 

The  characters  of  the  capillary  network  vary  in  different  tissues 
and  different  organs:  the  closeness  and  wideness  of  the  meshes 
may  be  said  to  be  in  proportion  to  the  functional  activity  or  iu- 

FiG.  127. 


Capillary  network  of  Fat  Tissue. 


activity  of  the  organ  or  tissue  in  questiou,  a  greater  amount  of 
blood  being  required  in  the  parts  where  energetic  duties  are  per- 
formed. 

The  venous  radicles  arise  from  the  capillary  network,  commenc- 
ing as  tributaries  which  unite  in  much  the  same  way  as  the  ar- 
terioles divide,  but  they  form  wider  and  more  numerous  channels. 
They  rapidly  congregate  into  comparatively  large  vessels,  which 
frequently  intercommunicate  so  as  to  form  coarse  and  irregular 
plexuses.  The  general  arrangement  of  the  structures  in  the  walls 
of  the  veins  is  like  that  of  the  arteries ;  they  also  have  three 
coats,  the^external,  middle,  and  internal ;  the  tissues  of  each  dif- 


RELATIVE   CAPACITY   OP   THE   VESSELS.  283 

fering  but  little  from  those  of  the  arteries.  The  middle  coat,  how- 
ever, iu  the  large  veins  is  distinguished  from  that  of  the  large 
arteries  by  being  much  thinner,  owing  to  the  paucity  of  yellow 
elastic  tissue.  It  is  also  characterized  by  its  relative  richness  in 
muscle  fibre,  while  the  structure  of  the  middle  coat  of  the  small 
veins  can  only  be  distinguished  from  that  of  the  arterioles  by  the 
comparative  sparseness  of  the  muscle  cells  running  around  the 
tubes. 

Fig.  128. 


Diagram  intended  to  give  an  idea  of  the  aggregate  sectional  area  of  the 
different  parts  of  the  vascular  system.— a.  Aorta  c.  Capillaries,  v.  Veins. 
The  transverse  measurement  of  the  shaded  part  may  be  taken  as  the  width 
of  the  various  i<inds  of  vessels  supposing  them  fused  together. 

The  veins  are  capable  of  considerable  distension,  but,  though 
possessed  of  a  certain  degree  of  elasticity,  they  are  much  inferior 
to  the  arteries  in  resiliency. 

In  a  large  proportion  of  veins,  valve-like  folds  of  their  lining 
coat  exist  which  prevent  the  backward  flow  of  blood  to  the  ca- 
pillaries, and  insure  its  passage  towards  the  heart.  These  valves 
resemble  in  their  general  plan  the  pocket  valves  that  protect  the 


284  MANUAL   OF   PHYSIOLOGY. 

great  arterial  orifices  of  the  heart.  They  vary  much  in  arrange- 
ment, there  being  commonly  but  two  or  sometimes  one  flap  or 
pocket  entering  into  the  formation  of  the  valve.  They  are  most 
closely  set  in  the  long  veins  of  the  extremities,  in  which  the  blood 
current  has  to  move  against  the  force  of  gravity. 

The  general  aggregate  diameter  of  the  different  parts  of  the 
vascular  system  varies  greatly.  The  combined  calibre  of  the 
branches  of  an  artery  exceeds  that  of  the  parent  trunk,  so  that 
the  aggregate  sectional  area  of  the  arterial  tree  increases  as  one 
proceeds  from  the  aorta  towards  the  capillaries.  After  the  mus- 
cular arterioles  are  passed  the  general  diameter  of  the  vascular 
system  suddenly  increases  enormously,  and  in  the  capillaries  it 
reaches  its  maximum,  which  is  said  to  be  about  eight  hundred 
times  as  great  as  the  diameter  of  the  aorta. 

The  aggregate  sectional  area  of  the  veins  also  diminishes  as  the 
tributaries  unite  to  form  main  trunks,  and  reaches  its  minimum 
at  the  entrance  of  the  vena  cava  into  the  right  auricle. 

The  capacity  of  the  veins  is,  however,  everywhere  much  greater 
than  that  of  the  corresponding  arteries,  the  least  difference  being 
near  the  heart,  where,  however,  the  calibre  of  the  veins  is  more 
than  twice  that  of  the  aorta. 

After  this  brief  anatomical  sketch  the  most  important  proper- 
ties of  each  different  part  of  the  vascular  system  may  be  sum- 
marized thus : 

1.  The  structure  of  the  walls  of  the  large  arteries  shows 

them  to  be  capable  of  sustaining  considerable  pressure, 
and  of  exerting  powerful  elastic  recoil. 

2.  In  the  small  arteries,  as  well  as  this  elasticity,  great  varia- 

bility in  their  calibre,  dependent  on  the  contraction  of 
their  muscular  coat,  occurs. 

3.  In  the  capillaries  we  find  extreme  thinness,  elasticity,  and 

permeability  of  their  wall,  which  presents  an  enormous 
surface,  so  as  to  allow  free  interchange  between  the 
blood  and  the  surrounding  textures. 

4.  The  veins  have  yielding  and  distensible  coats,  great  gen- 

eral capacity  to  accommodate  a  large  quantity  of  blood, 


PHYSICAL    FORCES   OF    THE   CIRCULATION.  285 

aud  valves  to  prevent  its  backward   flow   upon   the 
capillaries. 

Physical  Forces  of  the  Circulation. 

When  liquid  flows  through  a  tube  it  does  so  as  the  result  of  a 
diflTereuce  of  pressure  in  the  different  parts  of  the  tube  ;  the  liquid 
moving  from  the  part  where  the  pressure  is  higher  towards  that 
where  it  is  lower. 

The  energy  of  the  flow  corresponds  with  the  amount  of  differ- 
ence in  the  pressure,  and  varies  exactly  with  the  pressure-differ- 
ence, being  continuous  so  long  as  the  pressure  is  unequal  in  dif- 
ferent parts,  and  being  interrupted  when  the  pressure  is  equalized 
throughout  the  tube.  If  liquid  be  forcibly  pumped  into  one  ex- 
tremity of  a  long  tube,  such  as  a  garden  hose,  a  pressure-differ- 
ence is  of  course  established,  the  pressure  becoming  greater  at 
the  end  into  which  the  liquid  is  pumped,  consequently  a  current 
takes  place  towards  the  open  end.  So  long  as  the  free  or  distal 
end  of  the  tube  is  quite  open  and  on  the  same  level  as  the  rest, 
no  very  great  increase  of  pressure  can  be  brought  to  bear  on  the 
walls  of  the  tube,  no  matter  how  forcibly  the  pumping  may  go 
on,  as  the  liquid  easily  escapes,  and  therefore  flows  out  all  the 
more  quickly  as  the  pumping  becomes  more  energetic.  If,  how- 
ever, the  outflow  be  impeded  by  raising  the  distal  end  of  the 
tube  to  any  considerable  height,  or  by  partially  closing  the  orifice 
with  a  nozzle  or  rose,  then  the  pressure  within  the  tube  can  be 
greatly  increased  by  energetic  pumping,  and  the  tube  being  elastic 
will  swell  when  distended  by  the  pressure. 

It  can  be  further  observed  in  this  common  operation  that  the 
smaller  the  orifice  of  the  nozzle  be,  the  greater  the  pressure  in 
the  tube  with  a  given  rate  of  working  the  pump  ;  and,  the  orifice 
remaining  the  same,  the  pressure  will  increase  in  proportion  as 
the  pump  is  more  energetically  worked.  Or  in  other  words,  the 
pressure  within  the  tube  will  depend  on  (a)  the  force  used  at  the 
pump,  and  (6)  the  degree  of  impediment  oflfered  to  the  outflow. 

If  the  tube  be  resilient,  and  if  the  nozzle  have  a  small  orifice 
so  that  a  high  pressure  can  be  established  within  the  tube,  it  will 
be  found  that  the  liquid  will  flow  from  the  nozzle  in  a  continuous 


286  Manual  of  physiology. 

stream,  and  will  uot  follow  the  jerks  communicated  by  the  pump. 
That  is  to  say,  the  interrupted  energy  of  the  pump  is  stored  up 
by  the  elastic  tube  and  converted  into  a  continuous  pressure  ex- 
erted on  the  fluid.  But  if  the  tube  be  quite  rigid,  or  the  orifice 
too  wide  to  allow  the  pressure  within  the  tube  to  be  raised  suffi- 
ciently high,  then  the  fluid  will  flow  out  of  the  end  of  the  tube  in 
jets  which  correspond  with  the  strokes  of  the  pump ;  that  is  to  say, 
the  outflow  will  follow  closely  the  pressure-difierence  caused  by 
the  pump  at  the  point  of  inflow. 

Now  these  simple  facts  (which  can  be  verified  experimentally 
with  an  ordinary  enema  bag,  a  yard  of  elastic  tubing,  and  a  short 

glass  tube  drawn  to  a  point)  form 
Fig.  129.  the   key  to   the   most   important 

dynamic  principles  of  the  circu- 
lation. 

The  cause  of  the  blood's  motion 
^■""Hfll  t®^-'^-      is  simply  a  difierence  in  the  pres- 

sure within  the  various  parts  of 
the  vascular  system,  for  the  heart 
acts  as  the  pump  filling  the  tube 

^.  ,.    .      ,    .        1  represented   by  the  large  elastic 

Diagram  ol  (.Jirculation,  show-  ^      .  ,  .  ,  ,  i 

•  \i  i^^\      A  ^  f*  I,  T.  \     arteries,  which  can  be  more  or  less 
ing  right  (e.h.)  and  left  (l.h.)  ' 

hearts,  and  the  pulmonary  (p)  and  distended,  according  as  (1)  the 
systemic  (s)  sets  of  capillaries.         outflow  is  impeded  or  facilitated 

by  the  contraction  or  relaxation  of 
the  muscular  arterioles  which  form  the  outlet,  or  as  (2)  the  inflow 
is  increased  or  diminished  by  the  greater  or  less  activity  of  the 
heart's  action. 

From  the  foregoing  facts,  and  what  has  been  said  of  the  direc- 
tion of  the  blood  current,  namely,  that  it  flows  from  the  arteries 
through  the  capillaries  into  the  veins,  it  would  then  appear  that 
the  pressure  in  the  arteries  exceeds  that  in  the  capillaries,  and 
the  pressure  in  the  capillaries  must  in  turn  be  greater  than  that 
in  the  veins,  the  blood  flowing  in  the  direction  in  which  the  pres- 
sure becomes  less. 

The  difference  in  the  manner  in  which  the  blood  flows  from  a 
cut  artery  and  a  cut  vein  shows  that  a  great  difference  exists  in 
the  pressure  within  the  two  sets  of  vessels. 


PHYSICAL   FORCES   OP   THE   CmCULATION.  2S7 

When  a  small  artery  is  cut  across  and  the  orifice  directed  up- 
wards, the  blood  is  thrown  two  or  three  feet  in  jerks.  When  a 
vein  is  cut,  the  blood  only  trickles  gently  from  its  orifice,  the 
force  depending  much  upon  the  position  of  the  part;  and  it  is 
well  known  that  bleeding  from  a  vein  in  the  leg  can  easily  be 
stopped  by  placing  the  limb  in  a  position  more  elevated  than  the 
rest  of  the  body,  so  as  to  remove  the  force  of  gravity  from  acting 
on  the  blood. 

By  means  of  a  special  form  of  gauge  (the  mercurial  mano- 
meter)— which  will  presently  be  described — the  exact  difference 
in  the  pressure  exerted  by  the  blood  against  the  vessel  walls  in 
the  different  parts  of  the  circulation  can  be  accurately  estimated, 
and  it  has  been  found  by  direct  experiment  that  the  blood  pres- 
sure varies  just  as  one  would  be  led  to  expect  from  a  considera- 
tion of  its  physical  relationships,  namely,  the  direction  and  rate 
of  the  current  and  the  varying  width  of  the  bed  in  which  it 
flows. 

The  rate  of  the  fall  in  pressure  observed  in  the  vessels  passing 
from  the  left  ventricle  to  the  right  auricle  is  not  even,  but  in  the 
arterioles  it  falls  suddenly,  and  a  great  difierence  therefore  always 
exists  between  the  arterial  and  venous  pressure  (v.  p.  294).  Since 
there  is  a  permanent  high  pressure  in  the  arteries  as  compared 
with  that  in  the  capillaries  and  veins,  there  can  be  no  difficulty 
in  explaining  the  permanent  flow  through  the  capillaries  from 
arteries  to  veins. 

The  fundamental  problem  that  must  be  clearly  understood  in 
studying  the  dynamics  of  the  circulation  is,  how  the  high  pressure 
in  the  arteries  is  kept  up,  or,  in  other  words,  how  the  arteries  can 
exert  so  much  pressure  on  the  blood  when  the  capillary  out-flow 
is  so  wide  and  free. 

From  the  description  already  given  of  the  action  of  the  heart, 
it  appears  that  each  beat  of  the  ventricle  pumps  some  six  ounces 
of  blood  into  the  aorta,  which  blood,  though  coming  to  the  left 
ventricle  from  the  pulmonary  circulation,  may,  on  account  of 
the  exact  co-operation  of  the  two  sides  of  the  heart,  be  said  to 
be  pumped  out  of  the  systemic  veins,  and  thus,  as  far  as  the 
physical  forces  are  concerned,  the  pulmonary  circulation  may  be 


288  MANUAL   OF    PHYSIOLOGY. 

left  out  of  the  question.  This  occurs  some  seventy  times  a  min- 
ute, so  that  an  enormous  quantity  of  blood  is  removed  from  the 
veins  and  forced  into  the  arteries.  The  ventricles  in  filling  the 
arteries  have  to  work  against  considerable  pressure,  and  may  be 
said  to  pump  the  blood  from  the  low-pressure  veins  up  into  the 
high-pressure  arteries,  and  this  work  is  the  cause  of  the  pressure 
difference  between  the  two  sets  of  vessels.  During  the  contrac- 
tion of  the  heart  the  ventricular  pressure  must  exceed  that  of  the 
aorta,  while  during  the  diastole  it  falls  to  that  of  the  auricle  or 
even  of  the  great  veins.  The  heart  then  is  the  first  and  most 
important  agent  by  which  the  arteries  are  kept  stretched  and 
over-filled,  and  the  veins  are  emptied. 

A  second  important  factor  in  enabling  the  high  blood -pressure 
to  be  kept  up,  is  the  resiliency  of  the  middle  coat  of  the  arteries. 
It  is  only  on  account  of  the  great  elasticity  of  the  arterial  walls, 
that  these  vessels  are  capable  of  being  so  over-filled,  and  only  on 
account  of  the  perfect  resiliency  of  the  elastic  coat,  that  they  are 
able  to  exert  such  powerful  pressure  on  the  blood  for  such  an  un- 
limited time.  If  the  arteries  were  rigid  tubes,  overfilling  them 
with  a  fluid  itself  inelastic  would  be  out  of  the  question  ;  the  out- 
flow from  the  distal  extremity  would  take  place  exactly  when  the 
additional  charge  of  blood  was  injected  by  the  heart. 

With  each  contraction  the  ventricle  overcomes  arterial  pressure, 
and  further  stretches  the  elastic  artery.  But  the  act  of  injecting 
the  blood  into  the  aorta  only  occupies  about  one-quarter  of  each 
heart  beat.  The  serai-lunar  valves  bear  the  pressure  of  the  blood 
in  the  aorta  for  the  rest  of  the  time.  The  whole  force  of  the  ven- 
tricle is  therefore  used  up  in  causing  arterial  distension.  During 
the  greater  part  of  the  time  (about  three-quarters  of  the  heart's 
cycle)  the  arteries  are  in  the  condition  of  over-filled  elastic  tubes, 
with  their  cardiac  end  firmly  closed  by  the  aortic  valves,  and 
their  distal  ends  open. 

It  follows  that  the  blood  flowing  constantly  out  of  the  distended 
arteries  through  the  capillaries  into  the  veins  tends  to  equalize 
the  pressure  in  the  veins  and  arteries.  But  why  is  not  this  con- 
stant outflow  sufficient  to  allow  the  pressure  in  the  arteries  to  fall 
to  the  level  of  that  in  the  veins?     Or  in  other  words,  what  is  the 


BLOOD   PIVESSURE.  289 

impediment  offered  to  the  escape  of  the  blood  from  the  arteries 
that  thus  keeps  them  distended  ?  If  the  arteries  and  veins  were 
a  set  of  continuous  wide  tubes  of  similar  construction  and  capacity 
throughout,  it  would  be  impossible  for  the  heart  to  empty  the 
veins,  to  overfill  the  arteries,  and  to  establish  the  great  pressure 
difference  that  normally  exists.  Therefore  some  resistance  equal 
to  the  pressure  must  be  offered  to  the  flow  of  the  blood  from  the 
arteries  into  the  veins. 

This  resistance  is  made  up  of  several  items,  of  which  one  alone 
is  sufficient  to  keep  up  the  arterial  pressure,  namely,  the  active 
contraction  of  the  arterioles.     No  doubt  the  enormous  increase  of 

Fig.  130. 


Tracing,  showing  the  effect  of  Stimulation  of  Vagus  Nerve.  Stimulus 
applied  between  vertical  lines.  (Recording  surface  moved  from  left  to 
right.) 

surface  over  which  the  blood  has  to  move  in  the  capillaries,  and 
the  pressure  exercised  upon  them  by  the  surrounding  elastic  tis- 
sues, impede  the  emptying  of  the  arteries.  But  that  the  contrac- 
tility of  the  arterioles  is  the  most  important  item,  may  be  seen 
from  the  following  consideration.  The  resistance  offered  by  the 
capillaries  themselves,  whether  by  the  greater  friction  or  by  the 
elastic  pressure,  is  insignificant  when  compared  with  the  arterial 
blood  pressure,  and  is  far  from  sufficient  to  restrain  the  blood 
from  rushing  into  the  veins.  This  is  seen  when  the  arterioles  are 
paralyzed  by  the  destruction  of  the  nervous  mechanism  controlling 


290 


MANUAI>   OF   PHYSIOLOGY. 


them  ;  the  blood  then  flows  readily  through  the  capillary  network, 
the  veins  become  engorged,  the  arterial  blood  pressure  falls,  and 
the  circulation  comes  to  a  standstill,  in  spite  of  the  heart's  more 
rapid  beats.     We  know  also  that  after  the  arterioles  are  passed 


Fig.  131. 


Mercurial  ]\Ianometer  for  measuring  and  recorUuig  the  blood  pressure. — 
a.  Proximate  limb  of  manometer.  6.  Union  of  two  limbs  of  manometer. 
e.  The  rod  floating  on  mercury  and  carrying  the  writing  point,  d.  Stop- 
cock through  which  the  sodium  bicarbonate  can  be  introduced  between  the 
blood  and  mercury  of  manometer. 


the  pressure  falls  suddenly,  and  in  the  capillary  network  the  pres- 
sure is  always  very  low. 

The  four  great  factors  then,  in  keeping  up  the  arterial  blood- 
pressure,  are :  1,  the  heart-beat;  2,  perfect  aortic  valves:  3,  the 
elastic  resiliency  of  the  large  arteries;  4,  the  resistance  offered  by 
the  contraction  of  the  muscular  arterioles. 
•   If  any  of  these  fail,  the  mechanism  of  the  circulation  is  at  once 


MEASUREMENT   OF  THE   BLOOD   PRESSURE.  291 

impaired.  For  example,  the  heart's  beat  may  be  stopped  by  the 
stimulation  of  the  inhibitory  nerve  fibres  of  the  vagus,  iu  which 
case  the  blood  pressure  rapidly  falls,  as  shown  by  the  curve  taken 
by  the  graphic  method.  Or  weakness  of  the  heart-beat  may  arise 
from  disease  (fatty  degeneration)  of  the  muscle,  when  signs  of  low 
arterial  tension  can  be  recognized  in  the  human  subject. 

Any  insufficiency  of  the  aortic  valves,  whose  duty  it  is  to  close 
the  proximal  end  of  the  arteries,  that  permits  the  blood  to  flow 
backward  into  the  ventricle,  allows  the  pressure  in  the  arteries  to 
fall  between  each  ventricular  systole,  so  that  the  characteristic 
"pulse  of  unfilled  arteries"  is  recognized  by  the  physician. 

The  resiliency  of  the  arterial  coats  may  also  be  destroyed  to  a 
certain  extent  by  degeneration  of  the  tissue,  in  which  case  the 
large  arteries  become  greatly  distended,  and  unable  to  exert  their 
normal  steady  pressure  on  the  blood. 

Injuries  of  the  nerve  centres  are  often  associated  with  paralysis 
of  the  muscular  arterioles,  and  fall  of  blood  pressure ;  but  the 
effect  upon  the  blood  pressure  of  dilatation  of  the  small  arteries 
can  be  best  seen  by  experimenting  on  the  nerves  that  control  their 
contraction  in  the  lower  animals.  If  paralysis  or  inhibition  of 
the  vasomotor  mechanisms  be  experimentally  produced,  the  result 
on  the  arterial  pressure  is  the  same,  namely,  a  sudden  fall,  which 
may  reach  zero :  all  opposition  to  the  outflow  of  blood  from  the 
arteries  being  stopped,  they  cease  to  be  tense,  even  though  the 
ventricle  continue  to  beat  and  pump  the  blood  into  them. 

Measurement  of  the  Blood  Pressure. 

The  first  attempt  at  direct  measurement  of  blood  pressure 
was  made  by  the  Rev.  Stephen  Hales  about  the  middle  of  last 
century,  who,  wishing  to  compare  the  motion  of  fluids  in  animals 
with  that  of  plants,  connected  a  tube  in  an  artery  of  a  living 
animal,  and  found  that  the  blood  was  ejected  with  considerable 
force,  and  that  when  the  artery  of  a  horse  was  brought  into  union 
with  a  long  upright  tube,  the  blood  reached  a  height  of  about 
three  yards. 

The  blood  itself  is  not  now  used  as  a  measure,  because  so  much 


292 


MANUAL   OP   PHYSIOLOGY. 


blood  leaving  the  vessels  tends  to  empty  them  and  to  reduce  the 
pressure  in  the  arteries ;  besides  the  coagulation  of  the  blood  soon 
stops  the  experiment.  We  now  employ  the  mercurial  manometer, 
which  consists  of  a  column  of  mercury  in  a  U-shaped  tube.  To 
prevent  coagulation,  the  tube  between  the  mercury  and  blood  is 
filled  with  a  solution  of  sodium  carbonate,  the  pressure'being  regu- 
lated to  equalize  as  nearly  as  possible  that  of  the  blood.  A  rod 
is  made  to  float  upon  the  mercury,  in  the  open  side  of  the  tube, 

Fig.  132. 


The  ordinary  modern  form  of  rotating  blackened  cylinder  (r),  which  is 
moved  by  the  clockwork  in  the  box  (a)  by  means  of  the  disk  (d)  pressing 
upon  the  wheel  (n),  which  can  be  raised  or  lowered  by  the  screw  (l),  so  as 
to  rub  on  a  part  of  the  disk  more  or  less  near  the  centre,  and  thus  rotate  at 
different  rates.  The  cylinder  can  be  raised  by  the  screw  (v),  which  is  turned 
by  the  handle  (u).     (Hermann.) 


and  to  the  upper  extremity  of  this  a  writing  apparatus  can  be 
attached,  so  that  by  the  movements  of  the  mercury,  a  graphic 
record  of  the  blood  pressure  and  its  variation  can  be  traced  on  a 
regularly  moving  surface.     This  instrument,  known  as  Ludwig's 


KYMOGRAPHS. 


293 


Kymograph,  is  that  used  in  all  ordinary  measurements  and  ex- 
periments on  blood  pressure.  In  order  to  overcome  the  inertia 
of  the  mercurial  column,  another  instrument  has  been  devised 
which  will  be  mentioned  in  speaking  of  the  character  of  the  curve 


Fig.  133. 


Lxjdwig's  Kymograph  with  continuous  paper. — The  instrument  consists 
of  an  iron  table,  above  which  the  recording  surface  is  slowly  drawn  past  the 
writing  points  from  an  endless  roll  of  paper  on  the  left  by  the  motion  of  the 
cylinder  (c),  and  rolled  up  on  a  spindle  next  the  driving-wheel  on  the  right. 
The  mercurial  manometers  (d)  are  fixed  so  that  the  open  ends  come  in  front 
of  the  firm  roller  upon  which  the  paper  rests.  The  writing  style  can  be  seen 
rising  from  these  tubes  while  the  other  limbs  of  the  manometers  lead  through 
the  stop-cocks  to  the  tubes  which  are  in  communication  with  the  blood- 
vessels. The  time  is  recorded  by  means  of  a  pen  attached  to  the  electro- 
magnet (m),  which,  by  a  "breaking"  clock,  is  demagnetized  every  second. 
The  moment  at  which  a  stimulus  is  applied  is  marked  by  a  key  to  which 
another  pen  is  attached  near  the  time-marker. 


(p.  297).  When  an  experiment  of  long  duration  has  to  be  made, 
a  recorder  with  a  long  rolled  strip  of  paper  can  be  employed 
(Fig.  133). 


294 


MANUAL   OF   PHYSIOLOGY. 


The  modern  accurate  methods  of  research  have  taught  us  the 
differences  in  pressure  that  exist  in  the  various  parts  of  the  vascu- 
lar system.  However,  direct  measurement  can  only  be  accom- 
plished in  vessels  of  such  a  size  as  to  admit  a  cannula,  hence  the 
pressure  in  the  capillaries,  in  the  very  minute  arteries  and  veins 
can  only  indirectly  be  estimated.  The  pressure  in  all  parts  of 
the  vascular  system  is  subject  to  frequent  variation  to  be  presently 

Fio.  134. 


Diagram  sliowing  the  relative  height  of  the  blood  pressure  in  the  different 
regions  of  the  vessels.  H.  Heart.  A.  Arteries.  «.  Arterioles,  c.  Capil- 
laries. V.  Small  veins,  v.  Large  veins,  h.  v.  being  the  zero  line,  the  pres- 
sure is  indicated  by  the  elevation  of  the  curve.  The  numbers  on  the  left 
give  the  pressure  (approximately)  in  mm.  of  mercury. 

mentioned,  but  this  table  may  be  useful  iu  giving  a  general  idea 
of  the  average  permanent  differences  that  exist  in  the  different 
vessels  of  large  animals  and  man. 

Large  arteries  (Carotid,  Horse)  -f-  160  mra.,  mercury. 
Medium   "        (Brachial,  Man)  -f  120  mm.,        " 
Capillaries  of  Finger,  -f-    38  mm.,        " 

Small  Veins  of  Arm,  -{-      9  mm.,        " 

Large  Vein  of  Neck,  —  1  to  —  3  mm.,        " 


RECORD  OF  BLOOD  PRESSURE.  295 

If  the  different  parts  of  the  circulation  be  represented  on  the 
base  line  H.  A.  c.  v.,  these  letters  corresponding  to  heart,  arteries, 
capillaries,  and  veins  respectively,  and  if  the  height  of  the  blood 
pressure  be  represented  on  the  vertical  line  in  mm.  Hg,  the  curve 
A,  a,  c,  V,  would  give  about  the  relative  pressure  in  the  various 
parts  of  the  circulation.  This  shows  that  in  the  receiving  chamber 
of  the  heart  the  pressure  is  below  zero,  while  the  ventricular  pump 
drives  it  to  the  height  of  the  arterial  pressure  160  mm.  Hg.  In 
the  arteries  the  pressure  though  gradually  falling  is  everywhere 
high,  while  just  before  the  blood  reaches  the  capillaries,  a  sudden 
fall  occurs.  The  variation  after  this  is  merely  a  gentle  descent 
until  the  large  venous  trunks  are  reached,  where  the  pressure  is 
below  zero.  From  a  purely  physical  point  of  view  then,  the  ven- 
tricle may  be  regarded  as  pumping  the  blood  up  to  an  elevated 
high-pressure  reservoir  of  small  capacity  (the  arteries),  from  which 
it  rapidly  falls  by  numerous  outlets  into  an  expansive  low-lying 
irrigation  basin  (the  wide  capillaries),  whilst  it  slowly  trickles 
back  to  the  well  (the  auricle)  under  the  pump,  which  lies  below 
the  surface  pressure. 

From  this  diagram  the  following  points  can  be  gathered  : 

1.  The  great  difference  between  the  pressure  on  the  arterial 

and  venous  sides  of  the  circulation. 

2.  The  comparatively  slight  difference  in  pressure  in  the  dif- 

ferent parts  of  the  arterial  or  of  the  venous  systems  re- 
spectively. 

3.  The  suddenness  of  the  fall  in  the  pressure  between  the  small 

arteries  and  the  capillaries,  where  the  great  resistance  to 
the  outflow  is  met  with. 

4.  In  the  large  veins  the  pressure  of  the  blood  is  habitually 

below  that  of  the   atmosphere,  only   becoming   positive 
during  forced  expirations. 

Variations  in  the  Blood  Pressure. 

If  the  blood  pressure  be  recorded  with  Ludwig's  Kymograph,  a 
tracing  will  be  obtained  which  shows  that  the  pressure  undergoes 
periodic  elevations  and  depressions  of  two  different  kinds.     The 


296 


MANUAL  OF  PHYSIOLOGY. 


smaller  oscillations  are  found  to  correspond  with  the  heart-beat, 
the  larger  waves  have  the  same  rhythm  as  the  respiratory  move- 
ments, and  the  average  elevation  of  the  mercurial  column  is 
spcjkeu  of  as  the  mean  pressure.  In  the  large  arteries  of  the  warm- 
blooded animals  this  mean  pressure  varies  with  the  size  of  the 
animal  from  90  mm.,  mercury,  to  more  than  200  mm.  lu  cold- 
blooded animals  it  is  comparatively  low,  from  22  mm.  (frog, 
Volkmaun)  to  84  mm.  (large  fish). 

Fig.  13'. 


Blood-pressure  Curve,  drawn  by  mercurial  manometer.  0—x  —  zero 
line,  y—y'  =  curve  with  large  respiratory  waves  and  small  waves  of  heart 
impulse.     A  scale  is  introduced  to  show  height  of  pressure  in  c  c  of  Hg. 

The  general  mean  pressure  in  the  arteries  is  increased  by  :  (1), 
increased  action  of  the  heart;  (2),  increased  contraction  of  the 
muscular  coat  of  the  arteries  ;  (3),  sudden  increase  in  the  quan- 
tity of  blood.  When  the  change  is  gradual  the  vessels  adapt 
themselves  to  the  increase.  The  opposite  of  these  conditions  may 
be  said  to  have  an  opposite  effect. 


RESPIRATORY   WAVE   IN   BLOOD-PRESSURE   CURVE.      297 

The  character  of  the  change  in  pressure  which  accompanies  the 
heart's  systole  is  not  exactly  shown  in  the  tracing  obtained  by  the 
mercurial  manometer,  owing  to  the  sluggishness  of  the  movement 
of  the  mercurial  column,  which,  as  it  were,  rubs  off  the  apices  of 
the  curves.  But,  with  the  spring  Kymograph  of  Fick,  the  de- 
tails of  these  oscillations  are  marked.     They  are,  of  course,  syn- 

FiG.  136. 


Pick's  Spring  Manometer. — A  hollow  C-shaped  spring  (a),  made  of  ex- 
tremely thin  metal,  is  fixed  at  (6  b)  where  its  cavity  communicates  with 
the  tube  (k).  The  top  of  the  C  is  connected  at  (a)  with  the  writing  lever. 
Any  increase  of  pressure  in  the  tube  (k)  causes  the  spring  to  expand  and 
move  the  writing  point  (o)  up  and  down. 


chronous  with  the  arterial  pulse,  and  follow  the  variations  of  ten- 
sion, as  will  be  described  when  treating  of  that  subject.  (See 
Figs.  136  and  137.) 

The  cause  of  the  undulations  in  the  blood-pressure  curve  cor- 

25 


298  MANUAL   OF   PHYSIOLOGY. 

responding  to  the  respiratory  movements  is  not  quite  so  simple  as 
it  might  appear  to  be  at  first  sight,  and  it  has  often  been  misun- 
derstood. Though  many  causes  have  been  given,  no  single  one 
appears  to  explain  adequately  all  the  changes  that  may  occur  in 
this  phenomenon  under  varying  circumstances.  At  first  sight 
the  respiratory  movements  and  consequent  pressure  changes 
within  the  thorax  would  seem  to  give  a  simple  mechanical  ex- 
planation. But  if  the  change  occurring  in  the  iutrathoracic 
pressure  be  examined  carefully,  it  will 
Fig.  137.  be  found  not  to  correspond  exactly  with 

the  so-called  respiratory  wave  of  the 
pressure  curve  in  the  arterial  system. 
Owing  to  the  lungs  being  very  elastic 
and  constantly  tending  to  shrink  away 
from  the  costal  pleura,  as  may  be  seen 

_,     .        -,,     ,  when  the  thoracic  cavitv  is  opened  and 

Tracing  of  blood  pressure  -  ' 

taken    with    Pick's    mano-    the  lungs  collapse,  the  pressure  in  the 
meter.  pleural  cavity  is  less  than  that  of  the 

atmosphere  which  distends  the  lungs, 
i.e.,  the  pleuial  pressure  is  negative.  All  the  viscera  in  the 
thoracic  cavity  are  habitually  under  the  influence  of  the  negative 
pressure.  Thus  the  elastic  lungs  exert  a  kind  of  traction  on  the 
pericardium,  and  tend  to  cause  a  negative  pressure  within  the 
heart  and  great  systemic  vessels,  both  arteries  and  veins.  The 
influence  is  of  course  greater  in  the  thin-walled  veins,  in  which 
the  pressure  is  minimal,  than  in  the  thick-walled  arteries,  where 
the  pressure  is  so  high  that  they  feel  but  little  the  intrathoracic 
decrease. 

The  amount  of  traction  exercised  on  the  pericardial  contents 
bv  the  lungs  varies  with  the  respiratory  movements,  being  slightly 
increased  during  inspiration,  and  decreased  during  expiration. 
The  diflferences  which  are  thus  produced  however  during  ordinary 
respiration  are  very  slight  (probably  1  mm.,  mercury),  when 
compared  with  the  mean  negative  pressure,  which,  while  the 
thorax  is  in  an  intermediate  state  of  extension,  is  probably 
about  10  mm.,  mercury.  So  slight  a  variation  as  1  mm.,  mer- 
cury, could  not,  by  direct  action  on  the  aortic  arch,  cause  the 


RESPIRATOEY   WAVE   IN   BLOOD-PRESSURE   CURVE.      299 

change  of  several  millimetres  which  we  see  in  the  respiratory 
undulation  in  the  arterial  pressure.  We  must,  therefore,  seek 
the  explanation  in  the  changes  it  causes  in  the  great  veins. 

It  has  been  suggested  that  by  facilitating  the  flow  from  the 
great  veins  into  the  thorax,  by  a  kind  of  sucking  action,  the 
amount  of  blood  entering  the  right  auricle  during  inspiration  may 
be  increased,  and  thus  the  left  ventricles  may  be  better  filled  and 
made  to  beat  more  actively,  so  as  to  cause  an  elevation  in  the 
arterial  pressure. 

But  this  view  appears  to  leave  the  pulmonary  circulation  out 
of  the  question  in  a  way  hardly  justifiable,  since  it  must  be  tra- 

Fia.  138. 


Blood-pressure  and  Respiratory  Tracings  recorded  synchronously — record- 
ing surface  moving  from  right  to  left — showing  that  the  variations  in  press- 
ure in  the  arteries  (continuous  line)  and  in  the  thoracic  cavity  (dotted  line) 
do  not  exactly  correspond,  the  latter  continuing  to  fall  after  the  blood 
pressure  has  commenced  to  rise. 


versed  by  the  blood  before  the  increased  inspiratory  inflow  to  the 
right  auricle  can  affect  the  left  ventricle  or  the  systemic  arteries. 
The  sequence  of  events  may  be  read  as  follows.  During  inspi- 
ration the  negative  pressure  on  the  right  heart  is  increased  ;  the 
atmospheric  pressure  acting  on  the  tributaries  of  the  superior 
vena  cava  is  unchanged,  while  the  pressure  in  the  abdominal 
cavity  is  increased,  and  the  inferior  vena  cava  compressed  by  the 
muscular  action.  The  blood  then  flows  more  readily  during  in- 
spiration into  the  right  heart,  and  consequently  the  lungs  receive 


300  MANUAL   OF   PHYSIOLOGY. 

a  larger  supply  of  blood  during  this  period.  In  expiration  the 
negative  intrathoracic  pressure  becomes  less  negative,  the  com- 
pression of  the  abdominal  viscera  is  relieved,  and  the  flow  into 
the  auricle  loses  somewhat  in  force. 

It  must  be  carefully  borne  in  mind  that  the  left  side  of  the 
heart  works  under  different  conditions,  for  the  same  variations  of 
pressure  affect  both  the  pulmonary  veins  and  the  left  auricle 
equally,  since  they  are  both  included  in  the  thoracic  cavity,  and 
are  both  subjected  to  a  slightly  varying  negative  pressure.  The 
aid  given  to  the  flow  into  the  right  heart  by  the  low  intrathoracic 
pressure  is  quite  absent  on  the  left  side ;  so  that  the  thoracic 
movements  do  not  exert  any  influence  on  the  flow  of  blood  from 
the  pulmonary  to  the  systemic  arteries.  But  while  inspiration  is 
taking  place  the  lungs  receive  a  larger  supply  of  blood ;  and  from 
the  relative  amounts  of  blood  in  the  different  organs  it  is  prob- 
able that  this  slight  excess,  having  passed  the  lungs,  arrives  at  the 
left  ventricle  at  the  period  of  expiration.  Thus,  during  expira- 
tion the  left  ventricle  receives  from  the  lungs  and  ejects  to  the 
systemic  arteries  an  amount  of  blood  slightly  in  excess  of  that 
which  it  receives  and  ejects  during  inspiration.  This  may  have  a 
direct  effect  on  the  pressure  in  the  great  arterial  trunks.  But  it 
is  more  than  probable  that  excess  of  blood  in  the  heart  cavities 
acts  as  a  nervous  stimulus,  and  excites  the  inhibitory  centre  of  the 
heart  and  the  depressor  centres  which  control  the  arterioles. 

The  adoption  of  this  view  appears  necessary  from  the  following 
facts : 

(1.)  The  rise  in  pressure  is  not  exactly  synchronous  with  ex- 
piration or  inspiration  ; 

(2.)  The  heart  beats  more  slowly  during  expiration  than  in- 
spiration ; 

(3.)  This  difference  at  once  disappears  if  the  vagi  be  cut, 
and  the  respiratory  wave  becomes  greatly  modified ; 

(4.)  The  respiratory  wave  is  observed  when  artificial  respi- 
ration is  employed,  in  which  the  forcing  of  air  into 
the  lungs  is  the  cause,  and  not  the  result,  of  the  tho- 
racic movements,  so  that  the  pressure  effects  are  re- 
versed. 


RESPIRATORY    WAVE    IN    BLOOD-PRESSURE    CURVE.       301 

We  may  conclude,  then,  that  a  sympathy  in  action  can  be  dis- 
tinctly recognized  in  the  working  of  the  respiratory  and  cardiac 
nerve  centres. 

Since  the  undulations  known  asTraube's  curves  occur  in  cura- 
rized  animals  when  no  respiratory  movements  are  performed,  it  has 
been  proposed  to  explain  the  respiratory  undulations  in  the  same 
way,  namely,  by  referring  them  to  a  stimulation  of  the  vaSomoter 
centre  by  impure  blood,  which  by  rhythmical  impulses  increasing 
the  contraction  of  the  arterioles  causes  a  rhythmical  variation  in 
the  blood  pressure.  This  explanation  seems  to  be  rendered  unsat- 
isfactory by  the  fact  that  the  respiratory  undulations  go  on  even 
when  the  arterioles  are  cut  off  from  their  chief  nerve  centres  by 
sections  of  the  spinal  cord.  So  that  if  these  undulations  are  to 
be  referred  to  nerve  mechanism,  we  are  ignorant  of  the  course  the 
nerve  impulses  take,  for  any  rhythmical  sympathy  existing  be- 
tween the  respiratory  and  vasomotor  nerve  centres  in  the  medulla 
cannot  have  any  influence  when  the  cord  is  cut. 

The  blood  pressure  in  the  capillaries  cannot  be  directly  measured ; 
it  is  difficult  to  estimate,  and  very  variable.  The  slightest  change 
of  pressure  in  the  corresponding  veins  or  arteries  causes  the  press- 
ure in  the  capillaries  to  rise  or  fall.  Thus  variations  in  pressure 
are  constantly  occurring  in  the  capillaries,  which  cause  an  alter- 
ation in  the  rate  of  flow,  or  even  a  retrograde  stream  in  some 
parts  of  the  network. 

The  regulation  of  the  blood  supply,  and,  therefore,  of  the  press- 
ure in  the  capillaries,  is  under  the  control  of  the  small  arterioles 
which  supply  them  ;  a  slight  relaxation  of  the  muscle  of  the  arteri- 
oles causes  great  increase  in  the  amount  of  blood  flowing  through 
the  capillaries,  as  can  readily  be  seen  with  the  microscope. 

The  blood  pressure  in  the  veins  must  be  less  than  that  in  the 
capillaries,  and,  as  has  been  said,  must  diminish  as  the  heart  is 
approached,  where  in  the  great  veins  (superior  cava)  the  pressure 
is  said  to  be  rather  below  that  of  the  atmosphere  ( —  3  to  —  5 
ram.,  mercury).  During  inspiration  the  minus  pressure  may  be- 
come much  less,  whilst,  on  the  other  hand,  it  is  only  by  very 
forced  expiration  that  it  ever  becomes  equal  to  or  at  all  above 
that  of  the  atmosphere. 


302  MANUAL   OF   PHYSIOLOGY. 

This  is  a  most  important  fact,  as  the  suction  considerably 
helps  the  flow  of  blood  from  the  veins,  and  also  the  current  of 
fluid  from  the  thoracic  duct  that  bears  the  chyle  from  the  intes- 
tines and  the  fluid  collected  from  the  tissue  drainage  back  to  the 
blood. 

The  pressure  of  the  blood  in  the  veins  may  then  be  said  to  be 
generally  nil,  since  the  veins  are  nowhere  over-filled  with  blood. 
The  pressures,  on  the  other  hand,  that  can  be  registered  and 
measured  depend  upon  forces  communicated  from  without, 
namely  :  (1)  gravity  ;  (2)  the  elastic  pressure  of  the  surrounding 
tissue;  and  (3)  the  pressure  exerted  by  the  muscle  during  con- 
traction. This  pressure  is  increased  by  any  circumstance  which 
impedes  the  flow  of  blood  through  the  right  side  of  the  heart, 
through  any  large  vein,  or  through  the  pulmonary  circulation; 
but  when  no  abnormal  obstacle  exists  to  the  venous  blood-current, 
the  pressure  in  those  vessels  can  never  attain  any  great  height,  for, 
as  we  have  seen,  the  large  trunks  are  constantly  being  emptied 
by  the  heart's  action. 

Most  circumstances  which  tend  to  lower  arterial  pressure  also 
tend  to  raise  the  pressure  in  the  veins,  so  that,  when  the  heart's 
action  is  weak,  or  its  mechanism  faulty,  the  venous  pressure  rises. 

In  the  veins  of  the  extremities  the  pressure  greatly  depends  on 
the  position  of  the  limb,  as  it  varies  almost  directly  with  the  effect 
of  gravity. 

In  the  pulmonary  circulation  the  direct  measurement  of  the  in- 
travascular pressure  is  rendered  extremely  difficult,  and  possibly 
erroneous,  by  the  fact  that  to  ascertain  it  the  thorax  has  to  be 
opened.  It  has  been  found  in  the  pulmonary  artery  to  be  in  a 
dog  29.6  mm.,  in  a  cat  17.6  mm.,  and  in  a  rabbit  12  mm.,  mercury. 

The  Arterial  Pulse. 
Each  systole  of  the  ventricle  sends  a  quantity  of  blood  into  the 
aorta,  and  thus  communicates  a  stroke  to  the  blood  in  that  vessel. 
The  incompressible  fluid  causes  the  tense  arterial  wall  to  distend 
still  farther,  and  the  shock  to  the  column  of  blood  is  not  trans- 
mitted onward  directly  by  the  fluid,  but  causes  the  elastic  walls 
of  the  arteries  to  yield  locally,  and  thus  it  is  converted  into  a  wave 


THE   ARTERIAL   PULSE.  303 

which  passes  rapidly  along  those  vessels.  This  motion  in  the  walls 
of  the  vessel  can  be  felt  wherever  the  artery  can  be  reached  by 
the  finger,  but  best — as  in  the  case  of  the  radial  and  temporal  ar- 
teries— where  the  vessel  is  superficial  and  lies  on  some  unyielding 
structure  such  as  bone. 

This  motion  of  the  vessel  wall  is  called  the  arterial  pulse.  It 
consists  of  a  simultaneous  widening  and  lengthening  of  the  artery. 
The  arteries  near  the  heart  are  nvuch  more  affected  by  the  pulse 
wave  than  those  more  remote,  the  wave  becoming  fainter  and 
fainter  as  it  travels  along  the  branching  arteries.  In  the  smallest 
arteries  it  is  hardly  recognizable,  and  under  ordinary  circum- 
stances is  quite  absent  in  the  capillaries  and  veins. 

The  diminution  in  the  pulse-wave  in  the  smaller  arteries  chiefly 
depends  upon  the  fact  that  the  force  of  the  wave  is  used  up  in 
distending  the  successive  part  of  the  arteries.  In  the  small  arte- 
ries the  extent  of  surface  to  which  the  pulse-wave  is  communicated 
is  enormous,  and  thereby  the  wave  is  much  decreased.  Moreover, 
it  is  probable  that  reflected  waves  pass  from  the  peripheral  end 
of  the  arterial  tree,  and  meeting  the  pulse-wave  in  the  small  arte- 
ries help  to  obliterate  it. 

The  pulse-wave  can  easily  be  shown  to  take  some  time  to  pass 
along  the  vessels.  Near  the  orifice  of  the  aorta  the  arterial  dis- 
tension occurs  practically  at  the  same  time  as  the  ventricular 
systole,  but  even  with  comparatively  rough  methods  the  radial 
pulse  can  be  observed  to  be  a  little  later  than  the  heart-beat. 
The  difference  of  time  between  the  pulse  in  the  facial  and  the 
dorsal  artery  of  the  foot  has  been  estimated  to  be  one-sixth  of  a 
second,  and  the  difference  in  the  distance  of  these  vessels  from  the 
heart  is  about  1500  mm.,  so  that  the  rate  at  which  the  pulse-wave 
travels  is  nearly  10  metres  per  second.  The  velocity  of  the  wave 
is  said  to  be  regulated  by  the  degree  of  elasticity  in  the  walls  of 
the  vessels,  and  it  would  appear  to  be  quicker  in  the  lower  than 
in  the  upper  extremity. 

The  time  that  the  wave  takes  to  pass  any  given  point  must  be 
equal  to  the  time  taken  to  produce  it,  that  is  to  say,  the  time  the 
ventricle  occupies  in  sending  a  new  charge  of  blood  into  the  aorta, 
which  is  about  one-third  of  a  second.     Knowing  the  rate  at  which 


304  MANUAL   OF   PHYSIOLOGY. 

the  wave  travels  (10  m.  per  sec.)  and  the  time  it  takes  to  pass 
any  given  point  (^  sec.),  its  length  may  be  calculated  to  be  about 
three  metres,  or  about  twice  as  long  as  the  longest  artery.  Thus 
the  pulse  wave  reaches  the  most  distaut  artery  in  one-sixth  of  a 
second,  or  about  the  middle  of  the  ventricular  systole,  and  when 
the  wave  has  quite  passed  from  the  arch  of  the  aorta,  the  summit 
of  it  has  only  just  reached  the  arterioles. 

Numerous  instruments  have  been  invented  for  the  demonstration 
and  graphic  representation  of  the  pulse  in  the  human  being.  Of 
these  the  one   most  commonly  used  is  Marey's  Sphygmograph 

Fig.  139. 


Marey's  Spliyginograpli. — The  frame  (b,  b,  b)  is  fastened  to  the  wrist  by 
the  straps  at  B,  i?,  and  the  rest  of  tlie  instrument  lies  on  the  forearm.  The 
end  of  the  screw  (v)  rests  on  tlie  spring  (r),  the  button  of  which  lies  on  the 
radial  artery.  Any  motion  of  the  button  at  R  is  communicated  to  v,  which 
moves  the  lever  (l)  up  and  down.  When  in  position,  the  blackened  slip  of 
glass  (p)  is  made  to  move  evenly  by  the  clockwork  (h)  so  that  the  writing 
point  draws  a  record  of  the  movements  of  the  lever. 

(Fig.  139),  by  means  of  which  a  graphic  record  of  the  pulse  is 
made,  in  the  form  of  a  tracing  of  a  series  of  elevations  and  depres- 
sions (Fig.  140).  The  elevations  correspond  to  the  onset  of  a 
wave,  and  the  depressions  to  its  departure,  or  to  the  temporary 
rise  and  fall  of  the  arterial  pressure.  In  the  falling  part  of  the 
curve  an  irregularity  caused  by  a  slight  second  wave  is  nearly 
always  seen.  This  is  called  the  dicrotic  wave.  Sometimes  there 
are  more  than  one  of  these  secondary  waves,  the  most  constant  of 
which  is  a  small  wave  preceding  the  dicrotic,  called  jorerficro^ic; 
but  the  dicrotic  is  always  more  marked  than  any  other.     Several 


PUI^E    TRACINGS.  305 

waves  of  oscillation  can  be  seen  as  a  gradually  decreasing  series 
in  tracings  taken  from  elastio  tubes,  but  we  cannot  say  positively 
that  they  occur  in  the  arteries.  When  several  secondary  waves 
exist  in  the  pulse  curve,  the  smaller  ones  probably  depend  on 
oscillation  caused  by  the  lever  of  the  instrument. 

The  dicrotic  wave  does  not  depend  on  the  instrument,  because 
the  skilled  finger  laid  on  the  radial  artery  at  the  wrist  can  easily 
detect  it,  and  it  can  be  directly  seen  in  the  vessel  when  the  pul- 
sation in  the  arteries  is  visible,  or  when  a  jet  of  blood  escapes  from 
an  artery. 

When  a  new  charge  of  blood  is  shot  into  the  aorta  the  elastic 
wall  of  the  vessel  is  suddenly  stretched.  At  the  same  time  a 
shock  is  given  to  the  column  of  blood,  and  the  fluid  next  the 
valves  is  moved  forwards  with  great  velocity.  Owing  to  its  in- 
ertia the  fluid  tends  to  pass  onwards  from  the  valves,  and  thus 

Fig.  140. 


Tracing  drawn  by  Marey's  Spliygmograpli.  The  surface  moved  from 
right  to  left.  The  vertical  upstrokes  show  the  period  when  the  shock  is 
given  by  the  systole  of  the  ventricle.  The  upper  wave  on  the  downstroke 
shows  when  the  blood  has  ceased  to  enter  the  aorta.  Then  comes  the  di- 
crotic depression  which  is  a  negative  wave  produced  by  the  momentary 
backflow  in  aorta,  and  the  dicrotic  elevation  caused  by  the  closure  of  the 
valves. 

allows  a  momentary  fall  in  pressure,  which  is  at  once  followed  by 
the  reflux  of  the  blood  and  the  forcible  closure  of  the  valves. 

The  first  crest  or  apex  of  the  pulse  curve  corresponds  to  the 
shock  given  by  the  systole,  and  is  greatly  exaggerated  by  the  in- 
ertia of  the  lever.  The  crest  of  the  predicrotic  wave  marks  the 
moment  when  the  blood  ceases  to  flow  from  the  ventricle,  and 
therefore  it  is  the  real  head  of  the  pulse  wave. 

The  dicrotic  wave  has  been  explained  as  (1)  a  wave  of  oscil- 

26 


306  MANUAL   OF    PHYSIOLOGY. 

lation,  or  (2)  a  wave  reflected  from  the  periphery.  If  the  former, 
it  should  be  les.s  marked  tlian  the  prodicrotic,  which  by  this  theory 
is  said  to  be  the  iirat  wave  of  oscillatiou,  for  each  succeeding  os- 
cillaliou  is  less  than  its  forerunner.  But,  as  already  mentioned, 
the  dicrotic  is  invariably  the  larger. 

There  are  many  reasons  why  it  cannot  be  a  wave  of  reflection 
from  the  periphery  of  the  arterial  tree;  viz.:  1.  Its  curve  is  not 
nearer  the  primary  wave  when  the  peripheral  vessels  are  ap- 
proached. 2.  The  arterioles  which  form  the  peripheral  resistance 
are  at  too  irregular  distances  to  give  one  definite  wave  of  reflec- 
tion. 3.  It  is  seen  in  the  spurting  of  an  artery  cut  off  from  the 
periphery.  4.  It  increases  with  the  greater  elasticity  and  low  ten- 
sion, while  the  reflected  waves  diminish. 

The  dicrotic  notch  depends  upon  a  negative  wave  caused  by  the 
sudden  stop  of  inflow  and  the  momentary  reflux  of  blood  before 
the  valves  are  closed  ;  and  the  dicrotic  crest  is  no  doubt  produced 
by  the  closure  of  the  aortic  valves,  at  which  moment  the  sudden 
check  given  to  the  reflux  of  the  blood  column  causes  a  positive 
centrifugal  wave  to  follow  the  primary  wave  of  the  pulse. 

The  view  that  the  reflux  of  blood  and  the  closure  of  the  valves 
produce  the  dicrotic  wave  is  supported  by  the  fact  that  the  condi- 
tions which  increase  the  dicrotism — viz.,  (1)  sharp,  strong  systole, 
(2)  low  tension,  and  (3)  perfect  resiliency — promote  the  recoil 
and  closure;  and,  on  the  other  hand,  the  conditions  which  di- 
minish the  dicrotic  wave  in  the  most  marked  degree,  are  (1)  in- 
efficiency of  the  aortic  valves,  and  (2)  a  rigid  calcareous  condi- 
tion of  the  arteries. 

It  can  be  shown  by  means  of  an  elastic  tube,  fitted  with  a 
suitable  pump  and  sphygmographs,  that  when  its  outlet  is  closed 
a  positive  wave  is  reflected  from  the  distal  end  back  to  the  pump, 
and  when  it  is  open  a  negative  centripetal  wave  is  reflected.  This 
fact  assists  us  in  explaining  the  variations  in  the  character  of  the 
pulse  curve  of  the  radial  artery  where  the  equidistance  of  the 
derived  arterioles  enables  the  reflected  waves  to  have  considerable 
eflTect.  When  the  arterioles  are  constricted  (a  condition  corre- 
sponding to  the  closed  tube)  a  positive  wave  centripetal  is  re- 
flected and  is  added  to  the  pulse-wave  so  as  to  diminish  the  di- 


VARIATIONS    IN   THE    PULSE.  307 

erotic  notch,  and  give  the  curve  known  as  characteristic  of  "high 
tension  "  pulse,  as  in  Bright's  disease.  (Fig.  141,  II.)  On  the 
other  hand,  when  the  arterioles  are  widely  dilated  (corresponding 
to  the  open  condition  of  the  tube)  a  negative  wave  is  reflected, 
and  is  subtracted  from  the  force  of  the  pulse-wave  so  as  to  exag- 
gerate the  dicrotic  notch,  and  give  the  tracing  characteristic  of 
the  "low  teusion  "  pulse  seen  in  fever,  etc.     (Fig.  141,  III.) 

The  mean  rate  of  the  pulse  varies  in  different  individuals, 
seventy-two  per  minute  being  a  fair  average  for  a  middle-aged 
adult.  It  varies  also  with  many  circumstances,  which  must  be 
borne  iu  mind  in  taking  the  pulse  as  a  clinical  guide. 

Fig.  141. 


I.  Scheme  of  Normal  Pulse  Curve :  a,  Entrance  of  ventricular  stream 
into  the  aorta,  the  lever  is  jerked  too  high  to  reach  * ;  ab  sJiows  real  summit 
of  waves  ;  6,  point  at  which  stream  from  ventricle  ceases ;  c,  negative  wave 
caused  by  (1)  sudden  cessation  of  inflow  and  slight  reflux  of  blood  ;  d,  point 
of  closure  of  aortic  valves  ;  e,  positive  wave  from  valves  (dicrotic  wave).  The 
time  may  be  measured  on  abscissa  at  a^  b'  d' . 

II.  Scheme  of  High  Tension  Pulse  Curve :  A,  curve  of  radial  pulse,  which 
is  the  resultant  of  positive  reflected  wave  c,  added  to  the  primary  curve  B. 

III.  Scheme  of  Low  Tension  Pulse  Curve:  A,  radial  pulse  curve,  which 
is  the  resultant  of  the  negative  reflectant  wave  c,  subtracted  from  the  primary 
wave  B.     (After  Grashey.) 

1.  Age.  At  birth  it  is  about  140  per  minute,  and  is,  generally 
speaking,  quicker  in  young  than  in  old  people,  commonly  falling 
to  60  in  aged  persons. 

2.  Sex.  It  is  more  rapid  in  females  than  in  males. 

3.  Position.  It  is  quicker  standing  than  lying,  particularly  if  a 
patient  who  has  been  lying  down,  stand  or  sit, up,  the  pulse  be- 
comes more  rapid. 


308  MANUAL    OF    PHYSIOLOGY. 

4.  Tlie  time  of  day.  It  gains  in  rapidity  in  the  morning  till  9 
o'clock,  and  in  the  evening  till  G  o'clock,  and  falls  in  the  day- 
time, being  at  its  minimum  at  midnight. 

5.  Muscular  exercise  quickens  it. 

6.  It  is  quicker  during  inspiration  than  expiration. 

7.  It  increases  with  increase  of  temperature. 

8.  It  is  variously  affected  by  emotions. 

Velocity  of  the  Blood  Current. 

The  velocity  of  the  blood  must  not  be  confounded  with  the 
velocity  of  the  pulse  wave,  which  bears  to  it  the  same  relation  as 
the  surface  waves  on  a  river  do  to  the  rate  of  the  stream  of  water. 

It  has  already  been  mentioned  that  the  general  bed  of  the 
blood  increases  from  the  aorta  to  the  capillaries,  and  decreases 
from  the  capillaries  to  the  vena  cava,  on  account  of  the  branches 
or  tributaries  of  nearly  every  artery  or  vein  being  collectively 
of  larger  area  than  the  vessel  from  which  they  spring  or  to  which 
they  may  lead  ;  or,  in  other  words,  if  we  imagined  the  whole 
vascular  system  fused  together  into  one  tube  it  would  form  two 
somewhat  irregular  cones,  one  corresponding  to  the  arteries  and 
the  other  to  the  veins,  with  their  bases  placed  at  the  capillaries 
and  their  apices  at  the  heart.  Between  the  two  a  still  wider 
aggregate  would  represent  the  capillaries.  Compare  Fig.  128, 
p.  283. 

Since  the  same  quantity  of  blood  must  pass  through  each  sec- 
tion of  these  cones  in  a  given  time,  the  rate  at  which  it  flows  must 
vary  greatly  in  the  different  parts,  being  faster  in  proportion  as 
the  diameter  of  the  part  is  narrower,  in  accordance  with  the  well- 
known  physical  law  that  with  the  same  amount  of  liquid  flowing 
its  velocity  changes  inversely  with  the  diameter  of  the  tube. 
Thus,  the  mean  velocity  of  the  flow  in  the  arteries  becomes  slower 
and  slower  as  the  capillaries  are  approached,  and  in  the  wide  bed 
of  the  latter  the  rate  of  the  current  is  reduced  to  a  minimum.  In 
the  small  veins  the  rate  is  slower  than  in  the  larger  trunks,  but 
on  the  venous  side  its  rapidity  never  reaches  that  of  the  aorta, 
where  it  may  be  said  to  move  at  least  twice  as  quickly  as  in  the 
vena  cava. 


VELOCITY   OF   THE    BLOOD    CURRENT. 


309 


The  following  table  may  be  useful  in  giving  a  general  idea  of 
the  average  velocity  in  different  parts  of  the  circulation  : 

Near  valves  of  aorta — while  the  ventricles  are  con- 
tracting it  reaches  1200  mm.  per  sec. 
Descending  aorta,      .         .      300-600 
Carotid,     ....      205-357 


Radial,      .... 

100 

mm 

per  sec. 

Metatarsal, 

57 

Arterioles, 

50 

Capillaries, 

.5 

Venous  radicles. 

25 

Small  veins  on  dorsum  of  hand 

50 

Vense  cavse, 

200 

In  the  aorta  near  the  valves  the  blood  current  varies  in  rapidity, 
because  the  flow  through  the  aortic  orifice  is  intermittent,  and 
this  variation  must  be  more  or  less  communicated  to  the  neighbor- 
ing arteries  in  the  form  of  an  increase  of  rapidity  coincident  with 
the  beat  of  the  arterial  pulse.  The  variation  in  the  rate  of  the 
blood-flow  which  is  caused  by  the  heart-beat  diminishes  with  the 
force  of  the  pulse  as  the  smaller  arteries  are  approached,  and 
finally  ceases  completely  in  the  capillaries,  where  under  ordinary 
circumstances  the  flow  is  perfectly  continuous.  In  the  first  part 
of  the  aorta  the  velocity  of  the  blood-flow  is  reduced  to  nil  after 
each  ventricular  beat,  while  in  the  capillaries,  no  change  is  per- 
ceived. Between  these  two  extremes  all  gradations  may  be  found, 
which  follow  the  same  rules  as  the  pulse. 

The  general  mean  velocity  varies  directly  with  the  blood  pres- 
sure, which  bears  an  inverse  relation  to  the  calibre  of  the  arteries, 
and  further,  the  mean  velocity  in  any  one  artery,  and  its  branches, 
will  vary  with  the  diameter  of  the  vessels,  which  are  constantly 
undergoing  local  changes  in  size. 

Generally  speaking,  quick  heart-beats  cause  increase  in  velocity 
of  the  stream,  but  no  definite  or  invariable  relation  exists  between 
the  two,  the  vaso-motor  influences  having,  no  doubt,  much  more 
effect  than  the  heart-beat  on  the  rate  of  the  stream  in  the  smaller 
vessels. 


310 


MANUAL   OF   PHYSIOLOGY. 


In  lookiug  at  the  blood  passing  through  the  small  vessels  of  a 
transparent  tissue,  such  as  the  frog's  tongue  or  web,  it  appears 


Fig.  142. 


Small  portion  of  Frog's  AVeb,  very  liiglily  magnified.  (Huxley.) — A. 
Wall  of  capillary  vessels.  B.  Tissue  lying  between  the  capillaries,  c.  Epi- 
thelial cells  of  skin,  only  shown  in  part  of  specimen  where  the  surface  is  in 
focus.  D.  Nuclei  of  epithelial  cells,  e.  Pigment  cells  contracted.  F.  Red 
corpuscles  (oval  in  the  frog).  G.  H.  Red  corpuscles  squeezing  their  way 
through  a  narrow  capillary,  showing  their  elasticity,     i.  White  blood  cells. 


WORK    DONE   BY   THE   HEART.  311 

that  different  parts  of  the  column  of  fluid  move  with  different 
velocities.  Down  the  centre  of  the  stream  the  red  corpuscles  are 
seen  coursing  rapidly,  while  between  the  central  part  and  the 
vessel  wall  on  each  side  a  pale  line  of  plasma  can  be  recognized, 
which  seems  to  flow  more  slowly  and  to  carry  with  it  only  a  few 
white  corpuscles. 

In  the  veins  the  velocity  varies  enormously  with  a  variety  of 
circumstances  which  have  little  or  no  effect  on  the  arterial  flow. 
Thus,  the  position  of  the  body  or  limb,  the  activity  of  the  neigh- 
boring muscles  and  the  respiratory  movements  alter  it,  but  as  a 
general  rule  the  flow  in  the  veins  is  pretty  steady,  there  being  no 
pulsation  or  corresponding  variation  of  velocity.  In  the  large 
vessels  the  onward  flow  is  affected  by  the  contraction  of  the  auri- 
cles. During  the  auricular  systole  the  veins  cannot  empty  them- 
selves, and  therefore  there  is  a  slight  check  to  the  onward  flow, 
and  the  velocity  of  the  current  is  accordingly  reduced.  In  cases 
where  the  auricles  are  dilated  and  distended  with  blood  this  may 
cause  a  definite  pulsation,  which  becomes  visible  in  the  great  veins 
of  the  neck. 

Work  done  by  the  Heart. 

This  can  only  be  determined  when  the  mechanism  of  the  ves- 
sels is  understood.  The  amount  of  work  done  by  any  form  of 
engine  may  be  expressed  as  so  many  kilogrammetres  per  hour. 
That  is  to  say,  the  numbers  of  kilogrammes  it  could  raise  to  the 
height  of  one  metre  in  that  time. 

The  left  ventricle  moves  with  each  systole  about  0.188  kilo- 
gramme of  fluid  against  an  arterial  pressure  corresponding  to 
3.20  metres  height  of  blood,  i.e.,  0.188  x  3.21  =  0.604  kilogram- 
metres  for  each  systole.  This  at  75  per  minute  for  twenty-four 
hours  would  be — 

0.604  X  75  X  60  X  24  =  65,230  kilogrammetres. 

The  right  ventricle  does  about  one-third  as  much  work  as  the 
left,  making  a  total  of  86,970  kilogrammetres  for  the  ventricles, 
or,  in  other  words,  the  heart  of  a  man  weighing  twelve  stone  does 


812  MANUAL    OF    PHYSIOLOGY. 

as  much  work  in  twenty-four  hours  as  would  be  required  to  lift 
his  body  1248  yards  into  the  air,  i.e.,  nearly  ten  times  as  high  as 
the  top  of  St.  Paul's  Cathedral. 

Controlling  Mechanisms  of  the  Bloodvessels. 

Vaso- Motor  Nerves. 

That  the  arteries  possessed,  as  well  as  elastic  resiliency,  vital 
contractility,  which  regulated  the  amount  of  blood  flowing  to  any 
given  part,  was  well  known  to  John  Hunter. 

The  muscle  cells  have  also  been  long  since  clearly  demonstrated 
in  the  middle  coats  of  the  arteries,  but  nothing  was  known  of  the 
nervous  channels  which  bore  the  stimulus  to  the  vessels,  or  the 
nerve  centres  which  regulated  their  contraction,  until  compara- 
tively recent  times. 

The  first  knowledge  concerning  special  nervous  arrangements 
for  the  control  of  the  muscular  wall  of  the  vessel  was  given  to 
us  by  Claude  Bernard,  in  his  notable  experiment  of  cutting  the 
sympathetic  nerve  in  the  neck,  which  was  always  followed  by  an 
increase  in  temperature  of  that  side  of  the  head,  and  a  great  ex- 
pansion and  over-filling  of  the  arteries. 

It  was  further  observed  that  stimulation  of  the  superior  gan- 
glion of  the  sympathetic  brought  about  an  opposite  result,  namely, 
a  loss  of  temperature  and  contraction  of  the  vessels  on  the  same 
side  as  the  stimulus  was  applied.  If  the  stimulus  was  much 
increased,  the  vessels  contracted  much  more  than  the  normal 
amount,  but  on  cessation  of  the  stimulus  they  became  greatly 
dilated  above  the  normal  point  and  the  temperature  rose  again, 
but  after  a  time  the  eflfect  of  the  stimulus  gradually  passed  off. 
From  this  it  was  concluded  that  the  sympathetic  in  the  neck  con- 
veyed to  the  muscles  in  the  bloodvessels  impulses  which  caused  a 
certain  amount  of  habitual  contraction  of  the  vessel  wall,  which 
was  called  tonic  contraction,  corresponding  to  what  was  already 
recognized  as  arterial  tone.  When  the  nerve  was  divided  this 
tone  disappeared,  but  when  gently  stimulated  it  reappeared,  and 
when  more  strongly  stimulated  an  exaggerated  contraction  set  in 
causing  complete  occlusion  of  many  of  the  vessels. 


VASO-MOTOR    NERVES.  313 

Subsequent  experiraeuts  have  shown  that  all  the  vessels  of  the 
body  are  supplied  with  similar  vaso-raotor  nerves,  section  of 
which  destroys  their  tone,  while  their  stimulation  causes  contrac- 
tion of  all  the  vessels  in  the  territory  presided  over  by  the  stimu- 
lated nerve. 

Experiment  has  also  shown  that  these  nerves  come  from  the 
cerebro-spinal  axis,  passing  out  from  the  spinal  cord  as  "  commu- 
nicating nerves,"  commonly  becoming  associated  with  the  sym- 
pathetic chain,  and  are  distributed  to  the  vessels  either  as  special 
nerves,  branches  of  the  sympathetic  (as  the  splanchnics),  or  with 
the  general  peripheral  nerve  trunks. 

The  nerve  centre,  which  governs  the  vast  majority  of  the  vaso- 
motor channels,  lies  in  the  upper  part  of  the  medulla  oblongata 
in  the  floor  of  the  fourth  ventricle.  This  is  proved  by  two  facts: 
1st,  most  of  the  brain  may  be  removed  without  diminishing  the 
arterial  tone  ;  and  2d,  if  the  spinal  cord  be  cut  below  the  medulla 
(artificial  respiration  of  course  being  kept  up)  the  mean  blood 
pressure  is  found  to  fall  immediately  almost  to  zero,  which  is  due 
to  the  relaxation  of  the  smaller  arteries  consequent  on  the  paral- 
ysis of  their  muscular  coat. 

The  same  can  be  seen  in  the  web  of  a  frog,  in  which  the  me- 
dulla has  been  destroyed  (pithed)  while  the  circulation  is  being 
studied.  The  small  arteries  dilate  and  the  pulse  becomes  apparent 
in  the  capillaries,  and  even  in  the  veins.  From  these  facts  it 
seems  highly  probable  that  in  the  medulla  oblongata  a  vaso-motor 
centre  exists,  which  regulates  the  contraction  of  all  the  vessels, 
and  keeps  them  constantly  more  or  less  contracted  ;  the  centre 
receiving  some  continuous  stimulation,  which  results  in  a  slight 
general  vascular  constriction  or  arterial  tone.  The  existence  of 
such  a  centre  in  the  medulla,  and  of  nerve  channels  in  the  cord 
leading  from  such  a  centre,  is  made  certain  by  the  fact,  that  if  a 
gentle  stimulus  be  applied  to  a  certain  part  of  the  medulla,  or 
just  below  it,  simultaneous  general  vascular  constriction  sets  in, 
and  is  indicated  by  a  great  and  sudden  rise  in  the  blood  pressure. 

The  action  of  the  vaso-motor  centre  can  be  increased,  and 
thereby  the  tone  of  the  vessels  elevated,  and  the  pressure  raised, 
either  by  (1)  direct  or  (2)  reflex  excitation.     If  the  blood  flowing 


314 


MANUAL   OF   PHYSIOLOGY. 


through  the  medulla  contains  too  little  oxygen  or  too  much  car- 
bonic anhydride,  it  stimulates  the  centre  directly  and  the  blood 
pressure  rises.  This  may  be  seen  by  temporarily  suspending 
artificial  respiration  during  an  experiment  on  blood  pressure. 
Reflexly  the  activity  of  the  vaso-motor  centre  can  be  increased 
by  (1)  the  stimulation  of  any  large  sensory  nerve,  or  (2)  by  sud- 
den emotion  {fear). 

The  tone  of  the  arteries  may  be  diminished  by  lessening  the 
activity  of  the  vaso-motor  centre  by  the  stimulation  of  a  peculiar 
afferent  nerve,  the  anatomy  of  which  has  been  made  out  in  the 

Fig.  143. 


./UULXjUUUl^'LTI JJJIJ  f1  JJ.JLJ.JULJLAJIJUUUUL, 

Kymograjjliic  tracing  showing  llie  eliect  on  tlie  blood-pressure  curve  of 
stimulating  the  central  end  of  the  depressor  nerve  in  the  rabbit.  The  re- 
cording surfoce  moving  from  left  to  right. — (c)  Commencement  and  (o)  ces- 
sation of  stimulation.  There  is  considerable  delay  (latency)  in  both  the 
production  and  cessation  of  the  effect,  (t)  Marks  the  rate  at  which  the 
recording  surface  moves,  and  the  line  below  is  the  base  line.     (Foster.) 

rabbit,  and  probably  has  its  analogue  in  man,  and  which  passes 
from  the  inner  surface  of  the  heart  to  the  medulla.  The  effect 
of  stimulation  of  this  nerve  in  lowering  the  pressure  is  so  great 
that  it  has  been  called  the  depressor  nerve.  Some  emotions  {shame) 
may  also  reduce  the  activity  of  the  centre,  as  is  seen  in  blushing, 
which  is  simply  dilatation  of  the  facial  vessels. 


DEPRESSOR   NERVE.  315 

Besides  this  chief  vaso-motor  centre  it  is  probable  that  in  the 
higher  animals,  as  certainly  is  the  case  in  the  frog,  other  centres 
are  distributed  throughout  the  spinal  cord,  which  seem  to  be  able 
to  take  the  place  of  the  great  primary  centre.  For  after  the 
spinal  cord  has  been  cut  high  up,  the  hinder  extremities  more  or 
less  recover  their  vaso-motor  power  in  a  few  days,  and  destruction 
of  the  lower  part  of  the  spinal  cord  causes  renewed  vaso-motor 
paralysis.  In  frogs  this  is  very  well  marked,  the  centres  being 
less  confined  to  the  medulla  than  is  the  case  in  the  more  highly 
organized  animals. 

During  recent  times  numerous  investigations  on  the  subject  of 
vaso-motor  nerves  have,  no  doubt,  thrown  much  light  on  the 
subject,  but  these  inquiries  have  not  made  the  nerve  mechanism 
by  which  the  various  vascular  areas  are  governed  so  clear  or  so 
obvious  as  might  be  wished. 

In  order  to  explain  and  reconcile  the  various  experimental 
truths  on  this  subject  (too  numerous  to  be  mentioned  here),  we 
must  suppose  that  the  vaso-motor  nerve  mechanisms  are  very 
complex.  The  supposition  of  some  such  arrangements  as  the 
following  may  help  to  simplify  the  matter  in  some  degree  to  the 
student. 

1.  The  bloodvessels  have  muscular  elements  which,  though 
commonly  controlled  by  nerves,  are  capable  of  automatic  activity. 
A  free  supply  of  arterial  blood  is  a  sufficient  stimulus  for  their 
moderate  action,  and  mechanical  or  other  stimulus  is  capable  of 
exciting  increased  constriction.  We  know  that  such  automatic 
contractile  elements  exist  in  some  of  the  lower  animals  (snail's 
heart,  hydra,  etc.),  and  we  have  no  reason  to  doubt  their  exist- 
ence in  mammals.  Moreover,  such  a  hypothesis  obviates  the 
necessity  of  supposing  that  local  nerve  elements  exist,  which  we 
cannot  recognize  morphologically. 

2.  In  the  medulla  oblongata  (in  close  relation  to  the  centres 
governing  the  respiratory,  cardiac,  intestinal,  and  other  move- 
ments subservient  to  the  vegetative  part  of  the  economy)  there 
exist  nerve  centres  which  constantly  exert  an  important  influence 
over  the  activity  of  the  vessel  muscles.  These  groups  of  nerve 
cells,  called  the  vaso-motor  centres,  are  intimately  connected  with 


3iG  MANUAL,   OF    PHYSIOLOGY. 

the  centres  which  preside  over  the  functional  activity  of  various 
organs  and  parts,  and  are  also  closely  related  to  the  nerves  com- 
ing from  all  parts  of  the  circulatory  apparatus.  From  these 
centres  impulses  of  two  distinct  kinds  may  emanate,  the  one  in- 
creasing the  action  of  the  contractile  elements,  and  the  other 
inhibiting  it. 

3.  Direct  communication  between  this  vaso-motor  centre  and  the 
contractile  elements  in  the  middle  coat  of  the  bloodvessels  is  kept 
up  by  means  of  efferent  nerve,  channels  of  different  sorts,  some 
bearing  stimulating  (vaso-constrictor)  others  inhibitory  (vaso- 
dilator) impulses,  these  being  conveyed  by  nerve  fibres  which  run 
side  by  side  in  the  same  nerve  cord. 

4.  The  activity  of  the  contractile  elements  of  any  given  vascu- 
lar area  may  be  altered  by  impulses  arising  from  different  sources. 
(a.)  Local  influences  under  ordinary  circumstances  are  brought 
but  little  into  play,  but,  if  cut  off  from  the  nervous  centres,  are 
capable  of  controlling  the  local  blood  supply  by  changing  the 
degree  of  arterial  constriction,  (y?.)  Central  influences  from  the 
medulla,  are  habitually  in  action,  affecting  all  the  vessels  and 
keeping  up  the  vascular  tone.  These  impulses  are  variously  modi- 
fied by  changes  occurring  in  distant  parts  of  the  circulatory  appa- 
ratus, and  can  be  regarded  as  a  general  regulating  mechanism. 
They  probably  pass  through  the  sympathetic  chain,  (y.)  Special 
influences,  which  are  associated  with  the  functions  of  the  different 
parts  and  organs,  are  only  called  into  operation  during  the  per- 
formance of  the  function,  whatever  it  may  be.  These  impulses 
probably  are  conveyed  by  the  same  nerves  as  excite  the  various 
forms  of  functional  activity,  namely,  ordinary  peripheral  nerves. 

These  three  sets  of  influences  are  variously  brought  about  in 
different  parts,  and  thus  we  find  that  section  or  stimulation  of 
the  different  nerves  gives  vaso-motor  effects  which  appear  contra- 
dictory. 

Section  of  a  sensory  nerve  causes  temporary  vaso-motor  paraly- 
sis, owing  to  the  tonic  constrictor  influence  being  cut  off.  Stimu- 
lation of  the  peripheral  stump  causes  vaso-coustriction  from 
excitation  of  the  same  fibres. 

The  stimulation  of  a  motor  nerve-fibre  causes  an  increase  in  the 


NERVOUS   CONTROL    OF   THE    BLOODVESSELS.  317 

flow  of  blood,  or  in  other  words,  is  associated  with  a  vaso-dilator 
effect,  probably  dependent  on  the  inhibitory  effect  of  certain  centri- 
fugal fibres  which  control  the  local  agencies. 

Thus  we  must  suppose  that  there  exist  local  agents  under  the 
control  of  the  medullary  centres,  and  that  there  are  two  distinct 
efferent  and  afferent  sets  of  exciting  and  inhibitory  fibres  passing 
between  the  centre  and  periphery,  along  two  perfectly  distinct 
routes ;  one  being  in  the  direct  track  of  the  ordinary  functional 
nerve  of  the  part,  the  other  being  in  the  sympathetic,  which  to 
a  great  extent  runs  along  the  vessels  themselves,  and  forms  most 
intricate  networks  capable  of  carrying  impulses  in  all  imaginable 
directions. 


CHAPTER  XVIII. 
TPIE  MECHANISM  OF  RESPIRATION. 

In  its  course  through  the  circulation  the  blood  undergoes  a 
series  of  uece.ssary  modifications.  The  condition  of  the  fluid  is 
thus  constantly  being  altered  as  it  passes  from  one  part  and  organ 
to  another. 

It  has  already  been  seen  that  a  quantity  of  nutrient  material 
is  taken  up  by  the  blood  on  its  way  through  the  capillaries  of  the 
alimentary  tract.  Further,  a  stream  of  lymph  and  chyle  is  con- 
stantly pouring  into  the  great  venous  trunks,  so  that  from  two 
sources  the  blood  is  steadily  increased  in  quantity.  But  the  most 
urgently  essential  addition  to  the  circulatory  fluid  is  that  which 
it  receives  in  the  capillaries  of  the  lungs.  All  the  blood  passes 
through  these  organs,  in  order  that  the  changes  taking  place  in 
the  general  systemic  capillaries  may  be  counteracted  in  the  lungs. 
These  gas  interchanges  will  form  the  subject-matter  of  the  present 
chapter;  and  the  more  special  modifications  which  the  blood 
undergoes  in  the  ductless  glands,  the  spleen,  the  liver,  etc.,  as  well 
as  in  the  kidneys  and  other  excretory  glands,  will  be  considered 
in  subsequent  chapters. 

As  has  already  been  pointed  out  (Chapter  V.),  an  animal 
during  its  life  may  be  said  to  use  the  substances  supplied  to  it  in 
food  as  fuel,  and  thus  to  acquire  the  energy  which  is  bound  up 
in  them,  for  the  activities  of  the  various  tissues  are  really  com- 
bustions, being  invariably  associated  with  an  oxidation  of  some 
of  the  carbon  compounds,  so  as  to  produce  carbon  dioxide  and 
water.  In  order  that  the  structures  may  undergo  this  change 
they  must  have  a  ready  supply  of  oxygen  constantly  at  hand, 
and  moreover  the  carbon  dioxide  which  is  formed  in  the  process 
must  be  removed,  or  further  combustion  would  be  frustrated.  A 
regular  income  of  oxygen  and  a  regular  output  of  carbon  dioxide 
are  then  essential  to  life ;  hence  we  find  in  almost  all  animals 
special  arrangements  by  means  of  which  these  gases  can  find  their 


RESPIRATORY   MECHANISMS.  319 

way  to  atid  from  the  tissues  and  external  air  respectively.  These 
gas  interchanges  form  the  very  important  function  of  Respiration. 

Here,  as  in  the  case  of  the  nutritive  materials,  the  blood  acts 
as  the  carrier  between  the  tissues  and  the  outer  world.  The  pul- 
monary half  of  the  circulation  is  devoted  to  the  gas  interchange 
between  the  blood  and  the  atmosphere,  and  is  sometimes  spoken 
of  as  external  respiration.  The  gas  interchange  between  the  blood 
and  the  tissues  goes  on  in  the  general  systemic  capillaries,  and 
has  therefore  been  spoken  of  as  the  internal  or  tissue  respiration. 

The  special  arrangement  for  the  taking  up  of  oxygen  from  the 
air,  and  for  the  giving  up  of  carbonic  anhydride  to  the  air  is 
named  the  pulmonary  apparatus.  In  mammalia  this  is  so  far  per- 
fected that  all  the  necessary  gas  interchange  can  be  carried  on 
by  the  lungs,  and  the  respiratory  influence  of  the  external  skin 
or  the  mucous  passages  may  be  regarded  as  insignificant.  But  it 
should  be  remembered  that  whenever  the  blood  is  in  close  rela- 
tion to  oxygen,  as  in  the  case  of  swallowed  air,  the  oxygen  is  soon 
absorbed  by  the  blood. 

In  the  lower  animals  the  cutaneous  surface  aids  very  materially 
in  respiration,  and  thus  frogs  can  live  from  this  cutaneous  respi- 
ration alone  for  an  almost  indefinite  time. 

In  the  lungs  the  change  consists  in  oxygen  being  taken  from 
the  atmospheric*  air  by  the  blood  and  carbonic  anhydride  being 
given  oft'  from  the  blood  to  the  air.  In  the  capillaries,  on  the 
other  hand,  the  blood  takes  the  carbonic  anhydride  from  the  tis- 
sues, and  yields  to  them  a  great  portion  of  its  oxygen. 

In  the  lowest  class  of  animals  (e.g.,  amoeba)  we  find  no  special 
organs  for  the  purpose  of  respiration,  the  gas  interchange  being 
sufficiently  provided  for  by  the  exposure  of  the  general  surface 
of  their  bodies  to  the  medium  in  which  they  live,  namely,  water. 

Other  animals  have  some  special  apparatus  for  the  purpose  of 
respiration.    This  apparatus  has  always  the  same  essential  object, 

*  The  composition  of  the  atmosphere  is  everywhere  remarkably  constant, 
in  spite  of  its  oxygen  being  used  np  by  living  beings.     It  consists  of — 

Oxygen, 21  vols. 

Nitrogen, 79     " 

Moisture  (variai)le), 8  per  cent. 

Carbonic  acid  gas  (also  variable;,         ,        ,       .04      " 


320 


MANUAL   OF   PHYSIOLOGY. 


Fui.  144. 


that  of  exposing  their  tissues  to  a  medium  contaiuing  oxygen,  aad 
of  removing  the  carbonic  acid  gas. 

In  some  of  the  invertebrate  animals  it  suffices  to  distribute  the 
medium  containing  oxygen  throughout  the  tissues  of  the  animal 
by  means  of  tubes.  Thus  in  the  Echinodermata  a  water  vascular 
system  exists  which  seems  to  carry  on  the  function  of  respiration. 
A  similar  distribution  of  oxygen  takes  place  in  arthropoda,  deli- 
cately branching  open  tubes  (trachese)  distribute  air  to  the  tissues 
of  the  animal's  body. 

When  more  active  changes  occur  in  the  tissues  there  is  always 
a  perfect  blood  vascular  system,  and  the  blood  is  invariably  used 
as  the  distributing  and  collecting  agent  of  the  gases  in  the  tissues, 
and  by  flowing  through  some  special  organ  exposed  to  the  sur- 
rounding medium  it  insures  the  gas-interchange  between  the  body 

and  the  outer  world.  These  organs 
are  formed  on  two  general  types :  (1) 
external  vascular  fringes  ;  and  (2) 
internal  vascular  sacs. 

Animals  living  in  water  have  com- 
monly the  external  fringe  arrange- 
ment (gills),  whilst  those  living  in 
air  have  sacs  (lungs).  Some  animals 
(frogs,  toads,  etc.)  have  gills  in  the 
early  stages  of  their  life  and  lungs 
when  they  are  more  fully  developed. 
In  frogs  and  serpents  the  lungs  are 
simple  sacs,  with  the  inner  surface 
increased  by,folds  of  the  lining  mem- 
brane, which  gives  it  a  honeycomb 
appearance  ;  into  each  sac  opens  one 
of  the  divisions  of  the  air-tube.  In 
crocodiles  the  air-tubes  divide  into 
several  branches  which  open  into  a 
series  of  anfractuous  vascular  re- 
cesses which  communicate  one  with 
another. 

In    birds    wide    bronchial   tubes 
course  through  the  lung  tissue  to  reach  large  air  cavities,  and 


Diagram  of  the  Respiratory 
Organs.  The  windpipe  leading 
down  from  the  larynx  is  seen 
to  branch  into  two  large  bron- 
chi, which  subdivide  after  they 
enter  their  respective  lungs. 


STRUCTURE   OF    LUNGS. 


321 


their  walls  are  studded  with  the  openings  of  innumerable  air  cells, 
there  being,  however,  no  terminal  vascular  air  cavities  as  in  the 
mammalian  lung. 

The  respiratory  apparatus  of  mammals  consists  of  (1)  vascular 
sacs  filled  with  air,  known  as  the  lung  alveoli ;  (2)  channels  by 
which  these  sacs  are  ventilated — the  air-passages;  (3)  motor  ar- 
rangements, which  carry  on  the  ventilation  of  the  lungs — the 
thorax. 

1.  The  lungs  are  made  up  of  innumerable  minute  cavities 
(alveoli),  with  thin  septa  springing  from  the  inner  surface  so  as 

Fig.  145. 


Section  of  small  portion  of  Lung  in  which  are  seen  a  bronchial  tube  with 
its  plicated  lining  mucous  membrane  in  the  centre,  and  the  large  blood- 
vessels at  the  sides  cut  across.  Loose  areolar  tissue  and  numerous  lymphatics 
surround  the  large  vessels  and  separate  them  from  the  lung  tissue. 

to  divide  the  space  into  several  compartments^  or  air-cells.  Each 
of  these  cavities  forms  a  dilatation  on  the  terminal  twig  of  a 
branching  bronchus,  and  may  be  regarded  as  an  elementary  lung. 
The  aggregate  of  these  cavities,  and  the  branches  of  the  air-pas- 
sages and  vessels  distributed  to  them  make  up  the  structure  of  the 
lung. 

The  walls  of  the  cavities  are  formed  chiefly  of  fine  elastic  fibres, 
and  the  surface  is  lined  with  exceptionally  delicate  and  thin  celled 

27 


322 


MANUAL   OF   PHYSIOLOGY. 


epithelium.  Supported  in  the  delicate  frame-work  of  elastic  and 
connective  tissue  is  the  remarkably  close-set  meshwork  of  capilla- 
ries, in  which  the  blood  is  exposed  to  the  air.  The  delicate  wall 
of  the  vessel,  and  thin  body  of  the  epithelial  lining  cell  are  the 
only  structures  interposed  between  the  blood  and  the  air. 

2.  The  air-passages  are  kept  permanently  opeu  during  ordinary 
breathing  by  the  elasticity  of  more  or  less  rigid  tissues.  The 
trachea  and  bronchi  have  special  cartilaginous  springs  for  the 

Fig.  \iC). 


Muscles  of  Larynx,  viewed  from  above. — Th.  Thyroid  cartilage.  Or. 
Cricoid  cartilage.  V.  Edges  of  the  vocal  cords.  Arrj.  Arytenoid  cartilages. 
Th.  A.  Thyro-arytenoid  muscle.  C.a.l.  Lateral  crico-arytenoid  muscle. 
C  a.  p.  Posterior  crico-arytenoid  muscle.     Ar.  p.  Posterior  arytenoid  muscle. 

purpose.  These  are  closely  attached  to  the  fibro-elastic  tissues 
which  complete  the  general  foundation  of  the  walls  of  the  tubes. 
The  air-passages  are  throughout  lined  with  ciliated  cylindrical 
epithelium,  which  at  the  entrance  to  the  infundibula,  loses  its 
cilia,  and  forms  but  a  single  layer  of  flattened  cells. 

The  air-passages  are  supplied  with  muscle  tissue  of  different 
kinds.  Besides  the  ordinary  striated  muscles  that  control  the 
opening  of  the  anterior  and  posterior  nares  and  pharynx,  a  special 
set  surrounds  the  upper  part  of  the  larynx,  and  is  capable  of 
completely  closing  the  glottis,  and  thus  shutting  off  the  lung  cav- 
ities and  proper  air-passages  from  the  outer  air.     (  F,  Fig.  146.) 


STRUCTURE   OF   AIR-PASSAGES. 


323 


In  the  trachea  a  special  muscle  exists  which  can  narrow  the 
Fig.  147. 


Transverse  section  of  part  of  the  wall  of  a  medium-sized  bronchial  tube. 
X  30.  (F.  E.  Schultze.) — a.  Fibrous  layer  containing  plates  of  cartilage, 
glands,  etc.  b.  Coat  composed  of  unstriated  muscle,  c.  Elastic  sub-epithe- 
lial layer,    d.  Columnar  ciliated  epithelium. 


Fig.  148. 


d    e    ^    a. 


Section  of  a  portion  of  Lung  Tissue,  showing  part  of  a  very  small  bronchus 
cut  across.  (F.  E.  Schultze.) — a.  Fibrous  layer  containing  bloodvessels,  b. 
Layer  of  unstriated  nniscle.  c.  Layer  of  elastic  fibres,  d.  Ciliated  epithe- 
Kum. 


324 


MANUAL    OF    PHYSIOLOGY. 


Fig.  149. 


wiudpipe    by  approximatiug  the  extremities   of    the  C-shaped 

springs  that  normally  preserve  its  patency. 

In  the  bronchial  tubes  a  large  quantity  of  smooth  muscle  cells 

exist,  for  the  most  part  being  arranged  as  a  circular  coat,  which 

is  best  developed  in  the  small  tubes  (Fig.  148,  b).     As  we  pass 

from  the  large  to  the  smaller  bronchi  the  walls  become  thinner 
and  less  rigid,  and  the  cartilaginous  plates 
and  fibrous  tissue  gradually  diminish,  while 
on  the  other  hand  the  muscular  and  elastic 
elements  become  relatively  more  abundant. 
The  external  surface  of  the  lungs  is  com- 
pletely invested  by  a  serous  membrane — 
the  pleura,  which  is  reflected  to  the  wall  of 
the  thorax  from  the  roots  of  the  lungs,  and 
completely  lines  the  pleural  cavity  in  which 
they  lie.  Thus  the  lungs  are  only  attached 
to  the  thorax  where  the  air-passages  and 
great  vessels  enter,  the  rest  of  their  surface 
iDeing  able  to  move  over  the  inner  surface 
of  the  thorax,  and  to  retract  from  the  chest 
wall  if  air  be  admitted  into  the  pleural  sac. 
3.  The  thorax,  in  which  the  lungs  are 
placed,  is  a  bony  framework,  the  dimen- 
sions of  which  can  be  altered  by  the  muscles 
which  close  in  and  complete  the  cavity. 

The  framework  is  a  rounded  blunt  cone 
composed  of  a  set  of  bony  hoops — the  ribs, 
attached  by  joints  to  a  bent  pliable  pillar — 
the  vertebral  column,  and  held  together  in 
front  by  the  sternum,  to  which  they  are 
attached  by  resilient  cartilaginous  springs. 
The  ribs  slope  downwards  and  forwards, 
and  are  more  or  less  twisted  on  themselves 
about  the  middle  of  the  shaft. 

The  first  pair  of  ribs,  which  encircles  the 

apex  of  the  thoracic  cone,  forms  part  of  a  short,  flattened  hoop. 

It  slopes  downwards  in  front  to  reach  the  sternum.     Each  suc- 


Drawing  of  the  lit- 
eral view  of  Thorax  in 
the  position  of  gentle 
inspiration,  showing 
the  downward  slope  of 
the  ribs. 


CONSTRUCTION    OF   THORAX.  325 

ceeding  rib  from  above  downwards  increases  in  length,  in  the 
amount  of  its  slope  downwards  and  forwards,  and  in  the  obliquity 
of  its  shaft. 

The  floor  of  the  thorax  is  formed  by  a  dome-shaped  muscle — 
the  diaphragm,  which  bulges  with  its  convex  side  into  the  cavity, 
and  separates  the  thoracic  from  the  abdominal  viscera.  The 
upper  outlet  is  closed  arouud  the  trachea  by  several  muscles, 
which  pass  obliquely  upwards  from  the  first  rib  to  the  cervical 
vertebrae,  and  hold  the  upper  part  of  the  thorax  in  position. 
These  muscles  can  also  elevate  as  well  as  fix  the  first  rib,  as  will 
be  seen  when  speaking  of  the  muscles  in  detail.  The  intervals 
between  the  ribs  are  filled  up  by  two  sets  of  muscle  fibres,  which 
cross  one  another  at  right  angles,  and  are  attached  to  the  margins 
of  the  neighboring  ribs. 

The  base  of  the  thorax  is  connected  by  a  number  of  strong 
muscles  with  the  pelvis  and  the  spine,  whence  they  pass  upwards 
to  the  lower  ribs.  The  anterior  muscles  pull  down  the  sternum 
and  anterior  part  of  the  ribs.  The  posterior  fix  and  extend  the 
last  rib. 

From  a  mechanical  point  of  view  the  thorax  may  be  regarded 
as  a  specially  arranged  bellows,  the  dimensions  of  which  may  be 
increased  in  all  directions. 

Within  the  framework  of  the  bellows  is  an  elastic  bag,  with 
the  interior  of  which  the  outer  air  communicates  by  an  air-pipe, 
which  is  the  only  passage  between  the  atmosphere  and  the  in- 
terior of  the  bellows.  When  the  framework  enlarges  its  capacity 
the  pressure  of  the  atmosphere  pushes  a  stream  of  air  into  the 
elastic  sac  so  as  to  distend  it,  and  thus  fill  the  space  caused  by 
the  expansion  of  the  framework. 

By  the  motions  of  the  framework  a  stream  of  air  passes  in  or 
out  of  the  sac ;  a  small  quantity  of  the  air  contained  in  the  lungs 
is  thus  changed  at  each  breath,  and  a  certain  standard  of  purity 
kept  up. 

In  order  to  fully  understand  the  motions  by  which  the  thorax 
is  enlarged,  much  more  detailed  knowledge  of  the  anatomy  of  the 
bony  case  and  its  muscles  must  be  gained  than  can  possibly  be 
given  here. 


326  MANUAIi   OF    PHYSIOLOGY. 


Thoracic  Movements. 

Physiologically,  the  motions  are  divided  into  two  sets — (1) 
those  which  enlarge  the  thoracic  cavity,  and  cause  the  air  to  rush 
into  the  lungs,  called  inspiration ;  and  (2)  those  which  diminish 
the  size  of  the  thorax  and  force  out  the  air,  called  expiration. 

No  action  of  life  is  more  familiar  than  the  rhythmical  move- 
ments of  respiration.  The  slow  quiet  rise  and  fall  of  the  chest 
and  abdomen  are  the  signs  most  commonly  sought  as  indicative 
of  life ;  for  every  one  knows  that  constant  ventilation  must  go 
on  in  order  that  the  blood  may  readily  obtain  the  necessary 
amount  of  oxygen,  and  get  rid  of  the  carbonic  acid  gas,  the  or- 
dinary diffusion  that  takes  place  in  the  motionless  chest  being 
quite  insufficient  to  remove  the  heavy  carbonic  acid  gas  from  the 
lungs. 

The  rhythm  of  the  respiratory  movements  may  be  represented 
graphically  in  many  ways,  by  recording  either  the  changes  in  the 
diameter  or  circumference  of  the  thorax,  or  by  the  variations  of 
the  pressure  in  the  air-passages.  These  methods  more  or  less 
correspond,  and  give  curves  of  somewhat  the  same  character. 

The  respiratory  movements  are  up  to  a  certain  point  under  vol- 
untary control,  and  may  be  varied  by  the  will,  or  stopped  as  when 
one  holds  one's  breath. 

The  voluntary  control  of  the  respiratory  movements  is,  how- 
ever, limited ;  for,  if  we  hold  our  breath  for  any  length  of  time, 
a  moment  soon  arrives  when  the  "  necessity  of  respiration  "  over- 
comes the  strongest  will.  The  usual  respiratory  movements  are 
carried  on  without  our  being  conscious  of  them,  and  are,  there- 
fore, properly  involuntary. 

The  rate  of  the  respiratory  movements  varies  according  to  cir- 
cumstances, being  in  an  adult  man  about  18  per  minute ;  in  most 
of  the  lower  animals  it  is  much  more  rapid.  It  varies  with  age, 
being  very  rapid  at  birth,  decreasing  slowly  to  about  30,  and 
slightly  rising  towards  old  age.  The  following  table  (Quetelet) 
illustrates  this  : 


THORACIC   MOVEMENTS,  327 


A  new-born  infant 

respires 

44  times 

per 

minute 

5  years, 

26     " 

15—20     " 

20      " 

20—25     " 

18.7  " 

25—30     " 

16      " 

30—60     " 

18.1  " 

Muscular  exercise  increases  the  rapidity  of  the  respiratory 
movements,  and,  consequently,  the  effort  of  standing  produces  a 
more  frequent  respiration  than  is  found  in  the  recumbent  posture. 
Emotions  variously  affect  the  rate  and  rhythm  of  the  inspiration 
and  expiration  {e.g.,  sighing)  ;  and,  finally,  morbid  conditions, 
implicating  the  lungs,  usually  cause  a  greater  frequency  of  respi- 
ration, sometimes  attaining  a  rate  of  as  many  as  60-70  respira- 
tions per  minute. 

The  thorax  is  enlarged  in  all  directions  during  inspiration,  the 
motion  being  usually  referred  to  the  vertical,  transverse,  and  an- 
tero-posterior  diameters  respectively. 

The  vertical  diameter  is  increased  by  the  descent  of  the  lateral 
parts  of  the  diaphragm,  and  the  slight  elevation  of  the  parts  about 
the  apex. 

The  lateral  diameter  is  widened  by  the  side-droop  of  the  ribs 
beino'  lessened  ;  each  rib  is  rotated  upon  the  line  uniting  its  ex- 
tremities,  and  at  the  same  time  is  moved  upward  and  outward. 

The  antero-posterior  diameter  is  enlarged  by  the  general  eleva- 
tion of  the  ribs  and  sternum,  the  anterior  extremities  of  the  ribs, 
beinw  drawn  up  from  their  general  downward  incline,  push  the 
sternum  forwards. 

The  movements  of  the  diaphragm  depress  the  abdominal  vis- 
cera lying  beneath  it,  and  thereby  distend  the  elastic  abdominal 
wall  and  compress  the  gases  contained  in  the  intestines.  Thus, 
the  diaphragmatic  movements  cause  a  rhythmical  heaving  of  the 
abdomen.  Respiration  depending  chiefly  on  the  action  of  this 
one  muscle,  is,  therefore,  spoken  of  as  abdominal  respiration.  On 
the  other  hand,  when  the  ribs  are  the  chief  cause  of  expansion  of 
the  upper  parts  of  the  chest,  it  is  called  thoracic  or  costal  re^iration. 
These  two  types  of  respiratory  movements  may  be  imitated 
voluntarily,  and  are  variously  combined  in  different  individuals 


328  MANUAL   OF   PHYSIOLOGY. 

during  ordinary  respiration,  and  in  the  same  individual  under 
different  circumstances. 

In  men  the  general  character  of  the  ordinary  quiet  respiration 
is  abdominal,  the  movement  of  the  thorax  being  insignificant  in 
comparison  with  that  of  the  abdomen. 

In  women  the  reverse  is  the  case,  the  abdominal  movements 
are  slight  when  compared  with  those  of  the  upper  part  of  the 
thorax.  This  difference  is  only  well  marked  during  quiet  uncon- 
scious breathing  ;  any  forced  or  voluntary  respiratory  effJbrt 
changes  the  typical  character  of  man's  breathing,  and  the  costal 
movements  become  more  prominent.  In  a  forced  deep  inspira- 
tion the  upper  part  of  the  chest  shows  the  greatest  increase  in 
the  antero-posterior  diameter  in  both  sexes. 

This  difference  in  type  between  male  and  female  respiratory 
movements  has  been  ascribed  to  different  causes.  The  most  com- 
mon of  these  is  the  change  brought  about  by  the  costume  ordi- 
narily adopted  by  females.  This  can  hardly  be  an  adequate 
explanation  of  the  phenomenon,  for  we  find  the  same  type  exist- 
ing when  the  tight  garments  are  removed,  and  it  is  apparent  in 
those  who  have  never  been  constricted  by  tight  clothing,  and 
even  in  cases  where  no  clothing  at  all  has  been  used,  as  amongst 
the  inhabitants  of  hot  countries ;  so  that,  though  the  corset  may 
induce  an  exaggeration  of  the  costal  respiration,  by  constricting 
the  lower  ribs  and  interfering  with  the  action  of  the  diaphragm, 
it  would  not  seem  sufficiently  to  account  for  the  normal  physio- 
logical costal  type  of  breathing  found  in  women. 

The  occasional  distension  of  the  abdomen  during  pregnancy 
has  also  been  assigned  as  a  cause  of  the  female  type  of  breathing. 
That  this  type  of  breathing  should  be  transmitted  from  our  female 
ancestors  is  possible,  but  it  is  very  unlikely  that  pregnancy  is  the 
sole  agency  in  producing  it,  since  in  childhood  the  costal  type  is 
marked  in  both  sexes.  It  is  probable  that  the  abdominal  breath- 
ing of  the  male  is  also  acquired  and  increased  by  hereditary 
transmission,  and  is  really  due  to  the  gradual  increase  in  the 
development  of  the  muscles  of  the  upper  extremity  in  males, 
causing  a  greater  fixedness  of  the  upper  ribs  from  which  they 
take  origin. 


INSPIKATORY   MUSCLES. 


329 


Inspiratory  Muscles. 

The  act  of  inspiration  is  not  performed  by  any  single  muscle; 
indeed,  even  the  most  gentle  and  quiet  respiration  requires  the 
coordinated  action  of  many  sets  of  muscles.  Most  of  these  mus- 
cles have  other  duties  to  perform  besides  helping  to  produce  res- 
piratory movements. 

Fig.  150. 


Diagram  of  a  section  made  vertically  from  side  to  side  through  the  tho- 
racic and  part  of  the  abdominal  cavities  to  show  the  position  of  the  dia- 
phragm, which  is  indicated  by  the  dark  line  (d  d)  placed  on  the  parts  of 
the  muscle  that  descend  in  inspiration. — p.  Pericardial  cavity,  l,.  Liver. 
S.  Stomach.     R.  Koots  of  lungs  cut  through. 

Those  which  are  strictly  inspiratory  in  their  function  are  : 

1.  The  Diaphragm  with  its  accessory  Quadratus  Lumborum 

to  fix  its  origin  from  the  last  rib. 

2.  Levatores  costarum  (including  thescaleni)  with  their  ac- 

cessory intercostals,  which  act  chiefly  as  regulators. 

3.  The  Serratus  posticus  superior. 

The  Diaphragm  is  the  most  important  inspiratory  muscle.  It 
is  the  only  one  muscle  which  unaided  can  koep  up  the  necessary 

28 


330 


MANUAL   OF    PHYSIOLOGY. 


Fio.  151. 


thoracic  ventilation,  and,  in   injury  of  the  spinal  cord,  owing  to 
its  isolated  nervous  supply,  it  may  be  called  upon  to  do  so. 

During  ordinary  quiet  breathing  in  the  male  it  does  the  greater 
part  of  the  work. 

When  not  in  action,  a  great  part  of  the  muscular  sheets  of  the 
diaphragm  lies  in  direct  contact  with  the  inner  surface  of  the 
lower  costal  part  of  the  thoracic  wall,  the  rest  is  higher  than  the 
central  tendon  that  forms  the  floor  of  the  pericardium  and  is 
fixed  in  one  position.  During  in-spiration  these  lateral  parts  are 
separated  from  the  ribs  and  drawn  below  the  level  of  the  central 

tendon  by  the  contraction  of  the  mus- 
cular fibres.  The  separation  is  aided 
by  the  abduction  of  the  floating  rib.s, 
which  is  accomplished  by  the  quadratus 
lumborum  and  the  deep  dorsal  muscles. 
In  order  that  the  diaphragm  may 
act  to  the  best  advantage,  it  is  neces- 
sary that  its  attachments  be  fixed  by 
the  other  muscles;  for  when  the  quad- 
ratus lumborum,  levatores,  and  other 
fixing  muscles  are  not  acting,  the  lower 
floating  ribs  are  drawn  in  by  the  dia- 
phragm, and  the  power  of  that  muscle 
is  much  diminished  by  the  approxima- 
tion of  its  attachments.  This  may  be 
seen  in  spinal  injuries  when  the  respira- 
tion is  carried  on  by  the  diaphragm 
alone.  In  these  cases  a  circular  furrow 
marks  the  line  of  attachment  of  the  muscle  to  the  lower  ribs 
and  their  cartilages,  which  are  drawn  inwards  during  each 
inspiration,  the  breathing  being  of  course  purely  abdominal  in 
type. 

The  Quadratus  Lumborum,  which  passes  from  the  pelvis  to  the 
last  rib,  has,  besides  the  action  in  aid  of  the  diaphragm  just  men- 
tioned, the  power  of  drawing  down  the  lower  outlet  of  the  thorax, 
in  which  it  is  helped  by  other  abdominal  and  dorsal  muscles.     In 


Diagram  showing  inter- 
val between  the  position  of 
the  diaphragm  in  expiration 
(e,  e)  and  inspiration  (/,(')• 
The  increase  in  capacity  is 
shown  by  the  white  areas. 


RESPIRATORY   MUSCLES. 


331 


this  action  it  may  be  regarded  as  the  antagonist  of  the  next 
group. 

The  Scaleni  Muscles,  which  pass  down  from  the  lateral  aspects 
of  the  cervical  vertebrae  to  the  first  two  ribs,  which  they  raise  so 
as  to  draw  up  the  upper  outlet  of  the  thorax.  The  quadratus 
and  scaleni  muscles  thus  act  upon  the  thorax  in  the  same  way  as 
the  hands  when  extending  a  concertina. 

The  Levatores   Costarum  are  small  muscles,  but  on  account  of 
their  number,  their  aggregate  force  is  much  greater  than  is  com- 
monly thought.     They  are  short,  thick  muscles,  which  pass  ob- 
liquely   downwards    and    out- 
wards from  the  transverse  pro-  Fig.  152. 
cesses  of  the  dorsal  vertebra?  to 
the  angle  of  the    ribs.     Their 
only  action  is  to  raise  the  angle 
of  the  ribs,  and  thus    remove 
their  anterior  and  lateral  down- 
ward slopes ;  by  so  doing  they 
increase  the  intervals  between 
the  ribs  and  enlarge  the  lateral 
and  the  autero-posterior  diame- 
ters of  the  chest.   Thus  they  are 
purely  muscles  of   inspiration, 
and  probably,  acting  with  the 
diaphragm  and  the  scaleni,  are 
the  chief  workers  in  ordinary 
breathing. 

The  Intercostals  produce  dif- 
ferent effects  on  the  ribs  accord- 
ing to  the  different  sets  of  mus- 
cles with  which  they  act  in  association.  They  never  act  alone, 
and  it  is  therefore  idle  to  try  to  ascribe  to  them  any  constant 
specific  inspiratory  or  expiratory  action.  Generally  speaking, 
the  intercostals  approximate  the  ribs,  and  by  this  action  they 
stiffen  the  thoracic  wall  and  help  to  elevate  the  thorax  when  its 
upper  part  is  fixed,  or,  when  its  lower  part  is  fixed,  to  depress  it. 


View  from  behind  of  four  dorsal 
vertebrje  and  three  attached  ribs, 
sliowing  the  attachment  of  the  eleva- 
tor muscles  of  the  ribs  and  the  inter- 
costals. (Allen  Thomson.) — 1.  Long 
and  short  elevators.  2.  External  in- 
tercostal.    3.  Internal  intercostal. 


.'^32  MANUAL   OF   PHYSIOLOGY. 

Now,  if  both  the  upper  and  lower  margins  of  the  thorax  be  held 
firmly  by  strong  muscles,  as  really  occurs  in  inspiration — from 
the  action  of  the  quudratus  and  scaleni — the  iutercostals  cannot 
approximate  the  ribs.  Under  these  circumstances  the  results 
which  follow  their  contraction  will  be  twofold,  viz. :  CI)  the  ster- 
num will  be  pushed  forwards,  and  the  antero-posterior  diameter 
of  the  thorax  thus  increased ;  and  (2)  the  spaces  between  the 
ribs,  which  are  widened  by  the  other  muscles,  are  kept  rigid 
and  prevented  from  sinking  inwards  when  the  intrathoracic 
pressure  falls.  When  acting  with  the  elevators  of  the  ribs  both 
intercostal  layers  of  muscle  have  an  inspiratory  effect.  But  when 
the  elevators  of  the  ribs  are  passive  the  intercostals,  acting  with 
the  anterior  abdominal  muscles,  draw  down  the  ril)s,  and  act  as 
muscles  of  expiration. 

For  forced  breathing  an  enormous  number  of  muscles  may 
be  called  into  play  during  the  inspiratory  effort,  as  may  be 
seen  during  occlusion  of  the  air-passages,  where  all  the  thoracic, 
cervical,  facial,  abdominal  muscles,  and  even  the  muscles  of  the 
extremities,  one  after  another,  are  thrown  into  a  recurring  spasm 
before  suffocation  ends  the  patient's  life. 

Among  the  muscles  which  lend  their  aid  when  more  energetic 
inspiratory  movements  are  required,  may  be  mentioned  the  sterno- 
VKistoid,  which  helps  the  scaleni  to  elevate  the  front  of  the  tho- 
racic wall ;  the  pectoral  muscles  and  the  great  serratus,  which  assist 
when  the  arms  are  fixed  ;  and  also  the  deep  muscles  of  the  back, 
which  straighten  the  spine,  and  act  upon  the  vertebral  attachments 
of  the  ribs  so  as  to  elevate  them  and  widen  the  intervals  between 
them.  Owing  to  the  ribs  being  fixed  to  the  sternum  in  front,  they 
can  only  separate  laterally  when  the  dorsal  curve  is  lessened,  and 
this  tends  to  approximate  the  sternum  and  the  vertebrae,  thus 
narrowing  the  antero-posterior  diameter  of  the  thorax.  It  is  in 
preventing  this  flattening  of  the  chest  that  the  intercostals  are 
particularly  useful ;  by  holding  the  ribs  together  they  push  for- 
wards the  sternum,  when  the  dorsal  curve  is  extended. 

During  quiet  breathing  expiration  requires  no  muscular  effort, 
the  expulsion  of  the  air  from  the  chest  being  accomplished  by  the 
elasticity  of  the  parts. 


EXPIRATION. 


333 


Fig.  15:1 


The  most  powerful  force  is  the  elasticity  of  the  lungs,  which 
are  on  the  stretch  even  after  a  forced  expiration,  and  when  dis- 
tended by  inspiration  are  capable 
of  exerting  considerable  traction 
on  the  thoracic  wall. 

The  ordinary  shape  of  the  walls 
of  the  thorax,  when  the  muscles 
are  not  acting,  corresponds  with 
the  position  at  the  end  of  gentle 
expiration ;  therefore  the  resili- 
ency of  the  muscles,  costal  carti- 
lages, and  other  elastic  tissues 
which  are  stretched  during  inspi- 
ration tends  to  restore  the  ribs  to 
the  position  of  expiration. 

The  weight  of  the  thorax  itself, 
and  the  elastic  gases  in  the  intes- 
tinal tract,  which  have  been  com- 
pressed by  the  diaphragm,  may 
also  help  in  expiration. 

After  death,  when  the  elasticity 
of  the  expiratory  muscles  is  lost, 
the  traction  exerted  by  the  lungs 
on  the  thorax  reduces  it  below  the 
size  its  own  elastic  equilibrium 
would  tend  to  assume ;  when, 
therefore,  air  is  admitted  to  the 
pleural  cavity  by  puncture,  the  thorax  expands  slightly  as  the 
lungs  shrink,  and  the  pressure  on  the  pleural  surface  becomes 
equal  to  that  within  the  bronchi. 

Id  forced  expiration,  or  when  the  air  is  used  during  expiration 
for  any  purpose  such  as  the  production  of  voice,  or  any  blowing 
movements,  a  number  of  muscles  are  called  into  action.  The  only 
muscles  that  could  be  called  exclusively  special  muscles  of  expira- 
tion are  the  weak  triangularis  sterni,  serratus  posticus  inferior,  and 
parts  of  the  intercostals;  but  in  all  violent  and  forcible  expiratory 
efi'urts  these  are  aided  by  the  muscles  forming  the  anterior  wall  of 


Shows  the  position  of  the  Ribs 
and  the  Spinal  Cohuun  in  normal 
forna  of  the  thorax,  i.e.,  that  as- 
sumed in  expiration. 


334  MANUAT.   OF   PHYSIOLOGY. 

the  abdomen,  which,  associated  with  the  intercostals  and  quadratus 
lumboruni,  are  the  most  powerful  agents  in  drawing  down  the 
thoracic  wallf?. 

Function  of  the  Pleura. 

From  wliat  has  been  already  said  it  is  obvious  that  by  far  the 
"■reatest  amount  of  movement  takes  place  in  the  lower  part  of  the 
thorax,  while  the  capacity  of  the  apex  changes  but  little.  The 
space  formed  in  the  chest  during  inspiration  is  practically  formed 
between  the  costal  wall  and  the  diaphragm  (compare  Figs.  148,149). 
If  the  lungs  and  the  walls  of  the  thorax  were  fused  together,  with- 
out the  interposition  of  serous  membranes,  the  different  parts  of 
the  lungs  would  have  to  follow  the  movements  of  that  part  of  the 
thorax  to  which  they  are  attached.  Thus  the  lower  parts  of  the 
lung  would  be  much  distended  during  inspiration,  and  the  apices 
would  receive  but  little  addition  to  their  contained  air.  This  con- 
dition is  often  found  in  disease  of  the  pleura,  leading  to  adhesion 
of  the  visceral  and  parietal  layers.  When  such  cases  live  for  some 
time  after  the  pleurisy  and  the  adhesions  persist,  the  air-cells  of 
the  lower  margins  of  the  lungs  are  commonly  found  to  be  dis- 
tended and  bloodless  (i.e.,  local  emphysema  from  habitual  over- 
distension) ;  while  on  the  other  hand,  the  apices  become  abnormally 
dense,  and  the  alveoli  are  contracted  and  airless. 

The  surface  of  the  soft  elastic  lung  tissue  is  normally  quite  free, 
being  encased  in  a  serous  membrane,  the  smooth  surface  of  which 
can  slide  uninterruptedly  and  freely  over  the  similar  lining  of  the 
costal  wall.  That  this  motion  of  the  lung  actually  occurs  may  be 
seen  from  watching  the  lung  through  the  exposed  parietal  pleura, 
or  recognized  by  studying  the  sounds  produced  by  a  roughness  of 
the  pleura,  such  as  occurs  in  inflammation,  when  a  "friction"  can 
be  detected  by  the  ear. 

The  lungs  move  iu  a  definite  direction.  From  the  most  fixed 
points  of  the  thorax,  namely,  the  apex  and  vertebral  margin,  they 
pass  towards  the  more  movable  inferior  costal  and-sternal  regions. 
In  short,  the  anterior  part  of  the  lungs  passes  downwards  and  for- 
wards to  fill  up  the  gap  made  by  the  descent  of  the  diaphragm 
and  by  the  passing  of  the  costal  wall  upwards  and  forwards. 


FUNCTION    OF    THE    PLEURA.  335 

The  position  of  the  inferior  margin  of  the  lung  may  be  easily 
recognized  by  percussion  over  the  liver,  and  may  thus  be  shown 
to  be  moving  up  and  down  with  expiration  and  inspiration  re- 
spectively. By  percussion  we  also  find  that  the  space  between 
the  two  lungs  in  front  is  increased  during  expiration  and  dimin- 
ished during  inspiration,  so  that  the  heart  is  more  or  less  covered 
by  lung,  and  the  precordial  dulness  is  altered  every  time  we 
draw  a  breath. 

By  means  of  this  free  movement  of  the  lungs  in  the  serous  cavi- 
ties the  air  exerts  equal  force  on  the  walls  of  all  the  air-cells, 
whether  they  are  situated  in  the  apex  or  base  of  the  lung,  and 
the  alveoli  are  all  equally  filled  with  air. 

If  the  pleural  cavity  be  brought  into  contact  with  the  air, 
either  by  puncture  of  the  thoracic  walls  or  by  rupture  of  the  vis- 
ceral pleura,  the  lung,  owing  to  the  great  elasticity  of  its  tissue, 
shrinks  to  very  small  dimensions,  and  the  pleural  cavity  becomes 
filled  with  air  (pneumothorax). 

If  air  be  admitted  to  both  pleural  cavities,  so  as  to  produce 
double  pneumothorax,  death  must  ensue,  for  if  the  opening  remain 
free  the  motions  of  the  thorax  only  alter  the  quantity  of  air  in 
the  pleural  cavity,  and  cannot  ventilate  the  lungs.  This  demon- 
strates the  important  fact  that  it  is  the  atmospheric  pressure  which, 
having  access  to  them  only  through  the  trachea,  distends  the 
elastic  lungs,  and  keeps  them  pressed  against  the  wall  of  the 
thorax. 

The  power  with  which  the  lungs  can  contract  when  the  atmo- 
spheric pressure  is  admitted  to  the  pleura  has  been  found  after 
death,  without  inflation,  to  be  six  millimetres  of  mercury,  which 
is  probably  below  the  pressure  exerted  during  life,  when  the 
smooth  muscle  of  the  bronchi  is  acting  and  the  tubes  are  free 
from  mucus,  for  this  rapidly  collects  in  the  minute  air  tubes  at 
death,  and  impedes  the  outflow  of  air. 

When  the  lungs  are  inflated  before  the  pleura  is  opened  the 
pressure  can  easily  be  made  to  rise  to  nearly  1^  inches  (80  mm. 
mercury). 

From  this  it  would  appear  probable  that,  when  the  lungs  are 
stretched  by  inspiration,  they  exert  a  negative  pressure  equal  to 


336  MANUAL   OF   PHYSIOLOGY. 

30  ram.,  aud  when  the  lungs  are  in  a  position  of  expiration  they 
still  tend  to  contract  with  a  force  of  6  mm,  mercury. 

Pressure  Differences  in  the  Air. 

The  immediate  effect  of  the  increase  in  capacity  of  the  chest  is 
that  a  pressure  difference  is  established  between  the  interior  of 
the  thoracic  cavity  and  the  atmosphere. 

The  reduction  in  pressure  produced  in  the  lungs  aud  air- 
passages  by  inspiratory  movements,  or  the  increase  of  pressure 
accompanying  expiration,  is  very  slight  during  ordinary  quiet 
breathing  with  free  air-passages.  But  the  least  impediment  to 
the  entrance  or  to  the  exit  of  the  air  at  once  makes  the  difference 
very  notable. 

It  is  very  difficult  to  obtain  an  accurate  experimental  estimate 
of  the  variations  in  the  pressure  in  different  parts  of  the  air-pas- 
sages during  quiet  breathing,  because  even  the  most  careful  at- 
tempt to  measure  the  pressure  causes  an  increase  which  is  still 
further  magnified  by  the  sensitive  muscular  mechanism  of  the 
air-passages. 

The  variations  in  pressure  occurring  in  the  pulmonary  air  are 
greatest  in  the  alveoli,  and  gradually  diminish  towards  the  larger 
air  tubes,  so  that  they  disappear  at  the  nasal  orifice,  where,  if  no 
impediment  be  placed  to  the  course  of  the  air,  the  pressure  will 
remain  very  nearly  equal  to  that  of  the  atmosphere.  By  connect- 
ing one  nostril  with  a  manometer,  and  breathing  through  the  nose 
with  the  mouth  shut,  it  can  be  shown  that  inspiration  causes  a 
negative  pressure  of  about  1  mm.  mercury,  and  expiration  a  posi- 
tive pressure  of  2  to  8  mm. ;  these  results  must  be  divided  by  two, 
since  by  plugging  one  nostril  they  shut  off  half  the  normal  inlet. 
Forced  inspiration  and  expiration  give  respectively  — 57  and 
+  87  mm. 

This  great  difference  depends  on  the  elastic  forces  against  which 
the  inspiratory  muscles  act  in  distending  the  thorax,  all  of  which 
assist  in  expiration. 


VENTILATION    OF    AIR-PASSAGES.  337 


The  Volume  of  Air. 

During  ordinary  respiration  the  volume  of  the  inspiratory  and 
expiratory  stream  of  air  is  surprisingly  small  when  compared  with 
the  volume  of  air  sojourning  in  the  lungs. 

After  an  ordinary  expiratory  act  we  can  force  out  a  great 
quantity  of  air  by  a  voluntary  effort;  but  even  after  this  is  got 
rid  of  the  lungs  are  still  well  filled.  Some  of  this  residual  air, 
which  never  leaves  the  chest  during  the  life  of  the  animal,  is 
pressed  out  by  the  elasticity  of  the  lungs  when  the  pleura  is 
opened.  But  a  certain  amount  of  air  cannot  be  removed  in  any 
way  from  the  alveoli.  Even  when  the  lung  is  cut  out  of  the  chest 
and  divided  into  pieces,  enough  air  is  retained  in  the  air  cells  to 
render  it  buoyant.  This  fact  is  relied  on  by  medical  jurists  as  an 
evidence  that  an  infant  has  breathed  after  birth  and  distended 
the  lungs  with  air,  for,  except  breathing  has  been  well  estab- 
lished, the  tolerably  fresh  lung  of  an  infant  will  sink  in  water. 

In  order  to  have  a  clear  idea  of  the  volumes  of  air  at  rest  and 
in  motion  during  pulmonary  ventilation,  it  is  convenient  to  follow 
the  classification  from  which  the  nomenclature  in  common  use 
has  been  borrowed. 

Tidal  air  is  the  current  of  air  which  passes  into  and  out  of  the 
chest  in  quiet  natural  breathing.  It  amounts  to  about  500  cc. 
(30  cubic  inches). 

Reserve  air  is  that  volume  which  can  be  voluntarily  emitted 
after  the  end  of  a  normal  tidal  expiration,  and  which,  therefore, 
during  ordinary  respiration  remains  in  the  lungs;  it  is  estimated 
at  about  1500  cc.  (or  about  100  cubic  inches). 

Complemental  air  is  that  which  can  be  voluntarily  taken  in 
after  an  ordinary  inspiration  by  a  forced  inspiration;  it  also 
amounts  to  about  1500  cc,  but  is  not  used  during  ordinary 
breathing. 

Residual  air  is  the  air  volume  which  remains  in  the  lungs  after 
a  forced  expiration,  that  is  to  say,  which  no  voluntary  eftbrt  can 
remove  from  the  lungs;  it  includes  the  air  which  leaves  the  lungs 
when  the  pleura  is  opened  after  death  and  the  air  which  persist- 


338  MANUAL    OF    l'HVSI()LO(JY. 

eutly  remains  iu  the  lungs  after  they  have  collapsed.  This 
amounts  to  about  2000  cc.  (or  about  120  cubic  inches.) 

Vital  capacity  is  a  term  given  to  the  greatest  amount  of  air  that 
can  be  emitted  by  a  forced  expiration  immediately  following  a 
forced  inspiration,  so  that  it  equals  the  sum  of  the  tidal,  reserve, 
and  complemental  air.  The  vital  capacity  is  estimated  by  spiro- 
meters of  different  kinds,  and  gives  an  approximate  measurement 
of  (1)  the  capacity  of  the  chest ;  (2)  the  power  of  the  respiratory 
muscles ;  (3)  the  resistance  offered  by  the  elasticity  or  rigidity  of 
the  walls  of  the  thorax;  (4)  the  working  capacity  of  the  lungs, 
i.e.,  their  extensibility  or  freedom  from  disease.  It,  therefore, 
varies  greatly  according  to  the  age,  sex,  position  of  the  body,  the 
occupation,  weight,  height,  the  fulness  of  the  hollow  viscera  of 
the  abdomen,  and  the  pathological  condition  of  the  lungs.  It  can 
be  much  increased  by  practice,  and  this  fact,  apart  from  the 
injury  forced  respirations  may  produce  in  a  morbid  state  of  the 
lung,  renders  it  inapplicable  as  a  gauge  of  pulmonary  disease. 

From  the  foregoing  it  appears  that  the  volume  of  air  habitu- 
ally sojourning  in  the  lungs  during  natural  respiration,  or  station- 
ary air,  is  about  3500  cc.  (225  cubic  inches),  while  the  fresh  air 
introduced  by  each  inspiration  is  only  a  little  over  500  cc.  (30 
cubic  inches),  or,  in  other  words,  about  one-seventh  of  the  air  in 
the  lungs  is  changed  at  each  breath.  Indeed,  the  500  cc.  of 
air  is  only  just  sufficient  to  fill  the  trachea  and  larger  bronchial 
passages,  so  that  the  fresh  air  does  not  reach  the  pulmonary 
alveoli,  or  directly  replace  any  of  the  air  they  contain.  The  tidal 
stream  is,  however,  l)rought  into  immediate  relation  with  the  sta- 
tionary air,  and  the  thoracic  movements  cause  them  to  mix  me- 
chanically, so  that  rapid  diffusion  takes  place  in  the  minute 
bronchi.  Diffusion  is  also  constantly  occurring  between  the  air 
of  the  small  tubes  and  the  terminal  sacs,  and  it  alone  suffices  to 
maintain  the  necessary  standard  of  purity  in  the  air  of  the  alveoli. 
If,  during  breathing,  a  harmless  gas,  such  as  hydrogen,  be  in- 
haled during  one  inspiration,  it  requires  6  to  10  respirations  to 
get  rid  of  the  impurity  from  the  expired  air.  From  this  it  has 
been  inferred  that  this  number  of  respiratory  acts  would  be  neces- 
sary to  render  the  air  in  the  alveoli  quite  pure  even  if  no  fresh 
impurities  were  allowed  to  enter  from  the  blood. 


NERVOUS   MECHANISM   OF   RESPIRATION.  339 


Respiratory  Sounds. 

As  the  streams  of  air  enter  the  air-passages  and  lungs  they 
produce  sounds  which  are  of  the  greatest  importance  to  the  phy- 
sician, owing  to  the  manner  in  which  they  are  altered  by  disease. 

A  sound  called  "bronchial  breathing"  is  produced  in  the  large 
bronchi  and  trachea,  and  is  like  the  noise  of  air  blowing  through 
a  tube.  This  can  normally  be  heard  over  the  trachea,  or  at  the 
back  between  the  shoulder-blades  over  the  entrance  of  the  large 
bronchi  into  the  root  of  the  lung. 

Another  sound  called  "vesicular"  can  be  heard  all  over  the 
chest,  being  most  distinct  where  the  lung  is  most  superficial,  and 
where  other  sounds  are  absent,  as  in  the  sub-axillary  region.  It 
is  a  gentle  rustling  sound  caused  by  the  air  passing  into  the 
infundibuli.  It  varies  much  with  the  force  of  respiration  and 
many  other  circumstances.  In  children  up  to  ten  or  twelve  years 
of  age  it  is  remarkably  sharp  and  loud,  and  is  called  "  puerile 
breathing." 

Nervous  Mechanism  of  Respiration. 

The  movements  of  respiration  go  on  rhythmically  without  any 
voluntary  effort,  and  even  when  we  are  quite  awake  they  occur 
almost  without  our  being  conscious  of  them,  and  repeated  varia- 
tions take  place  in  the  rate,  depth,  and  general  type  of  our  respi- 
rations without  our  knowledge.  Indeed,  if  this  self-regulating 
arrangement  did  not  exist  we  should  have  to  devote  much  of  our 
attention  to  adapting  our  respiratory  movements  to  the  ever- 
changing  requirements  of  the  gas-interchange  of  the  blood. 

Like  all  other  groups  of  skeletal  muscles,  those  which  act  on 
the  thorax  are  regulated  by  nerves  and  work  together  in  harmony. 
These  coordinated  movements  are  so  far  under  the  control  of  the 
will  that  any  of  the  groups  of  muscles  may  be  employed  sepa- 
rately, or  in  conjunction. 

But  the  respiratory  differ  from  the  other  skeletal  muscles,  in 
that  they  undergo  rhythmical  coordinated  contractions  which  are 
not  directed  by  our  will,  and  can  be  influenced  by  it  only  to  a 
certain  extent,  for  they  cannot  be  made  to  cease  altogether. 


340  MANUAL   OF    PHYSIOLOGY. 

In  short,  the  rhythmical  coordinated  movements  of  respiration 
are  not  only  brought  about,  but  are  also  regulated  by  an  invol- 
untary nervous  mechanism.  Since  we  are  unconscious  of  its 
action,  it  certainly  is  not  dependent  on  the  voluntary  centres. 
Moreover,  we  know  that  the  upper  parts  of  the  brain  are  not 
needed  for  regular  breathing,  because  animals  born  with  deficient 
development  of  cranium  and  brain  can  breathe  quite  rhythmi- 
cally ;  and  removal  of  the  brain  of  birds,  etc.,  causes  no  inter- 
ruption of  the  respiratory  movements.  We  know,  however,  that 
an  injury  to  the  upper  part  of  the  spinal  cord  causes  death  by 
stopping  respiration.  The  regulating  centre  must  then  be  lower 
than  the  cerebral  centres,  and  higher  than  the  cervical  part  of 
the  spinal  marrow.  The  direct  evidence  of  the  seat  of  this  centre 
was  fi>und  by  Flourens,  who  showed  that  a  localized  spot  exists 
in  the  medulla  oblongata,  injury  of  which  causes  instant  cessa- 
tion of  the  respiratory  movement. 

This  vital  point,  or  noeud  vital,  is  situated  in  the  floor  of  the 
fourth  ventricle,  near  the  point  of  the  calamus  scriptorius,  and  is 
now  commonly  spoken  of  as  the  respiratory  centre. 

From  this  centre  the  impulses  which  give  rise  to  and  regulate 
the  all-important  respiratory  movements  rhythmically,  pass  down 
the  spinal  cord  and  nerves.  So  long  as  the  nervous  communica- 
tion between  the  centre  and  the  muscles  is  intact,  the  movements 
go  on  with  undisturbed  regularity;  if  it  be  cut  off,  or  the  centre 
destroyed,  they  instantly  stop. 

What  keeps  this  centre  active?  It  has  been  already  stated 
that  all  the  conditions  of  the  body  which  cause  an  increased  tissue- 
change  use  up  a  greater  amount  of  oxygen,  and  give  ofi"  more 
carbonic  acid,  therefore  are  accompanied  by  more  active  move- 
ments of  the  respiratory  muscles.  From  this  it  would  appear 
that  there  exists  some  relation  between  the  activity  of  the  respira- 
tory centre  and  the  condition  of  the  blood — a  deficiency  of  oxygen 
or  an  excess  of  carbonic  acid  gas  calling  forth  increased  action. 
One  has  only  to  hold  one's  breath  as  long  as  possible,  and  note 
the  series  of  rapid  and  deep  respirations  that  follow  such  a  tem- 
porary impediment  to  the  proper  oxygenation  of  the  blood,  in 
order  to  see  that  an  involuntary  respiratory  centre  is  profoundly 


NERVOUS    MECHANISM    OF    RESPIRATION.  341 

influenced  by  a  deficiency  of  oxygen.  Experimentally  it  can  be 
shown  that  the  effect  is  produced,  in  a  great  measure  at  least,  in 
the  medulla  itself,  by  the  blood  flowing  through  it,  and  not  by 
the  action  of  the  venous  blood  circulating  through  distant  organs, 
and  reflexly  affecting  the  centre.  It  has  also  been  shown  that 
the  temperature  of  the  blood  circulating  through  the  medulla 
changes  the  activity  of  the  centre,  for,  if  the  blood  in  the  carotids 
be  warmed,  the  respiratory  movements  become  more  rapid. 

The  respiratory  centre  is,  then,  a  good  example  of  what  is 
called  an  "  automatic  nerve  centre,''  not  depending  upon  nerve 
impulses  from  afar  for  its  energy,  nor  merely  reflecting  the  in- 
fluences of  other  centres,  but  acquiring  its  energy  from  the  ther- 
mal and  chemical  condition  of  the  blood  which  flows  through  it, 
and  thus  its  activity  is  intimately  related  to  its  nutrition  and  sup- 
ply of  oxygen. 

So  long  as  the  amount  of  oxygen  flowing  through  the  centre 
keeps  up  to  a  certain  standard,  the  normal  excitability  of  the 
centre  continues,  and  we  have  natural  quiet  breathing,  called 
Eupnoea.  When  the  oxygen  falls  below  the  normal  standard  the 
respiratory  centre  becomes  niore  excitable,  and  labored  breathing 
is  produced,  commonly  called  Dyspnoea. 

If  the  theory  that  a  deficiency  of  oxygen  is  the  normal  stimulus 
to  action  of  the  respiratory  centre  be  correct,  a  superabundant 
quantity  should  diminish  the  activity  of  the  centre,  and  a  con- 
dition the  opposite  of  dyspnoea  would  be  produced.  This  is  dif- 
ficult to  show  in  natural  breathing,  though  every  one  knows  the 
efiiciency  of  the  few  deep  breaths  one  takes  before  a  dive  into 
water  ;  but  with  artificial  breathing,  if  the  movements  be  carried 
on  very  energetically  for  some  time,  and  then  be  stopped,  the 
animal  will  not  at  first  attempt  to  breathe,  but  after  a  short  time, 
somewhat  less  than  a  minute,  gentle  and  slow  respiratory  move- 
ments commence.  This  cessation  of  breathing,  called  apncea,  de- 
pends upon  the  blood  being  so  charged  with  oxygen  that  it  no 
longer  acts  as  a  stimulus  to  the  centre. 

We  find  that  dyspnoea  is  produced  by  a  deficiency  in  the 
amount  of  oxygen  rather  than  by  an  excess  of  carbonic  acid  gas. 
This  is  proved  by  the  fact  that  it  occurs  when  the  carbonic  acid 


342  MANUAL   OF   PHYSIOLOGY. 

gas  is  removed  from  the  blood  by  breathing  freely  air  which  is 
only  deficient  in  oxygen,  and,  secondly,  because  an  excess  of  car- 
bonic acid  gas  in  the  air  causes  a  drowsy  condition  and  not  an 
active  dyspnoea. 

Although  the  respiratory  centre  is  in  the  strictest  sense  atdo- 
matic,  yet  it  is  profoundly  affected  by  many  influences  coming 
from  other  parts,  which  reflexly  modify  the  respiratory  move- 
ments. Thus,  mental  emotions  variously  influence  both  the  rate 
and  the  depth  of  breathing,  sometimes  causing  more  rapid  and 
sometimes  slower  respiratory  action.  The  application  of  stimulus 
to  almost  any  part  of  the  air-passages  completely  changes  the 
respiratory  rhythm.  The  ordinary  sensory  nerves  passing  from 
the  skin  are  also  capable  of  exciting  respiratory  movement?. 
This  is  well  seen  from  the  gasping  that  follows  the  sudden  appli- 
cation of  cold  to  the  body.  It  is  along  these  sensory  nerves  that 
one  tries  to  transmit  impulses  by  applying  mechanical,  thermal, 
or  other  stimulus  to  the  skin  of  a  new-born  infant,  whose  respira- 
tory centre,  having  been  kept  long  in  the  condition  of  apncea,  is 
slow  to  respond  to  an  exciting  influence  caused  by  a  deficiency  of 
oxygen. 

Experiment  shows  that  most,  if  not  all,  afferent  nerves  can 
affect  the  respiratory  centre,  either  by  increasing  or  reducing  its 
activity ;  but  there  is  one  special  nerve,  namely,  the  pneumogastric 
or  vagus,  and  its  branches,  which  have  both  these  capabilities 
developed  to  a  much  greater  degree  than  any  other. 

If  the  two  vagi  be  cut,  a  marked  change  takes  place  in  the 
respiratory  rhythm,  though  section  of  one  vagus  has  little  or  no 
effect  on  respiration.  The  rate  of  the  inspiration  is  reduced  to 
less  than  half,  while  each  breath  becomes  extremely  deep  and 
prolonged,  the  respiratory  function  of  the  lungs  goes  on  for  some 
time  unimpaired,  and  the  haemoglobin  of  the  blood  receives  the 
due  amount  of  oxygen.  Although  the  character  of  the  breath- 
ing is  completely  changed  from  the  rapid  gentle  motion  of  natural 
respiration  to  a  series  of  slow  deep  gasps,  the  air  volume  per 
minute  and  the  chemical  changes  remain  the  same.  If  the  cen- 
tral end  of  the  cut  vagus  be  now  stimulated  gently,  the  rate  of 
the  respiratory  movements  may  again  be  quickened  to  the  nor- 


NERVOUS   MECHANISM   OF   EESPIRATION.  343 


Fig.  154. 


Diagram  of  the  Nervous  Mechanisms  of  Respiration.  (After  Fick.) — 
sc.  Centre  for  inspiratory  movements,  from  wliich  pass  efferent  channels, 
represented  by  the  continuous  white  line  (o)  to  the  inspiratory  muscles  rep- 
resented by  the  diaphragm  (d).  ec.  Centre  for  expiratory  movements, 
from  which  efferent  channels  (p)  pass  down  the  cord  to  the  muscles  of  ex- 
piration, represented  by  the  abdominal  muscles  (a).  To  both  these  centres 
afferent  impulses  come  (1)  from  the  cerebral  centres  (a,  b,  c,  d)  to  check  or 
excite  activity.  These  voluntary  impulses  may  be  called  afferent  as  far  as 
the  respiratory  centres  are  concerned.  From  (2)  the  cutaneous  surface,  and 
(3)  the  nose,  impulses  (e,/,  fj)  arrive,  which  modify  the  action  of  the  inspi- 
ratory centre.  From  the  (4)  larynx  (o)  come  checking  impulses  (h)  to  the 
inspiratory,  and  exciting  impulses  (i)  to  the  expiratory  centre ;  and,  finally, 
(5)  from  the  lungs  come  both  exciting  and  inhibiting  impulses  {k,  I,  m,  n) 
to  both  the  expiratory  and  inspiratory  centres,  and  by  these  channels  the 
rhythm  of  ordinary  breathing  is  regulated. 


;;M  iMANUAI.   OF   PHYSIOLOGY. 

mal.  If  the  stimulus  be  very  strong,  respiratory  spasra  can  be 
produced.  On  the  other  hand,  if  the  central  end  of  the  superior 
laryngeal  branch  of  the  vagus  he  stimulated,  breathing  becomes 
slow,  and  can  be  made  to  cease  while  the  thorax  is  in  the  position 
of  ordinary  expiration,  a  spasm  of  tlie  laryngeal  and  expiratory 
muscles  is  caused. 

So  that  in  the  pneuraogastric  nerve,  fibres  exist  which  convey 
impulses  of  two  kinds  to  the  respiratory  centre,  the  one  increas- 
ing its  excitability  and  causing  more  rapid  discharges  of  inspira- 
tory impulses,  the  other  decreasing  its  irritability  and  causing  a 
slowing  of  the  respiratory  movements.  The  marked  change 
which  has  just  been  described  as  occurring  when  the  two  pneu- 
mogastrics  are  cut  proves  that  these  afferent  influences  are  con- 
stantly at  work  modifying  the  respiratory  rhythm.  We  may 
assume  that  the  slow,  deep  respirations  which  follow  section  of 
the  vagi  are  caused  by  the  unregulated  automatic  action  of  the 
centre.  No  impulse  is  discharged  until  the  venosity  of  the  blood 
in  the  centre  arrives  at  a  certain  point,  and  then  the  accumulated 
energy  is  sent  to  the  respiratory  muscles,  and  a  deep  gasping 
inspiration  occurs,  and  thus  each  respiratory  act  is  called  forth 
by  the  blood  becoming  so  venous  as  to  act  as  a  powerful  stimulus. 

So  long,  however,  as  the  centre  enjoys  the  regulating  influence 
of  the  vagi  this  venous  condition  is  not  allowed  to  occur,  and  the 
intense  excitation  of  the  centre  is  thereby  prevented,  and  the 
necessary  movements  are  performed  with  a  minimum  of  muscle 
energy. 

The  exact  mode  of  stimulation  of  the  pulmonary  terminals  of 
the  afferent  fibres  of  the  pneumogastric  is  not  certain.  It  has 
been  suggested  that  distension  or  retraction  of  the  lungs  may  act 
as  a  mechanical  stimulus  to  fibres  inhibiting  and  exciting  re- 
spectively the  inspiratory  centre.  Each  expansion  of  the  lungs 
calls  forth  the  ensuing  relaxation,  and  the  relaxed  state,  in  its 
turn,  induces  a  new  inspiration,  and  thus  the  lungs  themselves 
are  able  to  guide  the  thoracic  movements  by  means  of  the  pneu- 
mogastrics. 


MODIFIED   RESPIRATORY   MOVEMENTS.  345 


Modified  Movements  of  the  Respiratory  Muscles. 

Besides  the  ordinary  respiratory  motions  and  the  voluntary 
modifications  made  use  of  iu  speaking  and  singing,  etc.,  the  mus- 
cles of  respiration  perform  a  series  of  movements  of  an  involun- 
tary reflex  nature  indicative  of  certain  emotions  and  mental 
states. 

They  will  be  seen  to  resemble  each  other  in  the  mechanism  of 
their  production,  though  differing  essentially  in  expression.  The 
following  are  the  more  important: 

Coit^Am^  is  caused  by  a  stimulus  applied  to  certain  parts  of 
the  air-passages,  but  more  particularly  to  the  larynx  ;  the  stimulus 
passing  along  the  superior  laryngeal  branch  of  the  pneumogas- 
tric.  It  consists  in  a  deep  inspiration,  closure  of  the  glottis,  and 
then  a  more  or  less  violent  expiratory  effort,  accompanied  by 
two,  three,  or  more  sudden  openings  and  closures  of  the  glottis, 
so  that  rapidly  repeated  blasts  of  air  pass  through  the  upper  air- 
passages  and  mouth,  which  is  generally  held  open. 

Sneezing  is  caused  by  a  stimulus  applied  to  the  nose  or  eyes, 
the  impulses  being  carried  to  the  respiratory  centre  by  the  nasal 
and  other  branches  of  the  fifth  nerve.  It  consists  of  a  deep  in- 
spiration and  closure  of  the  glottis,  followed  by  a  single  explo- 
sive expiration  and  sudden  opening  of  the  glottis  and  posterior 
nares. 

Sneezing  is  a  purely  reflex  act,  it  being  impossible  to  produce 
it  voluntarily,  except  by  the  stimulation  of  the  nasal  mucous 
membrane  with  some  irritating  substance. 

Laughing  consists  in  a  full  inspiration,  followed  by  a  long  series 
of  very  short  rapid  expiratory  efforts.  The  facial  muscles  are  at 
the  same  time  thrown  into  a  characteristic  set  of  movements. 

Crying  is  made  up  of  a  series  of  short  sudden  expirations,  ac- 
companied with  peculiar  facial  contortions,  and  commonly  follow- 
ing or  associated  with  the  following  : 

Sobbing,  which  consists  of  a  rapid  series  of  convulsive  inspira- 
tory eflTorts,  causing  but  little  air  to  enter  the  chest,  and  followed 
by  one  long  expiration. 

29 


346  MANUAL   OF   PHYSIOLOGY. 

Sighing  is  a  long  slow  inspiration,  quickly  followed  by  a  cor- 
responding expiration. 

Yawning  is  a  very  long  deep  inspiration,  completely  filling  the 
chest.  It  is  accompanied  by  a  peculiar  depression  of  the  lower 
jaw,  wide  open  mouth,  facial  movements,  and  commonly  stretch- 
ing of  the  limbs. 

Hiccough  is  a  sudden  inspiratory  spasm  chiefly  of  the  dia- 
phragm, the  entrance  of  the  air  being  suddenly  checked  by  the 
sudden  closure  of  the  glottis. 


CHAPTER  XIX. 

THE  CHEMISTRY  OF  RESPIRATION. 

The  simplest  way  to  investigate  the  study  of  thegas-interchauge 
that  takes  place  in  the  luugs  between  the  air  and  the  blood  is  to 
compare  the  composition  of  the  expired  air  with  that  of  the  atmo- 
sphere, and  from  the  alteration  found  to  have  taken  place  in  the 
tidal  stream  we  can  arrive  at  the  changes  which  the  air  under- 
goes during  its  journey  in  and  out  of  the  air-passages,  and  we  can 
then  examine  the  venous  and  arterial  blood  in  order  to  ascertain 
the  change  the  blood  undergoes  in  becoming  arterial. 

The  atmosphere  is  made  up  of  a  mixture  of  nitrogen  and  oxy- 
gen, with  a  variable  amount  of  moisture  and  a  minute  proportion 
of  carbonic  acid. 

The  following  table  gives  the  volume^  of  the  gases  in  dried  air : 

Oxygen,  ....     20.96  per  cent.,  or  about  21  per  cent. 

Nitrogen,  .        .        .     79.02        "  "       79 

Carbonic  dioxide,     .       0.02-0.06         "  "         4  parts  in  10,000. 

The  amount  of  moisture  contained  in  the  air  is  very  variable, 
and  depends  in  a  great  measure  upon  the  temperature  and  the 
direction  of  the  wind.  The  dampness  of  the  air  depends  upon 
the  temperature,  so  that  air  containing  the  same  absolute  amount 
of  moisture  may  be  relatively  dry  or  damp,  according  as  the  tem- 
perature rises  or  falls.  As  a  general  rule  the  air  is  relatively  dry, 
that  is  to  say,  it  does  not  contain  so  much  moisture  as  it  is  capa- 
ble of  taking  up  in  the  form  of  aqueous  vapor  at  its  ordinary  tem- 
perature. At  certain  times  of  the  day  the  air  may  be  saturated 
owing  to  a  sudden  fall  of  temperature. 

The  temperature  of  the  air  which  we  breathe  of  course  varies 


*  On  account  of  the  difference  in  the  atomic  weights,  the  atmosphere  being 
only  a  mechanical  mixture  of  the  gases,  the  proportion  by  iveif/ht  is  slightly, 
different,  being  about — Oxygen  23  per  cent..  Nitrogen  77  per  cent. 


348  MANUAL   OF   PHYSIOLOGY. 

considerably,  according  to  the  season  of  the  year,  etc.,  but  almost 
always  in  this  country  it  is  lower  than  that  of  our  bodies. 

Expired  Aik. 

The  following  are  the  notable  characters  in  the  tidal  air  on  its 
leaving  the  air-passages  : 

1.  It  is  rich  in  CO^,  containing  on  an  average  4.38  per  cent,  in 
quiet  breathing. 

2.  It  is  poor  in  O,  containing  about  4.5  per  cent,  less  than  the 
atmosphere. 

3.  A  slight  increase  in  the  N  has  been  observed,  possibly  the 
outcome  of  nitrogenous  metabolism. 

4.  The  temperature  of  the  air  is  approximated  to  that  of  the 
body,  and  it  therefore  commonly  exceeds  the  temperature  of  the 
air  inspired.  The  air  on  leaving  the  air-passages  is  about  36.5° 
C.  This  is  not  much  influenced  by  the  temperature  of  the  atmo- 
sphere, as  may  be  seen  from  Valentine's  Table : 

Temperatures  of  Atmosphere.  and  of  Expired  Air. 
—  6.3°C.                     =  +29.8°  C. 

+  17.0°  C.  =  +36.2°  C. 

+  44.0°C.  =  +38.5°  C. 

It  can  be  seen  from  the  last  statement  that  very  hot  air  (+44° 
C.)  if  breathed  is  cooled  in  its  transit  through  the  air-passages. 

5.  In  quiet  breathing  the  expired  air  is  saturated  with  moisture; 
in  rapid  breathing  this  is  not  the  case.  It  must  be  remembered 
that  the  air  when  warm  is  capable  of  holding  a  greater  quantity 
of  vapor  than  when  it  was  inspired.  The  difference  can  be  best 
appreciated  in  cold  weather,  when  the  vapor  of  the  warm  expired 
air  is  condensed  on  meeting  the  cold  atmosphere.  Great  quanti- 
ties of  water  and  heat  are  given  off  in  producing  this  saturation. 

6.  If  the  tidal  air  be  dried  and  cooled  and  measured  at  a  cer- 
tain pressure  before  and  after  respiration,  it  is  found  that  the  ex- 
pired air  has  lost  about  ^^  of  its  volume.  But  owing  to  the  ex- 
pansion from  the  increased  temperature  and  the  presence  of  the 
vapor,  the  volume  of  air  expired  is  greater  than  that  inspired. 

If  the  oxygen  were  all  used  to  make  CO^,  these  volumes  ought 


RESPIRATORY   GAS-INTERCHAN(iE. 


349 


to  be  the  same,  for  the  volume  of  COj  is  equal  to  that  of  the  O  it 
contains,  if  set  free.  The  volume  CO^  given  off  is,  however,  only 
about  4.38,  to  4.5  volumes  of  O  taken  in,  so  that  part  of  the  O 
must  be  used  in  some  other  way  than  in  the  manufacture  of  CO^. 
7.  The  expired  air  is  also  said  to  contain  traces  of  the  following 
impurities  :  (1)  ammonia,  (2)  hydrogen,  (3)  carburetted  hydrogen. 
(CHJ,  (4)  organic  matter.  These,  and  probably  other  impurities, 
give  the  breath  its  peculiar  odor  and  noxious  properties,  for  an 
atmosphere  rendered  "stuffy"  by  expired  air  is  much  more  inju- 
rious to  health  than  an  atmosphere  in  which  a  similar  deficiency 
of  O  or  excess  of  CO^  has  been  artificially  produced  by  chemical 
means ;  this  fact  ought  to  be  remembered  when  calculating  the 
ventilation  required  for  hygienic  purposes.  The  following  Table 
may  assist  in  comparing  the  atmosphere  with  the  expired  air: 


Atmosphere. 

Expired  Air. 

Difference. 

C02 

0 

N 

Temperature 

Moisture 

.04  per  cent. 

20.81    "      " 

79.15    "      •' 

•    —6°  C—  +  35°  C. 

about  10  grm.'!.  to  1 

cubic  metre. 

4.38  per  cent. 
16.03    "      " 
79.55    "      " 
29.8°  a— 38.5°  C. 
about  40  grms.  to  1  cubic 

metre, 
f  Apparently     increased , 
1  absolutely  reduced  s^5th . 
j  NH3,  H,  CH4,  and  poison- 
\  ous  organic  matter. 

+4.34 
—4.78 
+  .40 

Impurities 

About  |th  of  the  O  which  is  used  does  not  take  part  in  the  pro- 
duction of  the  CO.j,  but  this  proportion  may  vary  greatly.  Thus 
the  estimation  of  the  CO^  can  give  no  sure  guide  to  the  amount  of 
O  taken  up ;  and  each  gas  has  to  be  estimated  separately  if  an 
accurate  measurement  be  required. 

The  average  amount  per  diem  may  be  said  to  be : 


Carbon  dioxide 
Oxygen  . 
Water 


given  off  about  800  grammes, 
consumed  about  700       " 
given  off  about  500         " 


The  amounts  of  O  taken  up  and  of  CO^  given  off  differ  in  dif- 
ferent individuals  and  in  the  same  individuals  under  varying 
circumstances,  among  which  the  following  may  be  enumerated : 

1.  Increase  in  the  rapidity  or  the  depth  of  respiratory  move- 


350  MANUAL   OF   PHYSIOLOGY. 

incuts,  accompanied  by  an  increase  in  the  tidal  stream,  pro- 
duces an  increase  of  tlie  total  amount  of  CO.,  given  off,  while  the 
percentage  iu  the  volume  of  expired  air  is  diminished. 

2.  It  varies  with  age.  The  amount  increases  with  age  up  to 
30  years,  and  then  remains  constant. 

3.  Sex  ;  is  less  in  women  than  in  men,  but  it  increases  in  preg- 
nancy. 

4.  With  muscular  activity  it  is  notably  increased. 

5.  Change  of  temperature  of  the  air  has  a  marked  influence  on 
the  COj  outpout  of  cold-blooded  animals,  in  which  it  increases  in 
direct  proportion  to  the  elevation  of  temperature.  The  effect  on 
warm-blooded  ones  is  the  opposite  so  long  as  they  can  regulate 
their  temperature.  The  sustentation  of  the  body  temperature  in 
cold  weather  is  accompanied  by  a  distinct  increase  in  the  output 
of  carbon  dioxide. 

6.  The  time  of  day;  a  maximum  is  arrived  at  about  midday, 
and  a  minimum  about  midnight. 

7.  An  increase  in  the  amount  of  carbon  dioxide  in  the  atmo- 
sphere diminishes  the  amount  given  off  from  the  lungs. 

Changes  the  Blood  undergoes  in  the  Lungs. 

In  order  to  understand  how  the  oxygen  and  the  carbonic  acid 
pass  to  and  from  the  blood  in  the  pulmonary  capillaries  we  must 
know  the  relationship  of  these  gases  to  the  blood  in  the  arterial 
and  venous  sides  of  the  circulation. 

In  the  chapter  on  the  blood  (pp.  239,  240)  it  is  stated  that  both 
the  oxygen  and  the  carbon  dioxide  can  be  removed  from  the 
blood  by  the  mercurial  air-pump,  and  that  the  greater  part  of 
these  gases  are  chemically  united  with  some  of  the  constituents  of 
the  blood,  and  that  a  different  quantity  of  each  gas  is  found  in 
arterial  and  venous  blood.  Now  that  we  know  that  the  change 
from  the  venous  to  the  arterial  condition  takes  place  during  the 
passage  of  the  blood  through  the  pulmonary  capillaries  where  it 
is  exposed  to  the  air,  we  may  assume  that  the  acquisition  of  oxy- 
gen and  the  loss  of  CO^  form  the  essential  difference  between 
venous  and  arterial  blood. 

From  either  kind  of  blood  about  60  volumes  per  cent,  of  gas 
may  be  extracted  with  the  mercurial  gas-pump.     The  composi- 


OXYHEMOGLOBIN.  351 

tion  of  this  varies  cousiderably  in  venous,  but  not  very  much  in 
arterial  blood.     An  average  is  given  in  the  following  table : 

O  per  cent.  vols.       CO2  per  cent.  vols.       N  per  cent.  vols. 
Arterial,     .         .         20  39  1-2 

Venous,      .         .    8-10  (about)  46  1-2 

The  more  rapidly  the  gases  are  removed  the  greater  is  the  pro- 
portion of  O  that  can  be  obtained  ;  as  delay  allows  some  of  it  to 
combine  with  easily  oxidized  substances  in  the  blood  itself.  The 
amount  of  oxygen  varies  in  different  parts  of  the  venous  system. 
In  the  blood  of  an  animal  which  has  died  of  slow  asphyxia  only 
traces  of  oxygen  can  be  found. 

The  proofs  that  O  is,  for  the  most  part,  in  chemical  combina- 
tion with  the  hpemoglobin  of  the  red  blood  corpuscles,  and  not 
merely  absorbed,  as  one  might  be  led  to  suppose  from  its  coming 
away  when  the  pressure  is  removed,  are  numerous  and  satisfac- 
tory. 

First.  When  arterial  blood  is  submitted  to  gradual  diminution 
of  pressure  in  the  mercurial  air-pump,  the  oxygen  does  not  come 
away  in  accordance  with  the  established  law  of  the  absorption  of 
gases  (Henry-Dalton)  by  coming  off  in  proportion  to  the  diminu- 
tion of  the  pressure,  as  at  first  only  traces  appear  (probably  the 
small  amount  really  dissolved),  and  when  the  pressure  has  been 
reduced  to  a  certain  point  the  oxygen  comes  off  suddenly ;  after 
which  little  more  can  be  obtained  by  further  reduction  of  pres- 
sure. Hseraoglobin  combines  with  0  in  the  same  way,  very 
rapidly  at  first  when  the  pressure  is  low,  and  then  with  a  much 
higher  pressure  a  smaller  quantity  is  taken  up. 

Secondly.  If  the  oxygen  were  only  in  a  state  of  absorption,  the 
blood,  while  passing  through  the  pulmonary  capillaries,  could 
only  take  up  about  0.4  volume  per  cent.,  which  would  be  inade- 
quate for  life.  We  know  that  the  quantity  of  O  going  to  the 
blood  from  the  air  in  the  alveoli  cannot  well  be  explained  on 
physical  grounds  alone ;  and,  moreover,  when  an  animal  is  allowed 
to  die  of  asphyxia  in  a  limited  space,  all  the  O  of  the  air  in  the 
space  is  ab.sorbed.  Since  the  partial  pressure  of  the  0  in  this 
case  must  fall  to  zero,  it  cannot  be  the  pressure  which  makes  the 
O  pass  into  the  blood. 


352 


MANUAL   OF   PHYSIOLOGY. 
Fig.  L55. 


*  :;=4  CJ  to  ■r'  "^  CO 

The  Spectra  of  Oxyliseiuoglobin,  reduced  htemoglobin,  and  CO-hsemo- 
globin.  (Gamgee.) — 1,  2,  3,  and  4.  Oxyhaemoglobin,  increasing  in  strength 
or  thickness  of  sohition.     5.  Reduced  haemoglobin.     6.  CO-haemoglobin. 


GASES   IN    THE   BLOOD.  353 

Another  conclusive  proof  that  the  union  of  the  O  with  the 
haemoglobin  is  really  a  chemical  one,  is  given  by  the  spectroscopic 
examination  of  a  hemoglobin  solution.  When  deprived  of  its 
O,  and  after  the  admixture  of  the  air,  quite  dissimilar  spectra 
are  seen,  as  already  pointed  out  in  Chapter  XIV.  (Fig.  107.) 

The  amount  of  O  taken  up  by  the  blood  is  not  always  in  pro- 
portion to  the  pressure  of  that  gas,  but  rather  to  the  amount  of 
hsemoglobiu  in  the  blood  ;  and  we  therefore  find  the  adequacy  of 
the  respiratory  function  of  the  blood  going  hand  in  hand  with 
its  richness  in  haemoglobin,  and  thus  the  "  shortness  of  breath  " 
of  ansemic  and  chlorotic  individuals  is  explained. 

Our  knowledge  concerning  the  relation  of  the  COj  to  the  con- 
stituents of  the  blood  is  less  definite  and  clear. 

It  does  not  altogether  exist  as  a  mere  physical  solution,  for  it 
comes  oft'  irregularly  under  the  air-pump,  and  does  not  obey  ex- 
actly the  Henry-Dalton  law  of  the  absorption  of  gases.  Part 
comes  off"  easily  and  part  with  difficulty.  It  is  not  associated 
with  the  corpuscles,  for  more  of  this  gas  can  be  obtained  from 
serum  than  from  a  like  quantity  of  blood.  It  is  more  easily  re- 
moved from  the  blood  than  from  the  serum,  a  certain  proportion 
(about  7  per  cent,  of  the  whole)  remaining,  in  the  serum  in  vacuo, 
until  dissociated  by  the  addition  of  an  acid  or  a  piece  of  clot 
containing  corpuscles.  If  bicarbonate  of  soda  be  added  to  blood 
from  which  all  the  gas  has  been  removed,  still  more  CO^  can  be 
pumped  out,  from  which  it  would  appear  that  something  exists  in 
the  blood  capable  of  dissociating  CO,  from  sodium  bicarbonate. 

It  has  been  suggested  that  the  CO^  is  in  some  way  associated 
(possibly  as  sodium  bicarbonate)  with  the  plasma  of  the  blood, 
and  that  the  corpuscles  have  the  power  of  acting  like  a  weak 
acid,  and  of  dissociating  it  from  the  soda,  and  thus  raising  its 
tension  in  the  blood. 

The  great  importance  of  the  chemical  nature  of  the  union 
between  the  0  and  haemoglobin  for  external  respiration  becomes 
most  striking  when  the  actual  manner  in  which  the  entrance  of 
the  O  is  eflfected  is  taken  into  account. 

It  must  be  remembered  that  the  further  we  trace  the  air  down 

30 


354 


MANUAL    OF    PHYSIOLOGY. 


the  passages,  the  less  will  be  the  percentage  of  O  found  in  it,  and 
therefore  a  less  pressure  exerted  by  that  gas.  This  is  shown  by 
the  fact  that  the  air  given  out  by  the  latter  half  of  a  single  Ex- 
piration has  less  0  and  more  CO.^  than  that  of  the  first  half.  The 
most  impure  air  lies  in  the  alveoli  of  the  lung,  for,  since  the  tidal 
air  scarcely  fills  the  tubes,  the  air  in  the  alveoli  is  only  changed 
by  mixture  and  diffusion  with  the  impure  air  of  the  small  bronchi. 
Any  impediment  to  the  ordinary  ventilation  of  the  alveoli  so 
reduces  the  percentage,  and  therefore  the  tension  of  the  O,  that 
it  would  probably  sink  below  that  in  the  blood,  and  in  that  case, 
were  it  not  a  chemical  union,  the  O  would  escape  from  the  blood 
in  proportion  as  its  tension  in  the  blood  exceeded  that  of  the  air 
of  the  alveoli.  We  know  it  does  not  do  this,  even  in  the  intense 
dyspnoea  of  suffocation. 

In  the  same  way  the  difference  of  tension  of  the  COj  in  the 
alveolar  air  and  in  the  blood,  hardly  explains  the  steady  manner 
in  which  the  CO^  escapes,  and  it  has  therefore  been  suggested 
that  this  escape  is  also  in  some  way  a  chemical  process,  possibly 
connected  with  the  union  of  the  O  and  haemoglobin  ;  because  the 
admission  of  O  to  the  blood  seems  to  facilitate  the  exit  of  the  CO,. 

The  following  table  gives  the  approximate  tension  of  the  two 
gases  in  the  different  steps  of  the  interchange,  and  shows  that  the 
tensions  are  such  as  to  enable  physical  absorption  to  take  some 
share  in  the  entrance  of  the  O  as  well  as  in  the  escape  of  the 
CO^.  A  separate  column  gives  the  volumes  per  cent,  of  each  gas, 
corresponding  to  these  tensions.  This  process  must  occur  before 
the  oxygen  and  the  haemoglobin  meet,  since  the  latter  is  bathed 
in  the  plasma,  and  further  separated  from  the  O  by  the  vessel 
wall  and  epithelium. 


CO2 

0 

Tpnsion        Correspond-  i 

inmm.Hg.     ing  volume  ' 

°    1     per  cent.     1 

Tension 
In  mm.  Hg. 

Correspond- 
ing volume 
per  cent. 

21.            1           2.8 
41.                       5.4 
27.            1           3..56 
0.38        !           0.04 

29.6 

22. 

27.44 

158. 

3.9 
2.9 
3.6 

20.8 

In  venous  blood 

RESPIEATION   OF   POISONOUS  GASES.  355 


Internal  Respiration. 

The  arterial  blood,  while  flowing  through  the  capillaries  of  the 
systemic  circulatiou  aud  supplying  the  tissues  with  nutriment, 
undergoes  changes  which  are  called  internal  or  tissue  respiration, 
and  which  may  be  shortly  defined  to  be  the  converse  of  pulmonary 
or  external  respiration.  In  the  external  respiration  the  blood  is 
changed  from  venous  to  arterial ;  whereas  in  internal  respiration 
the  blood  is  again  rendered  venous. 

There  can  now  be  no  doubt  that  these  chemical  changes  take 
place  in  the  tissues  themselves,  and  not  in  the  blood  as  it  flows 
through  the  vessels.  The  amount  of  oxidation  that  takes  place 
in  the  blood  itself  is  indeed  very  small.  The  tissues,  however, 
along  with  the  substances  for  their  nutrition,  extract  a  certain 
part  of  the  O  from  the  blood.  In  the  chemical  changes  which 
take  place  in  the  tissues,  they  use  up  the  oxygen,  which  rapidly 
disappears,  the  tension  of  that  gas  becoming  almost  nil ;  at  the 
same  time  other  chemical  changes  are  indicated  by  the  appear- 
ance of  CO.^.  The  disappearance  of  the  O  and  the  manufacture 
of  CO^,  need  not  exactly  correspond  in  amount,  and  they  doubt- 
less often  vary  in  different  parts  and  under  different  circum- 
stances. Of  the  intermediate  steps  in  the  tissue  chemistry  we 
are  ignorant.  We  do  not  know  the  way  in  which  the  oxygen  is 
induced  by  the  tissues  to  leave  the  haemoglobin  ;  we  can  only  say 
that  the  tissues  have  a  greater  affinity  for  O  than  the  haemoglobin 
has,  and  they  at  once  convert  the  O  into  more  stable  compounds 
than  oxyhiemoglobin,  and  ultimately  manufacture  COj,  which 
exists  in  the  tissues  and  fluids  of  the  body  at  a  higher  tension 
than  even  in  the  venous  blood. 

Respiration  of  Abnormal  Air,  etc. 

The  oxygen  income  and  carbonic  acid  output  are  the  essential 
changes  brought  about  by  respiration,  therefore  the  presence  of 
oxygen  in  a  certain  proportion  is  absolutely  necessary  for  life. 
The  21  per  cent,  of  O  of  the  atmosphere  suffices  to  saturate  the 
baJinoglobin  of  the  blood,  aud  14  per  cent,  of  O  has  been  found 


356  MANUAL   OF   PHYSIOLOGY. 

to  be  capable  of  sustaining  life  without  producing  any  marked 
change  in  respiration. 

Dyspnoea  is  produced  by  an  atmosphere  containing  only  7.5  per 
cent,  of  O.  This  dyspncea  rapidly  increases  as  the  percentage  of 
O  is  further  decreased,  and  when  it  gets  as  low  as  3  per  cent, 
suffocation  speedily  ensues. 

The  output  of  CO.^  can  be  accomplished  if  the  lungs  be  venti- 
lated by  any  harmless  or  indifferent  gas,  and  since  the  manufacture 
of  the  COj  does  not  take  place  in  the  lungs,  its  elimination  can 
go  on  independently  of  the  presence  of  O  in  them.  The  79  per 
cent,  of  N  contained  in  the  atmosphere  has  a  passive  duty  to  per- 
form in  diluting  the  O  and  facilitating  the  escape  of  the  CO^  from 
the  lungs. 

Indifferent  gases  are  those  which  produce  no  unpleasant  effect 
of  themselves,  but  which,  in  the  absence  of  O,  are  incapable  of  sus- 
taining life,  such  as  nitrogen,  hydrogen,  and  CH^. 

Irrespirable  gases  are  such  as,  owing  to  the  irritating  effect  on  the 
air  passages,  cannot  be  respired  in  quantity,  as  they  cause  instant 
closure  of  the  glottis.  In  small  quantities  they  irritate  and  pro- 
duce cough,  and  if  persisted  in,  inflammation  of  the  air  passages; 
among  these  are  chlorine,  ammonia,  ozone,  nitrous,  sulphurous, 
hydrochloric,  and  hydrofluoric  acids. 

Poisonous  gases  are  those  which  can  be  breathed  without  much 
inconvenience,  but  when  brought  into  union  with  the  blood  cause 
death.  Of  these  there  are  many  varieties.  (1.)  Those  which  per- 
manently usurp  the  place  of  oxygen  with  the  haemoglobin,  viz. : 
carbon  monoxide  (CO),  hydrocyanic  acid  (HON).  (2.)  Narcotic : 
(a)  carbonic  dioxide  (CO,),  of  which  10  per  cent,  is  rapidly  fatal, 
1.0  per  cent,  poisonous,  and  over  0.1  per  cent,  injurious,  (/^j  nitro- 
gen monoxide  (N.^O).  Both  of  these  gases  lead  to  a  peculiar 
asphyxia  without  convulsion,     (y)  chloroform,  ether,  etc. 

(3.)  Sulphuretted  hydrogen  (H,^S),  which  reduces  the  oxyhsemo- 
globin,  and  produces  sulphur  and  water. 

(4.)  Phosphoretted  hydrogen  (PH3),  arseniuretted  hydrogen 
(AsHg),  and  cyanogen  gas  (CjNj  also  have  specially  poisonous 
effects. 


VENTILATION.  357 


Ventilation. 


In  the  open  air  the  effects  of  respiration  on  the  atmosphere  can- 
not be  appreciated,  but  in  inclosed  spaces,  such  as  houses,  rooms, 
etc.,  which  are  occupied  by  many  persons,  the  air  soon  becomes 
appreciably  changed  by  their  breathing. 

The  most  important  changes  are  (1)  removal  of  oxygen,  (2) 
increase  in  carbonic  acid,  and  (3)  the  appearance  of  some  poison- 
ous materials  which,  though  highly  injurious,  cannot  be  deter- 
mined. The  deficiency  in  oxygen  never  causes  any  inconvenience, 
as  it  is  never  reduced  below  what  is  sufficient  for  the  saturation 
of  the  hsemoglobin.  The  excess  of  COj  seldom  gives  any  in- 
convenience, since  the  air  becomes  disagreeably  fusty  or  stuffy 
from  breathing  long  before  the  amount  of  COj  has  reached  0.1 
per  cent,  which  amount  of  pure  COj  can  be  inspired  without 
any  unpleasantness.  It  is,  then,  the  exhalations  coming  from 
the  lungs,  and  probably  skin,  some  of  which  must  have  a  poison- 
ous character,  that  render  the  proper  supply  of  fresh  air  impera- 
tive. 

The  difficulty  of  determining  the  presence  of  the  poisonous 
organic  materials,  makes  it  convenient  to  use  the  amount  of  CO^ 
present  in  the  air  as  the  means  of  measuring  its  general  purity. 
We  suppose,  then,  that  the  relation  between  the  poisonous  organic 
ingredients  and  the  CO2  is  constant. 

Air,  which  is  rendered  impure  by  breathing,  becomes  disagree- 
able to  the  sense  of  smell  when  the  CO,  has  reached  the  low  stand- 
ard of  .06  or  .08  per  cent. ;  that  is  to  say,  scarcely  twice  as  much 
CO2  as  is  contained  in  the  pure  atmosphere.  Supposing  that  air 
is  unwholesome  when  its  impurities  are  appreciable  by  the  senses, 
then,  if  the  animal  body  be  the  source  of  the  CO^,  .06  per  cent,  of 
this  gas  makes  the  air  unfit  for  use. 

An  adult  man  disengages  more  than  half  a  cubic  foot  of  CO.^ 
in  one  hour  (.6  Parkes),  and  consequently  in  that  time  he  renders 
quite  unfit  for  use  more  than  1000  cubic  feet  of  air,  by  raising 
the  percentage  of  CO2  to  .1  (.04  being  initial,  and  .06  respiratory). 

It  is  obvious  that  the  smaller  the  space  and  the  more  confined, 
the  more  rapidly  will  the  air  become  vitiated  by  respiration.  It 
becomes  necessary  for  health,  therefore,  to  have  not  only  a  certain 


358  MANUAL   OF   niYSIOLOGY. 

cubic  space  and  a  certain  change  of  air  for  each  individual,  but 
the  cubic  space  and  the  cliange  of  air  shoukl  bear  to  each  other 
a  certain  proportion  in  order  that  the  air  may  remain  sufficiently 
pure. 

The  space  allowed  in  public  institutions  varies  from  500  to  1500 
cubic  feet  per  head  in  such  apartments  as  are  occupied  by  the  in- 
dividuals day  and  night.  As  a  fair  average  1000  cubic  feet  may 
be  fixed  as  the  necessary  space  in  a  perfect  hygienic  arrangement. 
In  order  to  keep  this  perfectly  wholesome  and  free  from  a  stuffy 
smell,  and  the  CO,  below  .06  per  cent.,  it  is  necessary  to  supply 
some  2000  cubic  feet  of  air  per  head  per  hour. 

To  give  the  necessary  supply  of  fresh  air  without  introducing 
draughts  or  greatly  reducing  the  temperature  of  the  room  is  no 
easy  matter,  and  forms  the  special  study  of  the  hygienic  engineer. 

Asphyxia. 

If  the  proper  supply  of  oxygen  be  by  any  means  withheld  from 
the  blood  so  that  its  percentage  is  reduced  to  a  certain  point,  the 
death  of  the  animal  follows  in  3  to  5  minutes,  accompanied  by  a 
series  of  phenomena,  commonly  included  under  the  terra  asphyxia, 
which  may  be  divided  into  four  stages.  1.  Dyspnoea.  2.  Con- 
vulsion.    3.  Exhaustion.     4.  Inspiratory  spasm. 

If  the  air  passages  be  closed  completely  the  respirations  become 
deep,  labored,  and  rapid.  The  respiratory  efforts  are  more  and 
more  energetic,  and  the  various  supplementary  muscles  are  called 
into  play  one  after  the  other,  until  gradually  the  second  stage  is 
reached  in  about  one  minute. 

As  the  struggle  for  air  becomes  more  severe,  the  inspiratory 
muscles  lose  their  power,  and  the  expiratory  efforts  become  more 
and  more  marked,  until  finally  the  entire  body  is  thrown  into  a 
general  convulsion  in  which  the  traces  of  a  rhythm  are  hardly 
apparent.  This  stage  of  convulsion  is  short,  the  expiratory  mus- 
cles becoming  suddenly  relaxed  from  exhaustion. 

Then  the  longest  stage  arrives  in  which  the  animal  lies  almost 
motionless,  making  some  quiet  inspiratory  attempts.  These  be- 
come gradually  deeper  and  slower,  until  they  are  nothing  more 
than  deep  gasp.s  separated  by  long  irregular  intervals. 


ASPHYXIA.  359 

The  pupils  of  the  eyes  become  widely  dilated,  the  pulse  can 
hardly  be  felt,  and  the  animal  lies  apparently  dead,  when  often 
after  a  surprisingly  long  interval  one  more  respiratoiy  gasp  fol- 
lows, and  with  a  gentle  tremor  the  animal  stretches  itself  in  a 
kind  of  tonic  inspiratory  spasm,  after  which  it  is  no  longer  capa- 
ble of  resuscitation.  This  last  pulseless  stage  to  which  the  term 
asphyxia  is  more  properly  confined  is  the  most  irregular  in  dura- 
tion, but  always  the  longest. 

The  blood  of  an  animal  which  has  died  of  asphyxia  is  nearly 
destitute  of  oxygen,  the  haemoglobin  being  in  a  much  more  re- 
duced condition  than  is  found  in  venous  blood.  The  first  and 
most  obvious  eflfect  produced  by  the  circulation  of  blood  so  defi- 
cient in  oxygen  is  excessive  stimulation  of  the  respiratory  centre, 
which  causes  the  extreme  and  varied  actions  just  described.  At 
the  same  time  the  venous  blood  passing  through  the  systemic 
capillaries  affects  most  profoundly  the  vaso-motor  nerve  mecha- 
nism, so  as  to  cause  a  rapid  and  considerable  rise  in  blood  pressure 
in  the  first  stage  of  asphyxia.  The  general  constriction  of  the 
small  arteries  may  be  brought  about  by  the  venous  blood  acting 
directly  as  a  stimulus  to  the  medullary  and  spinal  vaso-motor 
centres,  to  thfe  local  centres,  or  as  direct  stimulation  of  the  muscle 
cells  of  the  arterioles  themselves.  The  centres  in  the  medulla 
which  govern  the  inhibitory  fibres  of  the  pneumogastric  are  also 
stimulated,  and  consequently  the  heart  beats  more  slowly.  The 
increase  in  arterial  tension  and  the  slow  beat  give  rise  to  disten- 
sion of  the  ventricle,  which,  when  a  certain  point  is  reached,  im- 
pedes the  working  of  the  heart,  and  its  muscle  begins  to  beat  more 
and  more  feebly,  so  that  in  the  third  stage  the  impulse  can  hardly 
be  felt.  The  muscular  arterioles  then  become  exhausted  and  re- 
lax, the  blood  pressure  falls  rapidly,  and  with  the  death  of  the 
animal  reaches  zero.  Both  sides  of  the  heart  and  great  veins  are 
engorged  with  blood  in  the  last  stage  of  asphyxia  ;  the  cardiac 
muscle,  being  exhausted  from  want  of  oxygen,  is  unable  to  pump 
the  blood  out  of  the  veins,  or  empty  its  cavity. 

Owing  to  the  force  of  the  rigor  mortis  of  the  left  ventricle,  and 
the  greater  capacity  of  the  systemic  veins,  the  left  side  is  found 
comparatively  empty  some  time  after  death,  and  at  post  mortem 
examination  the  right  side  alone  is  found  overfilled. 


CHAPTER  XX. 
BLOOD  ELABORATING  GLANDS. 

In  the  preceding  chapters  we  have  seen  that  the  blood  un- 
dergoes important  changes  as  it  courses  through  the  different 
parts  of  its  circuit.  Where  it  comes  in  contact  with  the  tissues 
it  yields  to  them  nutrient  material  for  assimilation,  and  oxygen 
for  their  metabolism,  and  carries  away  from  them  some  waste 
products.  In  the  lungs  it  receives  oxygen  and  gives  off"  carbonic 
acid.  While  it  flows  through  the  minute  vessels  of  the  alimentary 
tract  some  of  the  materials  elaborated  by  the  digestion  of  food  are 
absorbed  and  directly  added  to  the  blood  ;  at  the  confluence  of 
the  great  veins  in  the  neck  the  stream  composed  of  lymph  and 
chyle  is  poured  into  the  blood  before  it  enters  the  heart,  so  as  to 
be  thoroughly  mingled  with  it  on  its  return  from  the  general  cir- 
culation. Moreover,  in  various  glands,  different  substances  are 
used  in  the  manufacture  of  their  secretions. 

Thus  it  is  obvious  that  there  is  a  kind  of  material  circulation, 
a  constant  income  and  output  going  on  in  the  blood  itself  as  it 
passes  through  the  different  parts  of  the  body.  The  investigation 
of  the  exact  changes  which  take  place  in  the  blood  in  each  organ 
or  part  is  surrounded  with  difficulty,  and  in  many  cases  it  is  quite 
impossible  to  ascertain  what  changes  occur.  In  some  parts  it  may 
be  made  out  by  noting  the  results  produced,  or  the  substances 
given  off"  or  taken  up  by  the  blood,  as  seen  in  the  changes  found 
in  the  air  after  its  exposure  to  the  blood  in  the  lungs,  where  we 
can  definitely  state  that  the  blood  has  lost  or  gained  certain  mate- 
rials, and  is  so  far  altered.  In  other  parts,  such  as  the  muscles 
or  the  ductless  glands,  where  no  doubt  profound  changes  in  the 
blood  occur,  we  have  no  separate  outcome  which  we  can  analyze, 
and  we  must  therefore  trust  altogetheu  for  the  elucidation  of  the 
change  going  on  in  them  to  the  differences  which  may  be  found 
to  exist  in  the  blood  flowing  to,  and  that  flowing  from  such  an 


BLOOD  ELABORATING  GLANDS. 


361 


organ.  For  this  purpose  one  can  either  examine  samples  of  the 
blood  from  the  artery  and  vein  of  the  organ,  while  the  ordinary 
circulation  is  going  on,  or  immediately  after  the  removal  of  the 
organ,  by  causing  an  artificial 
stream  of  blood  to  flow  through 
it ;  then  the  changes  brought 
about  in  the  blood  by  its  passage 
through  the  organ  will  give  the 
required  information.  It  can 
be  seen  from  the  foregoing  enu- 
meration of  processes,  that  some 
organs  have  a  double  function 
as  regards  the  blood.  Thus,  in 
the  lung  there  is  both  renova- 
tion by  taking  in  oxygen,  and 
purification  by  getting  rid  of 
carbon  dioxide.  The  textures 
in  their  internal  respiration 
take  the  nutriment  and  oxygen, 
and  give  to  the  blood  CO.^  and 
various  other  waste  products  of 
tissue  change. 

Ductless  Glands. 

In    the    preceding    chapters 

the  chief  sources  of  income  to 

the  blood,  viz.,  the  alimentary 

tract  and  the  lungs,  have  been       -ir    .•    i      *•       ra    c  i 

°  '  Vertical  section  of  the  bupra-renal 

considered;     and    the    changes  Capsule.— 1.    Cortex.      2.    Medulla, 

the  blood  undergoes  in  its  pas-  a,  Fibrous  capsule;  h.  External  cell 

sage  through   the  tissues  in  the  masses;  c.Columnar  layer ;(/,  Internal 

systemic        capillaries         have  cell  masses;  <>,  Medullary  substance, 

been  alluded  to.     The  elimina-  ^"  ^'^'^'^  ^^^'  ^  ^^'^^  ^«^"'  P''^'"'^^  ^^^" 
,.  n  ^  ,,  .  .  .in  section/. 

tion  or  one  or  the  most  import- 
ant outcomes  of  tissue  change,  namely,  CO^,  has  been  described. 
It  has,  further,  been  shown  that  a  great  part  of  the  absorbed 


362 


MANUAL    OF    PHYSIOLOGY. 


nutrient  material  passes  through  a  special  set  of  vessels  called 
the  lacteals  or  lymphatics,  and  in  so  doing  has  to  traverse  pe- 
culiar organs  called  lymphatic  glands,  where  it  is  no  doubt  modi- 
fied and  has  added  to  it  a  number  of  cells  (lymph  corpuscles) 
whicli  subsequently  are  poured  into  the  large  veins  with  the 
lymph  and  become  important  constituents  of  the  blood. 

There  is  a  certain  set  of  organs,  which  have  but  slight  traits  of 
resemblance  to  one  another,  and  in  consequence  of  the  want  of 
more  accurate  knowledge  as  to  their  exact  function,  are  cora- 

FiG.  ir)7. 


Section  of  tiie  Thyroid  Gland  of  a  Cliild,  showing  two  complete  sacs  and 
portions  of  others.  The  homogeneous  colloid  substance  is  represented  as 
occupying  the  central  part  of  the  cavity  of  the  vesicles,  which  are  lined  by 
even  cubical  epithelium.     (Schafer.) 

monly  grouped  together  as  ductless  or  blood  glands.  Some  of 
these  are  doubtless  nearly  akin  to  the  lymphatic  glands,  their  duty 
being  the  further  elaboration  and  perfection  of  the  blood.  In 
this  gro\ip  are  commonly  placed  the  supra-renal  capsules,  the 
thyroid,  the  thymus,  and  the  spleen. 


Supra-renal  Capsule. 

With  regard  to  the  function  of  the  supra-renal  capsule  we  may 
say  that  nothing  definite  is  known.  The  cortical  part  is  said  to 
resemble  the  lymph  follicles  in  structure,  while  the  central  part. 


THYROID   GLAND. 


363 


on  account  of  its  numerous  peculiar  large  cells,  and  great  rich- 
ness in  nerves,  has  been  explained  as  belonging  to  the  nervous 
system. 

Thyroid  Body. 
The  thyroid  is  made  up  of  groups  of  minute  closed  sacs  im- 
bedded in  a  stroma  of  connective  tissue,  lined  with  a  single  row 

Fig.  158. 


Fig.  159. 


Fig.  158. — Portion  of  Thymus  removed  from  its  envelope  and  unravelled 
so  as  to  show  the  lobules  {b,  b)  attached  to  a  central  band  of  connective 
tissue  (a). 

Fig.  159. — Magnified  section  of  a  portion  of  injected  Thymus,  showing 
one  complete  lobule  with  soft  central  part  (cavity)  (6),  and  parts  of  other 
lobules.  (Cadiat.)  (a)  Lymphoid  tissue,  (c)  Bloodvessels,  {d)  Fibrous 
tissue. 


of  epithelium  cells,  and  filled  with  a  clear  fluid  containing  mucin. 
In  the  adult  the  sacs  are  commonly  much  distended  with  a  col- 


364 


MANUAL   OF    PHYSIOLOGY. 


loid  substance  and  peculiar  crystals,  and  the  epithelium  has  dis- 
appeared from  their  walls.  Although  said  to  be  rich  in  lymphat- 
ics and  to  contain  follicular  tissue,  positive  proof  of  the  relation 
of  the  thyroid  body  to  the  lymphatic  system  is  still  wanting. 


Thymus  Gland. 

The  functional  activity  of  tiie  thymus  is  restricted  to  that  pe- 
riod of  life  when  growth  takes  place  most  rapidly.  It  is  well 
developed  in  the  foetus,  and  increases  in  size  for  a  couple  of  years 
after  birth  ;  but  it  gradually  diminishes  in  bulk  and  loses  its 
original  structure  during  the  later  periods  of  childhood,  so  as  to 
become  completely  degenerated  and  fatty  in  the  adult.  It  is  com- 
posed of  numerous  little  follicles  of  lymphoid  tissue  collected  into 
groups  or  lobules  connected  to  a  kind  of  central  stalk.  The 
lymphoid  follicles  of  the  young  thymus  have  some  likeness  to 
those  of  the  intestinal  tract,  but  they  differ  from  these  agminate 
glands  not  only  in  arrangement  but  also  in  having  small  peculiar 
nests  of  large  cells  (corpuscles  of  Hassall)  in  the  midst  of  the 
adenoid  tissue  of  which  they  are  made  up. 
On  account  of  the  structure  of  the  lobules 
being  so  nearly  identical  with  that  of  a 
lympathic  gland,  and  from  its  great  richness 
in  lymphatic  vessels,  the  thymus  is  said  to 
be  related  to  the  lymphatic  system,  and  is 
supposed  to  play  an  important  part  in  the 
elaboration  of  the  blood  during  the  earlier 
stages  of  animal  life. 


Fig.  160. 


'i\e     '^sC      «c  l»* 

Elements  of  Thy- 
mus (high  power). 
(Cadiat.)  (a)  Lymph 
corpuscles.  (6)  p]pi- 
theloid  nests  of  Has- 
sall. 


Spleen. 

The  spleen  also  resembles  a  lymphatic 
organ  in  structure,  but  differs  from  it  in  the 
relation  borne  by  the  blood  to  the  elements 
of  the  follicular  tissue.     It  is  incased  in  a 

strong  capsule  made  of  fibrous  tissue  and  unstriated  muscle  cells. 

From  this  many  branching  prolongations  pass  into  the  substance 


SPLEEN. 


365 


of  the  organ,  so  as  to  traverse  the  soft  red  spleen  pulp.     In  these 
trabeculse   or   prolongations   from    the   capsule   are   found   the 


Fig.  162. 


(a)  Trabeculae  of  the  Spleen.     (6)  Artery  cut  obliquely.     (Cadiat.) 

branches  of  the  splenic  artery,  dividing  into  smaller  twigs  with- 
out anastomosis.  On  leaving  the  trabeculse  the  arteries  break  up 
suddenly  into  a  brush-like  series  of  small  branches,  ending  in 
capillaries  which  are  lost  in  the 
pulp  where  the  small  veins  may 
be  seen  to  commence. 

Between  the  trabeculse  are 
found  two  distinct  kinds  of  tissue  : 
(1)  Rounded  masses  of  lymphoid 
tissue,  which  are  here  and  there 
scattered  through  the  organ  (Mal- 
pighian  corpuscles)  ;  and  (2)  the 
peculiar  soft  splenic  pulp. 

The  small  rounded  masses  of 
lymph  follicular  tissue  are  situated 
on  the  course  of  the  fine  arterial 
twigs.  The  delicate  adenoid  re- 
ticulum which  holds  the  lymph 
cells  together,  is  intimately  con- 
nected with  the  vessel  wall.  The  pale  appearance  of  these  folli- 
cles, which  distinguishes  thera  from  the  surrounding  splenic  pulp, 


Reticulum  of  the  Spleen  Pulp 
injected  with  colorless  gelatine. 
(Cadiat.) — (a)  Meshes  made  of  en- 
dothelium. (6)  Lacunar  spaces, 
tlirough  which  the  blood  flows. 
(c)  Nuclei  of  endothelium. 


366  MANUAL   OF   PIIYSIOLOOY. 

depends  on  the  number  of  the  white  cells  which  are  packeil   in 
the  meshes  of  this  perivascular  adenoid  tissue. 

The  splenic  pulp  consists  of  a  system  of  comraunicatiug  lacunar 
spaces  lined  with  endothelium.  Into  these  spaces  the  blood  is 
poured  from  the  arteries,  and  thus  niinj^les  with  vast  numbers  of 
white  cells.  Besides  the  ordinary  blood  disks  and  the  white  cor- 
puscles or  lymph  cells,  many  peculiar  cells  are  found  in  the  spleen 
pulp.  Some  of  these  look  like  lymph  cells  containing  little  masses 
of  hicmoglobin,  and  appear  to  be  transitions  from  the  colorless 
to  the  red  corpuscles,  whilst  others,  small,  misshapen,  red  cor- 
puscles, are  regarded  as  steps  in  a  retrograde  change  in  the  disks. 
But  few,  if  any,  lymph  channels  lead  from  the  spleen  pulp,  and 
only  a  relatively  small  number  pass  out  from  the  hilus,  so  that 
the  splenic  artery  and  vein  must  be  regarded  as  taking  the  place 
of  the  afferent  and  efferent  lymph  channels. 

Chemical  examination  shows  the  splenic  pulp  to  have  remark- 
able peculiarities.  Although  so  full  of  blood,  which  is  generally 
alkaline,  the  spleen  is  acid  in  reaction,  and  contains  a  great  quan- 
tity of  the  oxidation  products  (so-called  extractives)  commonly 
found  as  the  result  of  active  tissue  change.  The  chief  of  these 
are  uric  acid,  leucin,  xauthin,  hypoxanthin,  inosit,  lactic,  formic, 
succinic,  acetic,  and  butyric  acids.  It  also  contains  numerous 
pigments,  rich  in  carbon,  but  little  known,  which  are  probably 
the  outcome  of  destroyed  hsemoglobin.  A  peculiarly  suggestive 
constituent  is  an  albuminous  body  containing  iron.  The  ash  is 
found  to  contain  a  considerable  quantity  of  oxide  of  iron,  to  be 
rich  in  phosphates  and  soda,  with  but  small  quantities  of  chlorides 
and  potassium. 

If  the  blood  flowing  in  the  artery  to  the  spleen  be  compared 
with  that  in  the  vein,  the  difference  gives  us  the  changes  the 
blood  has  undergone  in  the  organ,  and  hence  is  of  great  import- 
ance. In  the  blood  of  the  vein  is  found  an  enormous  increase  in 
the  number  of  the  white  corpuscles  (1  white  to  70  red  in  the  vein, 
as  against  1  to  2000  in  the  splenic  artery).  The  red  corpuscles 
from  the  vein  are  smaller,  brighter,  less  flattened  than  those  of 
ordinary  blood  ;  they  do  not  form  rouleaux,  and  are  more  capable 
of  resisting  the  injurious  influence  of  water.     The  blood  of  the 


FUNCTIONS   OP   THE   SPLEEN.  367 

splenic  vein  is  also  said  to  have  a  great  proportion  of  water,  and 
to  contain  an  unusual  proportion  of  uric  acid  and  other  products 
of  tissue  waste. 

The  amount  of  blood  in  the  spleen  varies  greatly  at  different 
times.  Shortly  after  meals  the  organ  becomes  turgid  and  remains 
enlarged  during  the  later  periods  of  digestion.  The  size  of  the 
spleen,  which  may  be  taken  as  a  measure  of  its  blood  contents,  is 
also  altered  by  many  abnormal  conditions  of  the  blood.  Thus, 
in  all  kinds  of  fever,  particularly  ague  and  typhoid,  and  in  syph- 
ilis, the  spleen  becomes  turgid,  and  in  some  of  these  diseases  it 
remains  swollen  for  some  time.  In  a  remarkable  disease,  leuco- 
cythiemia,  in  which  the  white  blood  cells  are  greatly  increased 
in  number,  and  the  red  ones  are  comparatively  diminished,  the 
spleen,  in  company  with  the  lymphatic  glands,  is  often  found  to 
be  profoundly  altered  and  diseased,  and  commonly  immensely  en- 
larged j  but  on  the  other  hand,  advanced  amyloid  degeneration 
of  the  spleen  may  occur  without  any  notable  alteration  taking 
place  in  the  number  or  properties  of  the  blood  corpuscles. 

The  spleen  may  be  removed  from  the  body  without  any  marked 
changes  taking  place  in  the  blood  or  the  economy  generally.  It 
is  said  that  if  an  animal  whose  spleen  is  extirpated  be  allowed 
to  live  for  a  certain  time,  the  lymphatic  glands  increase  in  size, 
or  become  SAVollen. 

In  attempting  to  assign  a  definite  function  to  the  spleen,  all  the 
foregoing  fticts  must  be  cai-efully  reviewed,  and  the  peculiarity  of 
its  (1)  structure,  (2)  chemical  composition,  (3)  the  chaiiges  the  blood 
undergoes  while  flowing  through  it,  (4)  the  variations  in  blood 
supply  which  follow  normal  and  pathological  changes  in  the 
economy,  and  (5)  the  absence  of  effect  following  its  extirpation, 
must  all  be  borne  in  mind. 

Its  structure  teaches  us  that  it  is  intimately  related  to  lymphatic 
glands.  The  Malpighian  bodies  are  simply  lymph  follicles,  and 
the  pulp  may  be  regarded  as  a  sinus  like  that  of  a  lymph-gland, 
with  this  difference  that  it  is  traversed  by  blood  instead  of  lymph. 
The  cell  elements  found  in  it  indicate  that  not  only  white  cells  are 
rapidly  generated,  but  also  that  these  cells  have  some  peculiar 
relationship  to  hsemoglobin,  as  they  are  often  found  to  contain 


368 


MANUAL   OF   PHYSIOLOGY. 


some.  The  varieties  in  size,  form,  and  general  appearance  of  the 
red  corpuscles  can  be  accounted  for  by  either  their  destruction  or 
their  formation  occurring  in  this  organ. 

Its  chemical  composition  also  shows  that  certain  special  changes 
go  on  in  the  pulp,  and  that  probably  stages  of  the  construction 
or  destruction  of  haemoglobin  are  here  accomplished  may  be 
inferred  from  the  peculiar  association  of  iron  with  albuminous 
bodies. 

From  the  characters  of  the  blood  flowing  from  the  spleen  it 
has  been  argued   that,  besides  an  enormous  production  of  white 


Fig.  1G3. 


Section  of  Spleen  through  a  lymph  follicle  (Malpighian  body)  (a)  injected 
to  show  the  vessel ;  (c)  entering  the  follicle,  tlie  lymphoid  tissue  of  which  is 
pale  in  comparison  with  the  pulp  (6),  the  meshes  of  which  are  filled  with  in- 
jection.    (Cadiat.) 

corpuscles,  the  destruction  of  the  red  disks  goes  on,  while  some 
new  disks  are  formed,  probably  by  means  of  the  white  cells  mak- 
ing hjemoglobin  in  their  protoplasm,  which,  gradually  disappear- 
ing, leaves  only  the  red  mass  of  hiemoglobin. 

The  increased  activity  of  the  spleen  after  meals,  and  in  certain 
abnormal  states  of  the  blood,  as  shown  by  its  containing  more 
blood,  distinctly  points  out  that  some  form  of  blood  elaboration 


GLYCOGENIC    FUNCTION    OF    THE    LIVER.  369 

goes  ou  iu  it,  which  is  nearly  related  to  or  associated  with  nutri- 
tion. 

The  swelling  of  the  lymphatic  glands  after  extirpation  of  the 
spleen  confirms  its  relation  to  these  organs,  and  the  fact  is  undis- 
puted that  it  is  a  source  of  the  white  corpuscles  of  the  blood  ; 
but  the  paucity  of  evidence  after  this  operation  as  to  changes  in 
the  number  or  character  of  the  red  disks  proves  that  if  the  spleen 
be  either  the  place  of  origin  or  destruction  of  the  red  corpuscles 
it  cannot  be  the  only  organ  in  which  they  are  produced  or  de- 
stroyed. 

Glycogenic  Function  of  the  Liver. 

Of  all  the  organs  that  modify  the  composition  of  the  blood  as 
it  flows  through  them,  the  liver  plays  the  most  important  part  in 
elaborating  the  circulating  fluid.  The  elimination  of  the  various 
constituents  of  the  bile,  which  has  already  been  mentioned  as 
necessary  for  the  purification  of  the  blood,  and  useful  in  aiding 
absorption,  is  probably  but  a  secondary  function  of  this  great 
gland.  The  production  of  a  special  material — animal  starch — 
essential  to  the  nutrition  and  growth  of  the  texture  is,  in  all 
probability,  the  main  duty  of  the  liver  cells,  and  possibly  the 
constituents  of  the  bile  are  but  the  by-products  which  must  be 
got  rid  of,  resulting  from  the  chemical  processes  of  the  manu- 
facture. 

In  the  chapter  on  the  digestive  secretions  the  structure  of  the 
liver  was  mentioned,  and  attention  was  directed  to  the  peculiari- 
ties of  its  double  blood  supply.  A  relatively  small  arterial  twig 
takes  blood  to  it  from  the  aorta,  while  the  great  portal  veins  dis- 
tribute to  it  all  that  large  supply  of  blood  which  flows  through 
the  intestinal  tract  and  the  spleen. 

The  blood  in  the  vena  porta  during  digestion  can  hardly  be 
called  venous  blood,  for  much  more  passes  through  the  intestinal 
capillaries  when  digestion  is  going  on  than  is  necessary  for  the 
nutrition  of  the  tissue  of  the  intestinal  wall.  The  portal  blood 
is  further  to  be  distinguished  from  ordinary  venous  blood  from 
the  fact  that  it  has  just  been  enriched  with  a  quantity  of  the  sol- 

31 


370 


MANUAL   OF   PHYSIOLOGY. 


uble  materials  taken  from  the  intestinal  canal,  namely,  proteids, 
sugar,  salts,  and  proba&ly  some  fats ;  and  it  has  been  profoundly 
modified  by  the  changes  taking  place  in  the  spleen. 

It  is  from  this  blood  that  the  liver-cells  manufacture  the  starch- 
like substance  above  mentioned.  This  substance  was  discovered 
by  Claude  Bernard,  and  called  by  him  Glycogen,  on  account  of 
the  great  facility  with  which   it  is  converted  into  sugar,  in  the 

Fig.  164. 


Diagram  of  tlie  Portal  Vehi  {p  v)  arising  in  the  alimentary  tract  and 
spleen  (.s),  and  carrj'ing  the  blood  from  these  organs  to  the  liver. 


presence  of  certain  ferments  which  exist  in  the  liver  itself  and  in 
most  tissues  after  death.  Shortly  after  the  death  of  an  animal 
the  tissue  of  the  liver,  and  also  the  blood  contained  in  the  hepatic 
veins,  are  extremely  rich  in  sugar,  which  has  been  formed  by  the 
fermentation  of  the  hepatic  glycogen.  The  quantity  of  sugar  is 
in  proportion  to  the  length  of  time  that  has  elapsed  since  the 


GLYCOGEN.  371 

death  of  the  animal,  and  is  minimal,  if  not  nil,  if  the  liver  or 
hepatic  blood  be  taken  for  examination  while  the  tissue  elements 
are  still  alive. 

The  peculiar  blood  of  the  great  portal  vein  coming  from  the 
stomach,  intestines,  and  the  spleen,  has  then  to  pass  through  a 
second  set  of  capillaries  in  the  liver,  and  undergoes  such  important 
changes  that  this  organ  must  be  regarded  as  occupying  a  foremost 
position  among  the  blood-glands.  Differences  of  the  utmost  im- 
portance have  long  been  thought  to  exist  between  the  blood  going 
to  and  that  coming  from  the  liver,  and  to  it  has  even  been  attrib- 
uted paramount  utility  as  a  blood  elaborator;  but  the  scientific 
knowledge  of  its  power  in  this  respect  must  date  from  the  dis- 
covery of  its  glycogenic  function. 

Glycogen  is  a  substance  nearly  allied  to  starch  in  its  chemical 
composition,  and  is  converted  with  great  readiness  into  grape- 
sugar  by  the  action  of  certain  ferments  and  acids.  Many  of  the 
animal  textures  contain  these  ferments,  among  others  the  liver 
itself,  at  least  when  its  tissue  is  dying  ;  and  consequently  the  liver 
with  the  blood  coming  from  it  (if  examined  in  an  animal  some 
time  dead)  does  not  contain  glycogen,  but  sugar  which  has  been 
formed  from  it.  If  a  piece  of  liver  taken  from  an  animal  imme- 
diately after  it  is  killed  be  plunged  into  boiling  water,  so  as  to 
check  the  action  of  the  ferment,  no  trace  of  sugar  is  found  in  it, 
but  only  glycogen.  After  the  lapse  of  a  little  time  another  piece 
of  the  same  liver,  which  has  lain  at  the  ordinary  room  tempera- 
ture, will  give  abundance  of  sugar. 

The  mode  of  preparation  of  glycogen  depends  upon  the  fore- 
going facts.  The  perfectly  fresh  liver  taken  from  an  animal 
killed  during  digestion  is  rapidly  subdivided  in  boiling  water. 
When  the  ferment  has  been  destroyed  by  heat  the  pieces  of  liver 
are  rubbed  up  to  a  pulp  in  a  mortar,  and  then  reboiled  in  the 
same  fluid.  The  liquor  is  then  filtered,  and  from  the  filtrate  the 
albuminous  substances  are  precipitated  with  potassio-mercuric 
iodide  and  hydrochloric  acid,  and  removed  on  a  filter.  From 
this  filtrate  the  glycogen  may  be  precipitated  by  alcohol,  caught 
on  a  filter,  washed  with  ether  to  remove  fat,  and  dried. 

Glycogen  thus  prepared  is  a  white  powder,  forming  an  opalcs- 


372  MANUAL   OF   PHYSIOLOGY, 

cent  solutiou  in  water,  which  becomes  clear  on  the  addition  of" 
caustic  alkalies.  It  is  insoluble  in  alcohol  and  ether.  With  a 
solution  of  iodine  it  gives  a  wine-red  color,  and  not  blue,  like 
starch,  which  it  otherwise  much  resembles  in  chemical  relation- 
ship. 

Glycogen  has  been  found  in  many  other  parts  besides  the  liver, 
namely,  in  all  the  tissues  of  the  embryo,  and  in  the  muscles,  tes- 
ticles, inflamed  organs,  and  pus  of  adults ;  in  short,  where  any 
very  active  tissue  change  or  growth  is  going  on,  some  traces  of 
glycogen  can  be  found. 

The  amount  of  glycogen  in  the  liver  depends  in  a  great  mea- 
sure on  the  kind  and  quantity  of  food  used.  It  rapidly  increases 
with  a  full,  and  decreases  with  a  spare  diet,  and  it  slowly  falls  to 
nil  after  prolonged  starvation.  The  formation  of  glycogen  is 
much  more  dependent  on  the  carbohydrate  food  than  on  the  pro- 
teid,  for  it  rapidly  rises  with  increase  in  the  quantity  of  sugar 
taken,  and  falls,  as  in  starvation,  when  pure  proteid  (fibrin)  with- 
out any  carbohydrate  is  used  either  with  or  without  fat.  Although 
the  large  supply  of  glycogen  normally  manufactured  in  the  liver 
is  probably  derived  from  the  sugar  of  the  food,  \ye  must  not  con- 
clude from  this  that  the  liver  cells  cannot  make  glycogen  from 
other  materials.  Possibly  anything  that  suffices  for  the  nutrition 
of  their  own  protoplasm  enables  the  cells  to  produce  glycogen. 
The  slowness  with  which  glycogen  disappears  in  starvation  would 
seem  to  point  to  this. 

The  ultimate  destiny  and  uses  of  glycogen  are  still  vexed  ques- 
tions. Much  trouble  has  been  taken  to  decide  whether  it  is  con- 
verted into  sugar,  and  as  such  carried  off  by  the  blood  to  the 
tissues.  One  set  of  observers  deny  the  existence  in  the  living 
tissues  of  the  amylolytic  ferment  necessary  for  its  conversion  into 
sugar,  and  think  that  it  is  distributed  simply  as  glycogen  to  the 
tissues ;  while  others  say  that  it  is  gradually  changed  to  sugar 
before  it  is  carried  off  in  the  hepatic  veins. 

The  difficulty  of  determining  the  exact  amount  of  sugar  in  the 
blood  with  sufficient  accuracy  may  account  for  the  remarkable 
disparity  of  opinion  on  this  subject,  and  makes  this  a  very  unsat- 
sfactory  means  of  determining  the  use  or  destiny  of  glycogen. 


GLYCOGEN.  373 

In  fact,  the  whole  controversy  seems  idle,  for  the  real  questions 
are,  not  whether  the  glycogen  is  distributed  to  the  tissues  as  sugar 
or  not,  but  is  the  glycogen  distributed  to  the  tissues  as  a  general 
carbohydrate  nutritive,  or  is  it  to  be  regarded  as  a  step  in  the 
manufacture  of  some  other  material  by  the  liver  cells;  in  other 
words,  is  the  glycogen  of  the  liver  a  store  of  special  animal  carbo- 
hydrate to  be  kept  till  called  for,  or  is  it  a  stage  in  the  formation 
of  fat  ? 


CHAPTER  XXI. 

SECRETIONS. 

The  secretions  which  are  poured  into  the  alimentary  tract  have 
been  already  described  in  the  chapter  on  digestion.  There  are 
other  glands  which  can  now  be  conveniently  considered,  since  they 
more  or  less  alter  the  blood  flowing  through  them,  and  thus  may 
be  said  to  aid  slightly  in  the  perfect  elaboration  of  that  fluid. 
They  are,  however,  subservient  to  very  dififerent  functions ;  some 
having  merely  local  offices  to  perform,  and  others  having  duties 
allotted  to  them  of  the  greatest  general  importance  to  the  economy. 
This  becomes  obvious  from  a  glance  at  the  following  enumeration 
of  the  remaining  glandular  organs. 

Secreting  glands  (other  than  those  forming  special  digestive 
juices) : 

Lachrymal. 

Mucous. 

Mammary. 

Sebaceous. 
Excreting  glands : 

Sudorific. 

Urinary. 

SURFACE  GLANDS. 
Lachrymal  Glands. 

Most  vertebrate  animals  that  live  in  air  have  a  gland  in  con- 
nection with  the  surface  of  their  eyes,  which  secretes  a  thin  fluid 
to  moisten  the  conjunctiva.  This  fluid  commonly  passes  from 
the  eye  into  the  nasal  cavity,  and  supplies  the  inspired  air  with 
moisture. 

The  lachrymal  fluid  is  clear  and  colorless,  with  a  distinctly  salty 
taste  and  alkaline  reaction.     It  contains  only  about  1  per  cent,  of 


MUCOUS    GLANDS.  375 

solids,  in  which  can  be  detected  some  albumin,  mucus,  and  fat 
(1  per  cent.),  epithelium  (1  per  cent.)  as  well  as  sodium  chloride 
and  other  salts  (.8  per  cent.). 

The  secretion  is  produced  continuously  in  sraallamount,  but  is 
subject  to  such  considerable  and  sudden  increase,  that  at  times  it 
cannot  all  escape  by  the  nasal  duct,  but  is  accumulated  in  the  eyes^ 
until  it  overflows  to  the  cheek  as  tears.  This  excessive  secretion 
may  be  induced  by  the  application  of  stimuli  to  the  conjunctiva, 
the  lining  membrane  of  the  nose,  or  the  skin  of  the  face,  or  by 
strong  stimulation  of  the  retina,  as  when  one  looks  at  the  sun.  A 
similar  increase  of  secretion  follows  certain  emotional  states  con- 
sequent on  grief  or  joy.  These  facts  show  that  the  secretion  of  the 
gland  is  under  nervous  control,  the  impulses  stimulating  secretion 
commonly  starting  either  from  the  periphery,  and  passing  along 
the  sensory  branches  of  the  fifth  or  along  the  optic  nerve,  or 
from  the  emotional  centres  in  the  brain,  and  arriving  at  the 
gland  in  a  reflex  manner.  The  amount  of  secretion  can  also 
be  augmented  by  direct  stimulation  of  the  lachrymal  nerves, 
so  that  in  all  probability  these  are  the  efierent  channels  for  the 
impulse. 

Mucous  Glands. 

In  connection  with  mouth  and  stomach  secretions,  mention  has 
been  made  of  glands  which  are  elongated  saccules  lined  with  re- 
fracting cells  (Fig.  165).  They  are  distributed  over  all  mucous 
membranes,  and  are  the  chief  source  of  the  thick,  tenacious,  clear, 
alkaline,  and  tasteless  fluid  called  mucus. 

This  material  contains  about  5  per  cent,  of  solid  matters,  of  which 
the  chief  is  mucin,  the  characteristic  material  of  mucus,  which 
swells  up  in  water  and  gives  the  peculiar  tenacity  to  the  fluid. 
It  is  precipitated  by  weak  mineral  and  acetic  acids;  and,  as  the 
precipitate  with  the  latter  does  not  redissolve  in  an  excess,  this 
acid  becomes  a  good  test  to  distinguish  it  from  its  chemical  allies. 
Mucin  is  not  precipitated  by  boiling.  Mucus  also  contains  traces 
of  fat  and  albumin,  and  inorganic  salts,  viz.,  sodium  chloride, 
phosphates  and  sulphates,  and  traces  of  iron. 

The  fluid  is  secreted  either  by  the  special  mucous  glands,  or  it 
may  be  produced   by  the  epithelium  of  the   mucous  surfaces. 


376 


MANUAl.   OF   PHYSIOLOGY. 


The  cells  produce  in  their  protoplasm  a  quantity  of  the  secretion, 
which  may  often  be  seen  to  swell  them  out  to  a  considerable 
extent.  This  clear  fluid  is  then  expelled,  and  the  altered  cells 
are  repaired  oi*  replaced.  Many  elements,  like  the  remains  of 
epithelial  cells,  are  found  in  the  secretion  ;  and  also  round  nu- 
cleated masses  of  protoplasm  similar  to  white  blood  corpuscles, 
after  the  imbibition  of  water.  In  the  abnormal  secretion  of  a 
mucous  surface   during  inflammation  these  mucous   corpuscles 

Fig.  165. 


Mall 


Section  of  the  Mucous  Membrane  of  upper  part  of  nasal  cavity,  showing 
numerous  Mucous  Glands  cut  in  various  directions. — a,  Surface  epithelium ; 
b,  gland  saccule;  c,  connective  tissue  lined  with  secreting  cells.     (Cadiat.) 


are,  as  well  as  the  general  amount  of  secretion,  enormously  in- 
creased, so  that  the  secretion  may  become  opaque,  and  may  appear 
to  be  purulent. 

The  chief  object  of  the  secretion  seems  to  be  to  protect  the  mu- 
cous surfaces,  which  are  rich  in  delicate  nerves  and  vessels,  and 
are  subjected  to  many  injurious  influences  of  a  chemical  or  me- 
chanical nature.  It  is  analogous  to  the  keratin  of  the  epidermis, 
and  may  be  regarded  as  an  excretion,  since  it  is  not  absorbed,  but 


SEBACEOUS   GLANDS. 


377 


is  cast  out  from  the  mucous  passages,  and  passes  from  the  iutes- 
tiual  tract  with  the  faeces,  and  from  the  air-passages  as  sputum, 
etc. 

Sebaceous  Glands. 

These  belong  to  the  outer  skin,  and  commonly  open  into  the 
follicles  of  the  hairs,  but  also  appear  on  the  free  surface  of  the  lips 
and  prepuce,  etc.,  where  no  hairs  exist. 

The  secretion  cannot  be  collected  in  great  quantity  in  a  normal 
condition,  but  as  far  as  can  be  made  out,  it  is  composed  of  neutral 


Fig.  im. 


Section  of  Skin  showing  the  roots  of  three  hairs  and  two  large  sebaceous 
glands  (rf).     (Cadiat.) 

fat,  soap,  and  an  albuminous  body  allied  to  casein,  and  organic 
salts  and  water,  about  60  per  cent. 

The  secretion  seems  to  be  made  up  of  the  remains  of  epithelial 
cells  which  are  thrown  off  from  the  inner  surface  of  the  glands, 
while  they  are  undergoing  a  peculiar  kind  of  fatty  degeneration. 
These  cells  gradually  get  quite  broken  down  during  their  sojourn 
in  the  gland  alveoli,  and  the  secretion  is  finally  pressed  out  by 

32 


378  MANUAL   OF   PHYSIOLOGY. 

the  smooth  muscle  band  which  commonly  embraces  the  gland  and 
squeezes  it  against  the  hair  follicle. 

This  secretion,  the  use  of  which  is  to  lubricate  the  surface  with 
a  fatty  material,  is  cast  off  with  the  desquamated  epithelium  and 
the  hairs.  The  Meibomian  glands  of  the  eyelids  are  analogous 
structures,  and  are  specially  elaborated  for  the  lubrication  of  the 
ciliary  margin.  The  glands  about  the  prepuce  and  clitoris  are 
also  analogous  to  the  sebaceous  glands;  in  some  animals  they  se- 
crete a  peculiarly  odoriferous  material  ((^astor). 

Mammary  Glands. 

The  secretion  of  milk  only  takes  place  under  certain  circum- 
stances, and  continues  only  for  a  limited  period.  As  the  name  of 
the  gland  implies,  they  are  present  in  all  mammalian  animals. 
The  activity  of  the  gland  commences  in  the  latter  stages  of  preg- 
nancy, and  then  continues,  if  the  secretion  be  regularly  withdrawn 
from  the  gland,  for  some  9  to  12  mouths. 

During  pregnancy  the  breasts  undergo  certain  preparatory 
changes  prior  to  the  appearance  of  the  milk.  They  increase  in 
bulk  owing  to  their  greater  blood  supply,  and  by  certain  changes 
in  the  cell  elements  of  the  glands,  which  are  compound  saccular 
glands.  Each  breast  contains  a  series  of  some  ten  to  twelve 
glands  with  distinct  ducts,  upon  which  are  dilatations  that  act  as 
reservoirs,  in  which,  during  active  lactation,  the  secretion  is  stored 
until  it  is  needed. 

The  alveoli  are  chiefly  saccular  in  form,  and  are  lined  with  a 
single  layer  of  glandular  epithelium,  and,  during  active  lactation, 
contain  but  little  fat,  though  in  the  later  stages  of  pregnancy 
before  the  secretion  is  established  the  cells  contain  quantities  of 
large  fat  globules. 

Milk  is  a  yellowish-white,  perfectly  opaque,  sweetish  fluid,  with 
an  alkaline  reaction,  and  a  specific  gravity  of  about  1030.  When 
exposed  to  the  air,  particularly  in  warm  weather,  the  milk  soon 
loses  its  alkalinity,  first  becoming  neutral,  and  then  markedly 
acid;  the  milk  is  then  said  to  have  "turned  sour,"  but  its  ap- 
pearance is  not  greatly  changed.  When  it  has  stood  a  very  long 
time  it  may  crack  or  curdle,  and  separate  into  two  parts,  one  a 


MAMMARY   GLANDS.  379 

thick,  white  curd,  and  the  other  a  thin,  yellowish  fluid.  This 
turniug  sour  and  ultimate  curdling  depends  upon  a  change  brought 
about  in  one  of  its  most  important  constituents,  namely,  milk- 
sugar,  by  means  of  a  process  of  fermentation.  The  milk-sugar, 
in  the  presence  of  certain  forms  of  bacteria,  ferments,  and  gives 
rise  to  lactic  acid.  When  the  quantity  of  lactic  acid  is  suflS- 
cient,  it  not  only  makes  the  milk  sour,  but  also  precipitates  an- 
other of  its  important  constituents,  namely,  casein.  This  albu- 
minous body  in  its  coagulation  entangles  the  fat  of  the  milk,  and 

Fig.  167. 


^//l 


<^^ 


"^{X^^  "  ■— 


Section  of  Mammary  Gland  during  active  lactation  (liuman). — (o)  Sac- 
cules lined  with  regular  epithelium.  (6)  Connective  tissue  between  the 
alveoli.     (Cadiat.) 

we  have  thus  formed  the  curd  of  cracked  milk,  whilst  the  whey 
consists  of  the  acid,  salts,  and  remaining  milk-sugar. 

Although  the  curdling  of  milk  depends  on  the  coagulation  of 
an  albuminous  body,  it  is  never  produced  by  boiling  fresh  milk, 
because  the  chief  proteid  is  casein,  a  form  of  derived  albumin 
(alkali-albumin),  which  does  not  coagulate  by  heat. 

When  milk  is  preserved  from  impurities,  and  kept  in  a  cool 
place,  a  thick,  yellow  film  soon  collects  on  the  top  of  the  fluid ; 
the  thickness  of  this  layer — the  cream — may  be  taken  as  a  rough 
gauge  of  the  richness  of  the  milk,  for  milk  consists  of  a  fine 


380  MANUAL   OF   PHYSIOLOGY. 

emulsion  of  fat,  the  suspended  particles  of  which  are  kept  from 
running  togetiier  by  a  superficial  coating  of  dissolved  casein. 
When  left  at  rest,  the  light,  fatty  particles  float  to  the  top  and 
form  the  cream. 

When  the  mammary  glands  commence  to  secrete,  the  milk 
contains  numerous  peculiar  structural  elements  which  finally  quite 
disappear  from  the  secretion,  but  which  are  of  considerable  in- 
terest in  relation  to  the  physiological  process  of  the  secretion. 
These  are  the  colostrum  corpuscles,  which  consist  of  large  spheri- 
cal masses  of  fine  fat-globules  held  together  by  the  remains  of  a 
gland  cell,  which  incloses  the  fat-globules  as  a  kind  of  sac  or 
case,  and  in  which  at  times  a  nucleus  can  be  made  out. 

The  most  remarkable  point  about  the  chemical  composition  of 
milk  as  a  secretion  is  the  large  proportion  of  proteid  and  fat  it 
contains.  It  appears  that  there  are  two  distinct  albuminous  bodies 
present,  viz. :  casein,  which  appears  identical  with  alkali-albu- 
min, and  another  form  of  albumin  allied  to  serum-albumin.  The 
fats  are  present  in  the  shape  of  globules  of  various  sizes,  being  in 
the  condition  of  a  perfect  emulsion,  as  above  stated.  They  con- 
sist of  glycerides  of  palmitic,  stearic,  and  oleic  acids. 

The  milk-sugar  is  very  like  glucose  or  grape-sugar,  but  not  so 
soluble.    It  has  the  peculiarity  of  undergoing  lactic  fermentation. 

Of  the  inorganic  constituents  of  milk  the  most  important  are 
sodium  chloride,  and  phosphates  and  carbonates  of  the  alkalies; 
and  it  is  a  remarkable  fact  that  the  potash  compounds,  which  are 
the  most  abundant  iu  the  red  blood  corpuscles,  are  present  in 
greater  quantity  than  those  of  soda. 

The  following  table  shows  the  composition  of  human  milk,  a 
comparison  of  which  with  that  of  some  domestic  animals  will  be 
found  on  page  94 : 

Albumin, 39.24 

Fat, 26.66 

Milk  sugar, 43.64 

Salts, 1.38 

110.92 
Water, 889.08 

1000.00 


COMPOSITION   OF   MILK.  381 

The  relative  quantity  of  the  several  ingredients  of  milk  varies 
with  the  kind  of  diet  used.  A  vegetable  diet  increases  the  per- 
centage of  sugar,  but  diminishes  that  of  the  other  constituents,  and 
also  the  general  quantity  of  milk.  A  rich  meat  diet  increases 
both  the  general  quantity  and  the  percentage  of  fats  and  proteids. 

The  quantity  of  milk  secreted  in  the  twenty-four  hours  is 
extremely  variable  in  different  individuals  and  under  different 
circumstances  in  the  same  individual ;  the  average  in  general 
being  about  two  pints. 

The  amount  of  the  different  materials  in  milk  varies  under  the 
following  rules.  The  proportion  of  albumin  increases  as  the  milk 
sugar  decreases,  and  the  fat  remains  the  same  as  the  period  of 
lactation  advances.  The  portions  of  milk  last  drawn  are  much 
richer  in  fats  than  that  which  is  first  taken  from  the  gland.  In 
the  evening  the  milk  is  richer  in  fat  than  in  the  morning.  The 
general  amount  of  solid  constituents  falls,  up  to  the  age  of  thirty 
years,  then  gains  slightly  until  thirty-five,  after  which  age  the 
milk  becomes  decidedly  thinner.  These  points  should  be  borne 
in  mind  in  the  selection  of  a  wet-nurse. 

Mode  of  secretion. — Although  the  blood  contains  albumins,  fats, 
etc.,  very  similar  to  those  which  form  the  solid  parts  of  the  milk, 
we  have  good  reason  for  thinking  that  the  constituents  of  milk  are 
not  merely  extracted  from  the  blood,  but  that  the  manufacture 
of  this  highly  valuable  secretion  is  due  to  the  activity  of  the 
protoplasm  of  the  gland  cells,  which  construct  the  various  ingre- 
dients out  of  their  substance. 

It  has  been  suggested,  as  a  simple  explanation  of  the  formation 
of  milk,  that  the  cells  undergo  fatty  degeneration,  and  the  secre- 
tion is  then  only  the  debris  of  the  degenerated  cells. 

Some  facts  support  this  view.  In  the  first  place  the  ingredients 
one  finds  in  milk  are  suggestive  of,  though  not  indentical  with, 
the  chemical  materials  which  can  be  obtained  from  protoplasm 
by  chemical  disintegration,  rather  than  of  any  group  of  substances 
found  in  the  blood.  Further  we  find  that  the  so-called  colostrum 
corpuscles,  which  appear  to  be  secreting  cells  filled  with  fat  parti- 
cles, are  thrown  off  from  the  gland  in  the  early  stages  of  the 
secretion,  and  appear  in  numbers  in  the  milk. 


382 


MANUAL   OF   PHYSIOLOGY. 


But  these  colostrum  corpuscles  soon  cease  to  be  thrown  off  in 
the  secretion,  and  the  saccules  of  the  glands  during  active  lacta- 
tion do  not  contain  any  sign  of  the  debris  of  cast-off  cells,  or  any 
gradations  in  degeneration.  Only  one  row  of  finely  granular 
cells  is  found  lining  the  saccules,  and  the  cavities  are  filled  with 
globules  of  various  sizes.  From  this  it  would  appear  that  in  the 
earlier  stages  of  the  production  of  the  secretion,  the  mammary 
cells,  after  a  long  period  of  inactivity,  are  so  unaccustomed  to  the 
duty  they  are  called  upon  to  perform,  that  they  succumb  in  the 
effort,  and,  being  unable  to  produce  the  rich  secretion  and  retain 


Fig.  168. 


Fig.  169. 


Section  of  the  Mammary  (iland  of 
a  Cat  in  the  early  stages  of  lactation. 
— (a)  Cavity  of  alveoli  filled  with 
granules  and  globules  of  fat.  1,  2, 
3.  Epithelium  in  various  stages  of 
milk  formation. 


Cells  of  Mammary  Gland 
during  lactation,  stained 
with  osmic  acid,  so  as  to 
show  the  various  sized  oil 
globules  as  black  ma.sses. 
(Cadiat.) 


their  vitality,  they  are  cast  off.  Their  offspring,  however,  after  a 
generation  or  two  acquire  the  necessary  faculty  of  making  within 
their  protoplasm  all  the  necessary  ingredients  of  the  milk,  and 
discharge  them  out  into  the  lumen  of  the  saccules  without  them- 
selves undergoing  any  destructive  change. 

The  influence  of  the  nervous  system  on  the  secretion  of  the  mam- 
mary glands  is  distinctly  shown  by  the  wonderful  sympathy 
between  the  action  of  these  glands  and  the  conditions  of  the 
generative  apparatus.  Further,  different  emotions  have  an  effect, 
not  only  on  the  quantity,  but  also  on  the  quality  of  the  secretion. 


SWEAT   GLANDS.  383 

Local  stimulation  also  promotes  the  secretion,  for  the  application 
of  the  child  to  the  breast  at  once  produces  this  effect,  partly, 
possibly,  through  mental  influences,  but  chiefly,  no  doubt,  by 
reflex  excitation  of  the  gland  following  the  local  stimulation. 

For  the  details  of  the  dietetic  value  of  milk,  see  Chapter  V.  on 
Food,  p.  94. 

Excretions. 

The  term  excretion  is  commonly  used  to  denote  a  gland-fluid 
the  chief  constituents  of  which  are  manufactured  by  other  tissues, 
and  are  of  no  use  in  the  economy,  but,  on  the  contrary,  require  to 
be  continually  removed  in  order  that  their  accumulation  in  the 
blood  may  not  give  rise  to  injurious  consequences.  These  effete 
matters  are  the  outcome  of  the  various  chemical  changes  in  the 
tissues  whence  they  are  always  collected  by  the  blood  and  carried 
to  the  glands  which  preside  over  their  elimination. 

The  next  form  of  cutaneous  glands  are  commonly  arranged 
among  the  excretory  organs,  though  their  more  important  func- 
tion, as  will  hereafter  appear,  is  to  supply  surface  moisture  for 
the  purpose  of  regulating  the  temperature. 

Sudoriferous  Glands. 

The  sweat  glands  are  distributed  all  over  the  cutaneous  surface, 
but  in  some  parts,  such  as  the  axilla,  perineum,  etc.,  they  are 
both  more  abundant  and  larger  than  elsewhere.  They  are  simple 
tubes  extending  in  a  more  or  less  wavy  manner  through  the  skin, 
and  ending  in  a  rounded  knot  formed  of  several  coils  of  the  tube 
some  way  beneath  the  corium,  where  they  are  surrounded  by  a 
capillary  plexus.  The  tube  is  lined  with  glandular  epithelium, 
and  its  basement  membrane  is  beset  with  longitudinally  arranged 
smooth  muscle  fibres. 

The  secretion  of  sweat  is  always  going  on,  though  it  does  not 
constantly  appear  as  a  moisture  on  the  surface,  because  the 
amount  produced  is  only  just  equal  to  the  amount  of  evaporation 
that  takes  place,  In  this  case  it  is  spoken  of  as  insensible  perspi- 
ration. Under  certain  circumstances  the  sweat  collects  on  the 
surface  and  becomes  obvious  as  fluid — sensible  perspiration — which 


384  MANUAL   OF   PPIYSIOLOOy. 

bathes  the  skin,  l)eing  produced  more  rapidly  than  it  can  be 
evaporated.  'J'lie  quantity  of  secretion  necessary  to  become  sen- 
sible varies  with  the  dryness  and  heat  of  the  air,  that  is,  with  the 
rapidity  with  which  evaporation  takes  place.  It  happens,  how- 
ever, that  the  very  circumstances  which  tend  to  assist  evaporation 
also  promote  the  secretion  of  sweat.  Indeed,  the  effect  of  great 
heat  and  dryness  of  the  air  is  to  increase  the  cutaneous  secretion 
more  rapidly  than  they  increase  the  capability  of  evaporation, 
and  therefore  when  the  air  is  hot  and  dry  and  evaporation  is  going 
on  very  actively,  we  have  the  secretion  of  sweat  made  sensible  to 
our  feelings.  When  dampness  is  associated  with  warmth  of  the 
atmosphere  the  sweat  collects  in  large  quantities  on  the  skin,  for 
the  heat,  as  we  shall  see  hereafter,  aids  the  secretion,  and  the 
damp  air  impedes  the  evaporation. 

The  quantity  of  perspiration  given  off  is  considerable,  but  the 
wide  limits  within  which  the  amount  may  vary  render  an  attempt 
to  express  an  average  in  numbers  useless.  The  amount  will  de- 
pend on  (1)  the  temperature  of  the  air,  (2)  the  quantity  and 
quality  of  fluids  imbibed,  (3)  the  amount  of  heat  generated  in  the 
body,  and  it  therefore  varies  directly  with  muscular  exercise. 
The  amount  that  becomes  perceptible  to  our  senses  depends  on 
the  impediments  to  evaporation  that  may  exist,  as  well  as  on  the 
amount  of  fluid  produced. 

The  chemical  composition  of  sweat  varies  with  the  amount  se- 
creted. When  collected  as  a  fluid  by  inclosing  a  part  of  the  body 
in  an  impervious  sac,  it  is  found  to  have  about  2  per  cent,  of  solid 
matters,  the  greater  quantity  of  which  is  made  up  of  inorganic 
salts,  sodium  chloride  being  by  far  the  most  abundant.  It  also 
contains  some  epithelial  debris,  traces  of  neutral  fats,  and  several 
volatile  and  fatty  acids  (butyric,  proprionic,  caproic),  to  which  it 
owes  its  peculiar  smell.  It  is  said  to  contain  urea,  but  this  has 
been  denied,  and  since  all  the  nitrogenous  income  is  accounted 
for  in  the  urea  excreted  by  the  kidneys,  it  is  probable  that  the 
cutaneous  elimination  of  urea  is  minimal,  if  not  exclusively  patho- 
logical. It  is  also  said  to  contain  salts  of  ammonia,  and  it  affords 
a  means  of  escape  to  many  drugs.  In  certain  parts  of  the  body, 
especially  in  some  individuals,  it  contains  a  considerable  amount 


DESQUAMATION.  385 

of  pigments,  varying  in  color  from  brick-red  to  bluish-black, 
which  need  not  be  here  further  described. 

The  effect  of  nervous  influence  on  the  secretion  of  sweat  is  so  as- 
sociated with  the  nervous  mechanisms  of  the  cutaneous  vessels 
that,  under  ordinary  circumstances,  it  is  a  difficult  matter  to  sep- 
arate them.  There  can  be  no  doubt,  however,  that  a  special  ner- 
vous control  is  exerted  over  the  production  of  sweat.  This  appears 
to  be  observable  in  some  diseases,  the  poisons  of  which  variously 
affect  the  two  sets  of  nerves.  Thus,  in  fever,  we  observe  a  dry 
red  skin  accompanied  by  an  increased  supply  of  blood,  and  a  sup- 
pression of  the  secretion  of  the  sweat  glands  ;  whilst  in  certain 
stages  of  acute  rheumatism,  the  exact  opposite  is  seen,  i.e.,  a  pro- 
fuse sweat  drips  from  the  pale  bloodless  skin.  It  has,  moreover, 
been  recently  shown  that  in  some  animals  (cats)  the  stimulation 
of  the  sciatic  nerve,  causing  contraction  of  the  bloodvessels,  pro- 
duces at  the  same  time  a  copious  secretion  of  sweat;  and  a  warm 
atmosphere  is  said  to  have  no  effect  on  the  secretion  of  a  limb  the 
nerve  of  which  has  been  cut,  although  the  warmth  be  so  great  as 
to  make  the  rest  of  the  animal's  body  sweat  profusely. 

The  effect  of  drugs  upon  the  cutaneous  secretion  is  well  known. 
There  is  a  large  group  of  medicines,  especially  pilocarpin,  which 
produce  an  increased  flow,  while  many  others,  notably  atropin, 
have  a  contrary  effect. 

Cutaneous  Desquamation. 

Together  with  cutaneous  excretion  should  be  mentioned  the  con- 
tinuous and  extensive  loss  all  over  the  surface  of  the  body  from 
the  casting  off  of  the  superficial  layers  of  the  dried  horny  cells  of 
which  the  outer  part  of  the  skin  is  composed. 

The  way  in  which  the  cells  of  the  mammary  gland  produce 
their  important  secretion  is  by  their  protoplasm  adopting  a  pecu- 
liar method  of  fat  manufacture,  whilst  all  the  strength  of  its  nu- 
tritive powers  is  devoted  to  the  elaboration  of  the  constituents  of 
milk.  In  a  similar  way  the  cells  of  the  epidermis  devote  their 
nutritive  activity  to  the  production  of  a  certain  material — keratin, 
which  cannot  be  called  a  secretion  in  the  ordinary  acceptation  of 


386  MANUAL   OF   PHYSIOLOGY. 

the  term,  but  which  is  certainly  elaborated  as  the  result  of  the  nu- 
tritive changes  going  on  in  the  protoplasm  of  the  cell  during  its 
life-history,  just  as  we  know  that  many  other  substances  are  pro- 
duced as  the  result  of  the  working  of  gland  cells. 

The  work  of  the  epidermal  cells  supplies — not  a  peculiar  chemi- 
cal reagent,  as  do  some  of  the  gland  cells  of  the  digestive  tract, 
nor  yet  a  nutrient  fluid  like  milk — but  the  exterior  of  the  body 
with  an  insoluble,  impervious,  tough  coating,  which  though  thin 
and  elastic,  is  very  strong  and  resisting. 

The  nearest  analogy  to  the  secretion  of  the  keratin  in  the  epi- 
dermal cells,  is  the  production  of  mucin  in  the  cells  of  the  epithe- 
lial lining  of  the  mucous  membranes.  Both  substances  may  be 
looked  upon  as  excretions,  as  they  never  reenter  the  system  and 
are  cast  off,  but  each  of  them  performs  a  definite  function,  and  is 
produced  by  special  protoplasmic  elements,  like  the  secretions 
more  generally  recognized  as  such. 

The  amount  of  nitrogenous  substances  thus  excreted  cannot 
well  be  reckoned,  but,  having  regard  to  the  great  extent  of 
surface  from  which  they  are  derived,  it  must  be  considerable. 


CHAPTER  XXII. 

URINAKY  EXCEETION. 

The  urine  is  the  most  important  fluid  excretion,  for  by  it,  in 
mammalia,  nearly  all  the  nitrogen  of  the  used-up  proteid  leaves 

Fig.  170, 


^...Mm 


Section  of  Kidney  of  Man. — a.  ('ortical  substance  composed  chiefly  of  con- 
voluted tubules;  the  portions  between  the  medullary  pyramids  form  the 
columns  of  Bertin  (e).  h.  Pyramids  of  medullary  substance,  composed  of 
straight  tubes,  etc.,  radiating  towards  cortex,  to  form  the  pyramids  of  Ferrein. 
d.  Commencement  of  ureter  leading  from  central  sac  or  pelvis,  c.  Papillae 
where  the  tubes  open  into  pelvis.     (Cadiat.) 

the  body  in  the  form  of  urea.     The  construction  of  the  urinary 
glands  requires  the  special  notice  of  the  physiologist. 


388 


MANUAL   OF   PHYSIOLOGY. 


Structure  of  the  Kidneys. 
The  kidueys  may  be  called  complex  tubular  glands,  because  the 
tubes  of  which  they  are  composed  are  made  up  of  a  number  of 


Fig.  171. 


Fio.  172. 


Fig.  17L — Diagram  of  the  Tubules  of  the  Kidney.  (Cadiat.) — a.  Large 
duct  opening  at  papilla.  6  and  c.  Straight  collecting  tubes,  dande.  Looped 
tubule  of  Henle.  /.  Convoluted  tubules  of  cortex,  y.  Capsule  from  which 
the  latter  spring. 

Fig.  172. — Portions  of  various  Tubules  highly  magnified,  showing  the 
relation  of  the  lining  epithelium  to  the  wall  of  the  tube.  (Cadiat.) — a.  Large 
duct  near  the  papilla.  6.  Commencement  of  Henle's  loop.  c.  Thin  part  of 
Henle's  loop. 


RENAL   CIRCULATION.  389 

parts  esseutially  differing  from  one  another  both  in  their  structure 
and  in  their  relation  to  the  bloodvessels. 

The  tubes  begin  by  a  small  rounded  dilatation  (Malpighian 
capsule)  which  is  lined  by  thin  flattened  epithelium.  Opening 
from  this  capsule,  Fig.  171  {g),  is  found  a  tortuous  tubule  (/), 
lined  by  peculiar  large  rod-beset  epithelial  cells,  which  occupy  the 
greater  portion  of  its  diameter.  This  convoluted  tubule  (/)  leads 
into  a  tube  (e)  of  much  less  external  diameter,  but  about  equal 
lumen,  owing  to  the  thinness  of  its  lining  epithelium,  the  cells  of 
which  are  more  flattened  and  much  thinner  than  those  in  the  tor- 
tuous tubes.  This  thin  tube  forms  a  loop  extending  down  into 
the  medullary  pyramid  and  returning  to  the  cortex,  where  it  can 
be  seen  to  become  again  convoluted  (d)  and  then  to  open  into  a 
straight  collecting  tube.     The  collecting  tubes  (e,  b)  receive  many 

Fig.  173. 


Portion  of  Convoluted  Tubule,  showing  peculiar  fibrillated  epithelial  cells. 
(Heidenhain.) 

similar  tributary  tubes  on  their  way  towards  the  apex  of  the  Mal- 
pighian pyramid,  where  they  pour  their  contents  into  the  pelvis 
of  the  kidney.  The  epithelial  lining  of  these  collecting  tubes  is 
of  the  ordinary  cylindrical  type. 

We  thus  find  four  kinds  of  epithelial  cells  in  the  various  parts 
of  the  urinary  tubules,  viz.,  scaly  cells  in  the  capsule;  peculiar 
rod-beset  glandular  cells  in  the  convoluted  tubes ;  flattened  cells 
in  a  great  part  of  the  loop,  and  ordinary  cylindrical  cells  in  the 
large  straight  tubes.  (Figs.  172  and  173.) 

Bloodvessels. 
The  renal  artery,  on  its  way  from  the  hilus  to  the  boundary 
between  the  cortical  and  medullary  portions  of  the  kidney,  breaks 


390 


MANUAL   OF   PHYSIOLOGY. 


up  suddenly  into  numerous  small  branches;  these  vessels  then 
form  arches  which  run  along  the  base  of  the  pyramids.  From 
the  latter,  straight  branches,  called  interlobular  arteries,  pass  to- 
wards the  surface,  and  give  off  lateral  brauchlets  which  form  the 
afferent  vessels  to  the  neighboring  Malpighian  capsules.  Within 
the  capsules  the  afferent  arteries  at  once  break  up  into  a  series  of 
capillary  loops,  forming  a  kind  of  tuft  of  fine  vessels — the  rjlom- 
erulus,  which  fills  the  cavity  at  the  beginning  of  the  tubules,  and 

Fig.  174. 


Glomerulus,  treated  witli  silver  nitrate,  showing  the  endothelium. 

is  only  covered  by  thin  scaly  epithelial  cells,  and  thus  separated 
from  the  urine.  It  is  a  singular  fact  that  in  the  renal  circulation 
the  efferent  vessel  on  leaving  the  glomerulus  does  not,  like  most 
veinlets,  unite  with  others  to  form  a  larger  vein  ;  but  again  breaks 
up  into  capillaries,  which  form  a  dense  meshwork  around  the 
convoluted  tubules.  The  blood  is  thence  conveyed  to  small 
straight  veins  corresponding  to  the  intralobular  arteries. 

Another  striking  peculiarity  of  the  renal  vessels  is  that  a  dis- 
tinct set  of  arteries,  starting  from  the  same  point  as  the  inter- 


URINE.  391 

lobular  (between  the  cortex  and  medulla),  pass  towards  the  centre 
of  the  gland  into  the  pyramids.  They  consist  of  bunches  of 
straight  arterioles,  which  lie  between  the  straight  and  the  looped 
tubules.  Corresponding  with  these  straight  arteries  are  minute 
straight  veins,  which  carry  the  blood  back  to  the  vessels  at  the 
base  of  the  pyramids. 

In  the  kidney,  then,  we  have  three  sets  of  capillary  vessels, 
which  differ  in  their  position,  the  form  of  their  meshes,  and  their 
relation  to  their  parent  artery.  Probably  the  pressure  exerted 
by  the  blood  in  them,  and  the  rapidity  of  its  flow  through  them, 
differ  also : 

1.  The  capillaries  in  the  glomeruli  are  loops  collected  into  a 
tuft  by  their  covering  of  delicate  epithelium.  On  account  of 
their  relation  to  the  afferent  artery  which  ends  abruptly  in  these 
capillaries,  and  to  the  smaller  efferent  vessel  that  leads  to  a  sec- 
ondary plexus  of  capillaries,  the  pressure  within  the  glomerulus 
must  be  very  great  compared  with  that  of  the  general  capillaries 
of  the  body,  and  must  vary  much  with  changes  in  local  blood- 
pressure. 

2.  The  secondary  capillary  plexus,  with  its  narrow  raeshwork 
closely  investing  the  tubules,  can  only  be  under  comparatively 
trifling  pressure  which  varies  but  little,  on  account  of  the  blood 
having  first  to  pass  through  the  capillaries  of  the  glomerulus. 
Their  current  of  blood  must  also  move  slowly,  since  the  bed  of 
the  stream  is  here  very  great. 

3.  The  straight  vessels,  with  long-raeshed  capillaries,  in  the 
pyramids  between  the  looped  and  straight  tubules  are  unlike  the 
two  preceding.  In  these  straight  vessels  the  blood  probably  flows 
with  greater  velocity  than  in  those  around  the  convoluted  tubes  ; 
and  their  blood-pressure  is  less  than  that  in  the  glomeruli,  but 
greater  than  that  in  the  intertubular  capillaries. 

The  Urine. 

When  freshly  voided  the  urine  of  man  in  health  is  a  clear 
straw-colored  fluid,  with  a  peculiar  aromatic  odor.  The  intensity 
of  the  color  varies  with  the  amount  of  solids — the  color  being  a 


392 


MANUAL   OF    PHYSIOLOGY. 


rough  indication  of  the  degree  of  concentration.  On  standing 
and  cooling  a  slight  cloud  of  mucus  often  appears  floating  in  the 
fluid.  This  comes  from  the  lining  membrane  of  the  bladder,  and 
it  usually  entangles  a  few  flattened  epithelial  cells,  which  are  the 
only  organized  structural  elements  found  in  it  in  health. 

Fig.  175. 


Diagram  showing  the  relation  borne  by  the  bloodvessels  to  the  tubules  of 
the  kidney.  The  upper  half  corresponds  to  the  cortical,  the  lower  to  the 
medullary  part  of  the  organ.  The  plain  tubes  are  shown  separately  on  the 
right,  and  the  vessels  on  the  left.  The  darkly  shaded  arteries  send  off 
straight  branches  to  the  pyramid  and  larger  interlobular  branches  to  the 
glomeruli,  the  efferent  vessels  of  which  form  the  plexus  around  the  convo- 
luted tubes. 


The  fresh  urine  has  a  distinctly  acid  reaction.  This  does  not 
depend  upon  the  presence  of  free  acid,  as  is  suggested  by  the  fact 
that  no  precipitate  is  formed  on  the  addition  of  sodium  hyposul- 
phite, but  upon  the  large  amount  of  acid  salts,  particularly  acid 
sodium  phosphate,  which  it  invariably  contains,     A  strictly  veg- 


SECRETION  OF  THE  URINE.  393 

etable  diet  renders  man's  urine  alkaline,  and  it  is  said  to  become 
less  acid  after  meals.  In  the  herbivorous  mammalia  the  urine  is 
normally  alkaline  so  long  as  their  digestion  is  going  on,  but  when 
they  are  deprived  of  food  for  some  time  it  becomes  acid,  showing 
that  the  alkalinity  depends  upon  their  diet. 

The  specific  gravity  of  urine  varies  greatly  at  different  times, 
commonly,  however,  ranging  between  the  figures  1015-1020. 
After  copious  drinking  it  may  go  as  low  as  1003,  and  after  pro- 
longed abstinence  from  liquids,  or  very  active  sweating,  it  may 
attain  1040. 

The  quantity  of  urine  secreted  is  also  very  variable,  that  pro- 
duced by  an  adult  usually  amounting  to  about  2  pints  per  diem 
(1000-1500  c.c).  The  amount  is  increased  by — (1)  elevation  of 
the  general  blood-pressure,  or  the  pressure  in  the  renal  vessels 
from  any  cause  whatever  ;  (2)  contraction  of  the  cutaneous  vessels 
from  cold;  (3)  copious  drinking;  (-4)  excess  of  nitrogenous  diet; 
(5)  the  presence  of  soluble  matter  in  the  blood,  such  as  sugar, 
salt,  etc. ;  and  (6)  the  presence  of  urea,  as  well  as  various  medi- 
caments, has  a  special  action  on  the  renal  secretion,  greatly  in- 
creasing the  amount  of  urine  passed. 

Although  the  quantity  of  urine  differs  so  much  under  different 
circumstances,  the  amount  of  solids  excreted  by  the  kidneys  in 
the  twenty-four  hours  remains  pretty  much  the  same,  being  on  an 
average  over  li  ounces  (50  grammes)  for  an  adult  man. 

From  this  it  is  obvious  that  the  height  of  the  specific  gravity 
must  vary  inversely  with  the  amount  secreted,  so  that  the  more 
scanty  the  urine  the  higher  we  expect  to  find  the  percentage  of 
solids. 

Secretion  of  the  Urine. 

We  have  just  seen  that  the  arterial  twig,  or  afferent  vessel, 
which  enters  the  capsule  of  Malpighi,  breaks  up  into  a  set  of 
capillary  loops,  which  are  only  covered  by  a  single  layer  of  ex- 
tremely thin  epithelial  cells  separating  them  from  the  lumen  of 
the  urinary  tubule,  and  that  the  pressure  in  the  vessels  of  the 
glomerulus  is  habitually  higher  than  that  in  most  capillaries,  and 

33 


394  MANUAL   OF   PHYSIOLOGY. 

constantly  greater  than  that  of  the  second  capillary  network 
around  the  convoluted  tubules. 

The  general  arrangement  of  these  vessels,  and  the  high  pressure 
in  the  glomerulus,  give  the  impression  that  it  is  simply  a  filtering 
apparatus  by  means  of  which  the  fluid  parts  of  the  blood  pass 
into  the  urinary  tubules.  This  view  seems  supported  by  the  fact 
that  the  quantity  of  urine  secreted  bears  a  direct  proportion  to 
the  blood-pressure  in  the  minute  renal  vessels,  whether  the  change 
in  pressure  depends  on  local  vascular  mechanisms  or  on  changes 
in  the  general  blood-pressure. 

Such  a  theory,  however,  cannot  adequately  explain  the  forma- 
tion of  urine,  because  the  urine  differs  so  materially  from  the  fluid 
one  could  obtain  as  a  filtrate  from  the  blood.  In  health  it  con- 
tains no  albumin,  a  substance  in  which  the  blood  is  very  rich  ; 
and  it  has  enormously  more  urea  and  salts  than  the  blood.  There 
is,  therefore,  both  a  quantitative  and  qualitative  difl^erence,  which 
implies  a  distinct  process  of  selection,  and  although  filtration 
cannot  be  altogether  excluded  from  the  process,  it  must  be  com- 
pletely modified  by  other  forces. 

Moreover,  in  the  general  description  of  the  organ  we  have  just 
seen  that  in  a  great  part  of  the  tubules,  both  the  epithelial  and 
vascular  supply  give  the  idea  of  actively  secreting  gland  tubes. 
From  the  mere  construction  of  the  different  portions  of  the  gland 
it  has  been  concluded  that  there  are  two  distinct  departments,  each 
of  which  plays  a  different  part  in  the  production  of  the  urine. 
One  is  a  simple  filtering  mechanism,  and  the  other  a  definitely 
secreting  glandular  tubule. 

It  is  not  surprising  that,  with  such  a  complex  arrangement  as 
the  tubules  above  mentioned,  there  should  exist  diflfereut  views 
as  to  the  exact  mode  in  which  the  urine  is  secreted.  As  these  are 
more  or  less  at  variance  in  their  explanation  of  the  method  of 
secretion,  and  as  it  is  difficult  to  put  any  of  them  aside  as  quite 
erroneous,  it  becomes  necessary  to  enumerate  each  somewhat  in 
detail. 

Feeling  convinced  of  the  filter-like  function  of  the  glomerulus, 
and  recognizing  the  fact  that  some  other  agency  was  also  at  work 
in  the  formation  of  urine.  Bowman  explained  the  process  thus: 


SECRETION   OF  THE   URINE.  396 

From  the  glomerulus  the  watery  parts  of  the  fluid  are  filtered, 
while  the  glandular  epithelium  selects  the  important  solid  constit- 
uents which  it  is  necessary  to  remove  from  the  blood. 

Ludwig  takes  a  different  view.  He  believes  that  the  watery 
part  of  the  plasma,  bearing  with  it  the  salts,  etc.,  is  filtered  from 
the  glomerulus.  As  this  fluid  passes  through  the  tortuous  urinary 
tubules,  a  large  portion  of  the  water  is  reabsorbed  into  the  capil- 
lary networks  surrounding  them.  This  reabsorption  is  assisted 
by  the  high  specific  gravity  of  the  blood  and  the  low  pressure  in 
these  capillaries  as  compared  with  the  glomeruli,  where  the  fil- 
tration of  the  liquid  occurs.  The  role  of  the  epithelium  is  not 
then  selection  from  the  blood  of  specific  materials,  but  possibly 
the  prevention  of  the  return  of  the  solids  with  the  water  back  to 
the  bloodvessels. 

Heidenhain  attempted  to  settle  the  question  as  to  the  function 
of  the  renal  epithelium,  by  introducing  into  the  blood  a  blue 
coloring  matter — pure  sodium  sulphindigotate — which  he  found 
to  be  eliminated  by  the  kidneys,  giving  rise  to  blue  urine.  On 
examining  the  organ  with  the  microscope  at  a  suitable  time  after 
the  injection  of  the  color  into  the  blood,  the  tubules  are  found  to 
be  filled  with  the  pigment,  and  in  some  cases  the  peculiar  epi- 
thelium of  the  convoluted  tubules  is  stained  with  the  blue  sub- 
stance, while  the  glomerulus  and  capsule  are  entirely  free  from 
the  color.  If  the  stream  of  fluid  from  the  glomerulus  be  stopped 
in  any  way — tying  the  ureter,  section  of  the  spinal  cord,  or  local 
destruction  of  the  glomeruli — the  blue  color  is  only  to  be  found 
in  the  convoluted  tubes  and  their  epithelium,  and  hence  it  has 
been  concluded  that  its  presence  in  the  looped  and  collecting  tubes 
of  the  kidneys  and  urinary  bladder,  depends  upon  its  being  washed 
out  of  the  convoluted  tubes  by  the  stream  of  fluid  filtered  from 
the  blood  at  the  glomerulus. 

The  following  facts  may  also  be  adduced  in  further  support  of 
the  view  that  the  glandular  epithelium  bears  no  mean  share  in 
the  removal  of  the  more  important  solid  constituents  of  the  urine. 

The  epithelium  in  the  tubules  of  the  kidney  of  birds  is  found 
impregnated  with  acid  urate  of  potassium,  which  insoluble  sub- 
stance forms  the  chief  constituent  of  the  solid  urine  of  birds. 


396  MANUAL  OF   PHYSIOLOGY'. 

The  amount  of  liquid  passing  out.  at  the  kidneys  is  in  direct 
proportion  to  the  blood-pressure,  whereas  the  excretion  of  the 
specific  constituents  of  urine  is  independent  of  the  pressure,  but  is 
related  to  the  amount  existing  in  the  blood,  and  the  condition  of 
the  epithelium.  This  is  shown  by  the  increased  elimination  of 
urea  when  that  substance  is  artificially  introduced  into  the  circu- 
lation, even  after  the  flow  of  the  fluid  has  been  checked  by  sec- 
tion of  the  spinal  cord. 

Another  view  has  been  put  forward,  which,  with  some  modifi- 
cation, appears  plausible,  or  at  least  worthy  of  mention.  Paying 
attention  to  the  fact  that  where  vascular  filtration — i.e.,  the  pas- 
sage of  liquid  under  pressure  through  the  capillary  wall — occurs 
elsewhere  in  the  body  it  is  not  only  water  and  salts,  but  plasma 
that  passes  out  of  the  vessels  into  the  interstices  of  the  tissues,  we 
may  then  assume  that  the  fluid  part  of  the  blood,  as  such,  and 
not  merely  its  watery  part,  escapes  at  the  glomerulus.  That  is  to 
say,  the  solid  ingredients  of  the  urine  in  a  diluted  form,  plus 
serum-albumin,  pass  into  the  tubules.  But  on  its  way  down  the 
long  and  circuitous  route  through  the  tubules  the  albumin  with 
much  water  is  reabsorbed  by  the  capillaries  of  the  convoluted 
tubes.  The  first  step  in  this  case  is  a  mechanical  filtration  ;  the 
second  is  a  vital  process  of  reabsorption  of  a  solution  of  serum- 
albumin  carried  on  by  the  gland  cells  in  the  tubules,  aided  by  the 
low  pressure  in  the  peri-tubular  capillary  plexus.  This  view 
seems  supported  by  pathological  experience,  which  teaches  that 
the  removal  of  the  epithelium  of  the  tubes  (the  glomeruli  remain- 
ing perfect),  is  followed  by  the  appearance  of  albumin  in  the 
urine,  and  cysts  formed  by  the  destruction  of  the  epithelium  and 
occlusion  of  the  tubules  commonly  contain  a  fluid  somewhat  like 
plasma. 

Doubtless  much  remains  to  be  found  out  as  to  the  exact  method 
of  secretion  of  the  urine,  and  possibly  future  research  may  show 
us  that  all  the  views  here  enumerated  have  some  truth  in  them. 
That  a  filtration,  not  mere  osmosis,  takes  place,  is  made  certain 
by  the  special  vascular  mechanism  of  the  glomerulus.  Why 
simply  water  and  salts  without  albumin  should  pass  through 
the  capillaries  of  the  glomerulus,  and  not  through  any  other 


CHEMICAL   COMPOSITION    OF    URINE.  397 

capillaries,  is  not  sufficiently  explained  to  make  it  sure  that  this 
filtration  differs  from  others.  That  the  glandular  epithelium  does 
take  an  active  part  in  the  elimination  of  the  urea  is  rendered 
almost  indisputable  from  the  researches  of  Heidenhain.  And  yet 
there  remain  other  parts,  e.g.,  the  loops  of  Heule,  which  are  con- 
stantly found  in  the  kidney,  and  have  a  special  vascular  mechan- 
ism, and  to  which  none  of  the  foregoing  theories  assign  any  spe- 
cial or  peculiar  function. 

From  the  foregoing  evidence  we  may  fairly  suppose  that  most 
of  the  urea,  and  possibly  some  other  solid  constituents  of  the 
urine,  are  selected  from  the  blood  by  the  epithelial  cells  of  the 
convoluted  tubules,  that  the  fluid  part  of  the  blood  escapes  at 
the  glomerulus,  and  flows  along  the  varied  and  circuitous  route 
of  the  tubules,  carrying  with  it  the  matters  poured  into  the  tubes 
by  the  cells,  and  that  in  some  part  of  the  tubules  the  dilute 
filtrate  loses  much  of  its  water  and  all  its  albumin. 

Chemical  Composition  of  Urine. 

The  percentage  of  the  various  materials  in  urine  varies  as  the 
secretion  differs  in  strength,  as  mentioned,  but  on  an  average 
it  may  be  said  to  contain  about  4  per  cent,  of  solids  and  96  per 
cent,  water. 

The  following  are  the  more  important  solid  matters: 
Urea  is  the  most  important,  and  at  the  same  time  most  abundant 
solid  constituent,  commonly  forming  about  2  per  cent,  of  the  urine. 
It  is  regarded  as  the  chief  end-product  of  the  oxidation  of  the 
nitrogenous  matter  in  the  body,  so  that  the  amount  excreted  per 
diem  gives  us  the  best  estimate  of  the  amount  of  chemical  change 
taking  piace  in  the  tissues.  It  is  readily  soluble  in  alcohol  and 
water,  but  insoluble  in  ether.  It  forms  acicular  crystals  with  a 
silky  lustre.     From  a  chemical  point  of  view  it  may  be  regarded 

as  the  diamide  of  carbonic  acid,  with  the  formula  CO  -I  ^rx^  or 
H,^  y  N,^.  It  is  isomeric  with  ammonium  cyanate  ^jr  [  0,  from 
which  it  was  first  prepared  artificially.     It  is  also  isomeric  with 


398  MANUAI.   OF   PHYSIOLOGY. 

the  amide  of  carbamic  acid,  with  which  it  is  considered  by  some 
to  be  identical. 

On  exposure  to  the  air  bacteria  develop  in  the  urine,  and,  act- 
ing as  a  ferment,  change  the  urea  into  ammonium  carbonate,  two 
molecules  of  water  being  at  the  same  time  taken  up,  thus  : 

CO(NH,),+  2H,0  =  (NH,),C03. 

This  gives  rise  to  a  change  in  the  reaction  of  the  urine,  which 
after  a  time  becomes  increasingly  alkaline,  and  the  change  is 
commonly  spoken  of  as  the  alkaline  fermentation  of  the  urine. 
This  change  is  extremely  slow  in  solutions  of  pure  urea,  which 
do  not  support  bacterial  life. 

With  nitric  and  oxalic  acids  urea  forms  sparingly  soluble 
salts — a  fact  made  use  of  in  its  preparation  from  urine. 

The  amount  of  urea  eliminated  in  the  twenty-four  hours  is 
about  500  grains  (35  grammes).  The  amount  varies  (1.)  in  some 
degree  with  the  amount  of  urine  secreted  ;  an  increase  in  the 
amount  of  water  being  accompanied  by  a  slight  increase  in  the 
urea  eliminated.  Some  materials,  such  as  common  salt,  increase 
the  water,  and  thereby  also  increase  the  urea.  (2.)  The  character 
and  quantity  of  the  diet  influences  most  remarkably  the  quantity 
of  urea  given  off,  the  amount  increasing  in  direct  proportion  to 
the  quantity  of  proteid  consumed.  Fasting  causes  a  rapid  fall  in 
the  amount  of  urea ;  even  in  the  later  days  of  starvation  it  con- 
tinues to  fall,  but  very  slowly.  (3.)  The  amount  differs  with  age, 
beinof  relatively  greater  in  childhood  than  in  the  adult  (about  half 
as  much  again  in  proportion  to  the  body  weight).  (4.)  Many  dis- 
eases have  a  marked  influence  on  the  amount  of  urea.  In  most 
febrile  affections  it  increases  with  the  intensity  of  the  fever,  while 
in  diseases  of  the  liver  it  often  notably  decreases. 

In  diabetes,  if  the  consumption  of  food  be  very  great,  the  daily 
excretion  of  urea  may  reach  nearly  4  oz.  (100  grammes),  or 
three  times  as  much  as  normal. 

Preparation. — To  obtain  urea  from  human  urine  it  is  evapo- 
rated to  one-sixth  of  its  bulk,  an  excess  of  nitric  acid  is  added, 
and  it  is  left  to  stand  in  a  cool  place.  Impure  nitrate  of  urea 
separates  from  the  fluid  as  a  yellow  crystallized  precipitate.    This 


URIC    ACID,    ETC.  399 

insoluble  salt  is  caught  ou  a  filter,  dried,  dissolved  in  boiling 
water,  mixed  with  animal  charcoal  to  remove  the  coloring  mat- 
ter, and  filtered  while  hot;  when  the  filtrate  cools,  colorless  crys- 
tals of  nitrate  of  urea  are  deposited.  The  precipitate  is  dissolved 
in  boiling  water,  and  barium  carbonate  added  as  long  as  effer- 
vescence takes  place,  barium  nitrate  and  urea  being  produced. 
This  is  evaporated  to  dryness,  and  the  urea  extracted  with  abso- 
lute alcohol,  which  on  evaporation  leaves  crystals  of  pure  urea. 

Esiimation. — Urea  can  be  estimated  volumetrically  by  the 
method  of  Liebig,  which  depends  on  the  power  of  mercuric  nitrate 
to  give  a  precipitate  with  it.  The  sulphates  and  phosphates  must 
be  first  removed  by  the  addition  of  40  c.c.  of  a  mixture  of  1 
volume  saturated  barium  nitrate  and  2  volumes  saturated  solu- 
tion of  caustic  baryta,  to  an  equal  volume  of  urine.  This  is 
filtered,  and  from  the  filtrate  an  amount  corresponding  to  10  c.c. 
urine  is  taken.  Into  this  known  volume  of  urine  a  standard  so- 
lution of  mercuric  nitrate  (of  which  1  c.c.  corresponds  to  1  cen- 
tigramme of  urea)  is  dropped  until  a  sample  drop  of  the  fluid, 
mingled  on  a  watch-glass  with  a  drop  of  concentrated  sodium 
carbonate  solution,  gives  a  yellow  color,  which  indicates  that  some 
free  mercuric  nitrate  I'cmains.  For  every  cubic  centimetre  of  the 
standard  mercuric  solution  used  there  will  be  1  centigramme  of 
urea  in  the  sample  of  urine;  a  small  reduction  has  to  be  made 
for  the  chlorides,  which  are  present  in  tolerably  constant 
amount. 

Another  simple  method  consists  in  mixing  together  known 
(juantities  of  urine  and  sodium  hypobromite  (NaBrO)  with  ex- 
cess of  caustic  soda.  The  urea  is  decomposed  in  the  presence  of 
this  salt,  and  free  nitrogen  evolved  : 

CON.H,  +  3(NaBrO)  +  2('NaOH)  =  3NaBr  -f  Na,C03  -f 
3H,0  -f  2N. 

The  quantity  of  urea  may  be  determined  by  ascertaining  the 
volume  of  nitrogen,  which  can  be  measured  directly  in  a  grad- 
uated tube. 

Uric  acid,  of  which  the  formula  is  C^H^N^O.,  or  C^H.^OiCNH.- 
CN)j,  is  present  only  in  extremely  small  quantities  in  the  normal 


400  MANUAL   OF   PHYSIOLOGY. 

urine  of  mammalia,  but  in  birds,  reptiles,  and  insects  it  forms 
the  chief  ingredient  of  the  renal  secretion.  It  is  sparingly  soluble 
in  water,  and  insoluble  in  alcohol  and  ether.  However,  in  solu- 
tions of  the  neutral  phosphates  and  carbonates  of  the  alkalies 
it  combines  with  some  of  the  base  so  as  to  form  acid  salts,  and 
at  the  same  time  converts  the  neutral  into  acid  phosphates,  to 
which,  as  has  been  already  stated,  the  urine  owes  its  acid  reac- 
tion. These  salts  are  more  soluble  in  warm  than  in  cold  water, 
and  hence  generally  fall  as  a  sediment  when  the  urine  cools. 
Uric  acid  is  readily  converted  into  urea  by  oxidation,  and  is 
probably  one  of  the  steps  in  the  formation  of  urea  which  com- 
monly occurs  in  the  body  during  the  gradual  oxidation  of  the 
proteid  bodies. 

The  presence  of  uric  acid  may  be  recognized  by  the  murexide 
test.  The  substance  to  be  tested  is  gently  heated  in  a  flat  cap- 
sule with  some  nitric  acid.  A  decomposition  occurs,  N  and  CO, 
going  off,  urea  and  alloxan  remaining  as  a  layer  of  yellow  fluid. 
If  this  be  cautiously  evaporated,  and  a  drop  of  ammonia  added, 
a  striking  purple-red  color  is  produced,  which  the  addition  of 
potash  turns  blue. 

The  amount  of  uric  acid  normally  follows  pretty  closely  the 
variations  in  urea,  but  is  usually  only  about  8  grains  (.5  gramme) 
per  diem.  In  certain  diseases  the  quantity  may  be  much  in- 
creased. For  the  quantitative  estimation,  which  is  seldom  de- 
cided by  the  practitioner,  the  student  must  consult  the  text-books 
of  physiological  chemistry. 

Kreatinin  (C^H^NgO)  is  always  present  in  urine,  probably  being 
formed  from  kreatin  by  the  loss  of  one  molecule  of  water.  About 
15  grains  (1  gramme)  is  excreted  per  diem. 

Xanthin  (CgH^N^Oj)  also  occurs  in  urine,  but  in  extremely  small 
quantities. 

Hippuric  acid  (CgHjNOj)  is  a  normal  constituent  of  human 
urine,  occurring,  however,  in  very  small  quantities.  On  the  other 
hand,  it  is  one  of  the  most  important  nitrogenous  constituents  of 
the  urine  of  the  herbivora,  where  it  takes  the  place  of  uric  acid. 
Its  presence  depends  on  the  existence  of  certain  ingredients  (ben- 
zoic acid,  etc.)  in  the  food,  which  are  capable  of  combining  with 


COLORING    MATTERS    OF    URINE.  401 

glycin,  and  forming  a  conjugated  acid,  a  molecule  of  water  being 
formed  at  the  same  time,  thus  : 

Benzoic  acid.         Glveiii.  Ilippuricacid.     Water. 

The  amount  of  hippuric  acid  increases  with  increased  consump- 
tion of  vegetable  food,  in  the  cellulose  of  which  the  materials 
exist  that  are  required  for  its  formation.  It  is  in  the  liver  that 
the  union  between  the  glycin  and  the  benzoic  acid  takes  place,  as 
is  proved  by  the  removal  of  that  organ,  when  benzoic  acid  in- 
jected into  the  portal  vein  appears  unchanged  in  the  urine. 

Oxalic  acid  (C^HjOJ  occurs  often,  but  not  constantly,  in  the 
urine.  It  is  generally  united  with  lime.  It  is  said  to  appear  in 
greater  quantity,  together  with  an  excess  of  uric  acid,  after  meals, 
and  therefore  to  be  related  to  the  production  of  the  latter  in  the 
body  ;  but  it  probably  is  chiefly  derived  from  oxalates  being  con- 
tained in  some  material  taken  with  the  food. 

Coloring  Matters. 

It  appears  probable  that  the  color  of  the  urine  depends  on  the 
presence  of  small  quantities  of  distinct  substances  which  have 
different  origins  in  the  body.  Three  such  have  been  described, 
and  may  be  taken  provisionally  to  represent  our  knowledge  of 
the  subject : 

1.  Urobilin,  which  is  an  outcome  of  the  coloring  matter  of  the 
bile,  and  therefore  a  remote  derivative  of  the  coloring  matter  of 
the  blood,  is  frequently  present  in  the  urine.  It  is  probably  the 
same  as  hydrobilirubin,  some  of  which  is  occasionally  absorbed 
from  the  intestinal  tract  and  eliminated  by  the  kidneys. 

2.  Urochrom  is  said  to  be  the  special  pigment  of  the  urine.  It 
oxidizes  on  exposure,  forming  a  reddish  substance  that  gives  the 
dark  color  to  some  urinary  sediments  {Uroerythrin). 

3.  A  certain  material  (Indican)  capable  of  producing  Indigo, 
is  commonly  present  in  the  urine  of  man,  and  in  greater  quantity 
in  that  of  some  animals,  particularly  the  horse.  It  is  supposed 
to  be  formed  from  the  indol   that  arise.s  from  the  putrefactive 

34 


402  MANUAL   OF   PHYSIOLOGY. 

changes. consequent  on  the  pancreatic  digestion.  The  indol  is 
absorbed  and  unites  with  sulphuric  acid  to  form  Indican,  which 
is  a  yellow  c^ubstancc.  Under  certaiu  conditions  it  can  be  con- 
verted by  oxidation  into  indigo-blue. 

Inorganic  Salts. 

The  urine  is  the  great  outlet  for  all  inorganic  salts.  The  most 
important  of  these  are  : 

Comvxon  salt  (NaCl),  of  which  a  very  variable,  but  always 
considerable  amount  passes  away  in  the  urine.  The  average 
quantity  excreted  per  diem  may  be  said  to  be  about  half  an  ounce 
(15  grammes).  It  depends  greatly  on  the  quantity  taken  with  the 
food,  and  falls  during  starvation,  but  does  not  completely  disap- 
pear. It  is  said  that  if  absolutely  no  sodium  chloride  be  taken 
with  the  food  the  quantity  excreted  diminishes  greatly,  and  that 
albun)in  appears  in  the  urine  about  the  third  day.  The  amount 
of  salt  eliminated  follows,  with  striking  accuracy,  the  changes 
that  take  place  at  different  times  and  under  different  circum- 
stances, in  the  quantity  of  urea  excreted.  These  facts  seem  to 
indicate  that  there  is  some  relationship  between  the  secretion  of 
the  two  bodies,  or  that  sodium  chloride  participates  in  the 
chemical  changes  of  the  nitrogenous  tissues.  In  many  diseases 
there  occur  variations  in  thequantity  of  common  salt  in  the  urine, 
which  can  hardly  be  explained  by  the  change  in,  or  absence  of 
food. 

Phosphates. — About  60  grains  (3  to  4  grammes)  of  phosphoric 
acid  is  excreted  daily  in  the  urine,  being  combined  with  alkalies 
to  form  salts,  viz.,  potassium,  sodium,  calcium,  and  magnesium 
phosphates. 

Sulphates. — Nearly  40  grains  (2  to  3  grammes)  of  sulphuric 
acid,  as  sulphates  of  alkalies,  are  daily  got  rid  of  in  the  urine. 
The  acid  comes  partly  from  the  food,  but  chiefly  from  the  oxida- 
tion of  the  sulphur  contained  in  the  proteids  of  the  tissues. 

A  considerable  quantity  of  potassium,  sodium,  calcium,  and 
magnesium,  combined  as  already  mentioned,  or  with  chlorine,  is 
contained  in  the  urine. 

Small  traces  oiiron  are  also  always  present  in  the  urine. 


ABNORMAL   CONSTITUENTS   OP   THE   URINE.  403 

Gases. — The  uriue  also  contains  free  CO,,  N,  and  some  0.  100 
volumes  of  gas  pumped  out  of  fresh  urine  have  been  found  to 
consist  of — 

CO,  =  05.40  per  cent. 
N  =31.8(5       " 
O  =    2.74       " 

Abnormal  Constituents. 

Different  kinds  of  substances  occur  in  urine  under  circumstances 
of  special  physiological  interest,  and  therefore  may  be  here  enu- 
merated, although  their  accurate  study  belongs  rather  to  pathol- 
ogy.    First  amongst  these  to  be  named  is — 

Albumin,  which  occurs  from  (1)  any  great  increase  in  the  blood- 
pressure  in  the  renal  vessels,  whether  caused  by  increased  inflow 
or  impeded  outflow.  (2.)  Excess  of  albumin  in  the  blood,  and, 
strange  to  say,  some  forms  of  albumin  escape  much  more  readily 
than  others.  Thus  egg  albumin,  globulin,  or  peptone,  if  intro- 
duced artificially  into  the  blood,  is  soon  found  in  the  urine. 
(3.)  A  watery  condition  of  the  blood,  such  as  would  give  rise 
to  oedema  elsewhere.  (4.)  Total  abstinence  from  NaCl  for  some 
time.  (5.)  Extensive  destruction  of  the  epithelium  of  the  urinary 
tubes. 

Next  in  importance  to  albumin  are  the  following: 

Grape  sugar;  of  which  normally  only  the  merest  trace  occurs 
in  the  urine  although  there  is  always  a  certain  quantity  in  the 
blood.  It  is  present  in  large  quantities  in  (1)  the  disease  known 
as  diabetes,  when  a  great  quantity  of  pale  urine  with  a  very  high 
specific  gravity  is  passed.  (2.)  After  injury  of  a  certain  part  of 
the  floor  of  the  fourth  ventricle  of  the  brain.  (3.)  After  poisoning 
by  curara,  carbonic  oxide,  and  nitrate  of  amyl.  In  short  any 
disturbance  of  the  circulation  of  the  liver  gives  rise  to  an  increase 
of  sugar  in  the  blood,  and  when  the  amount  reaches  6  per  cent, 
it  appears  in  the  urine. 

Bile  Acids  and  Pi^Hie?i<s  appear  in  the  urine  when,  from  occlu- 
sion of  the  bile  ducts,  they  find  their  way  into  the  blood. 

Lencin  and  Ti/rosin  also  occur  in  the  urine,  l)ut  only  after  pro- 
found interference  with  the  function  of  the  liver. 


404  MANUAL    OF    PHYSIOLOGY. 

The  urine  undergoes  important  changes  after  being  voided,  the 
explanation  of  which  is  of  much  interest  to  the  practitioner,  and 
must  be  understood  by  the  student  of  medicine.  (1.)  Com- 
monly enough  the  urine  loses  its  transparency  as  soon  as  it  gets 
cold,  though  perfectly  clear  when  passed,  or  when  again  healed 
to  the  body-temperature,  for  the  urates  are  soluble  in  warm  but 
almost  insoluble  in  cold  water.  This  "muddiness,"  which  soon 
settles  down,  as  a  more  or  less  brightly  colored  sediment,  is 
chiefly  caused  by  the  precipitation  of  acid  sodium  urate,  stained 
with  a  coloring  matter  derived  from  the  urochrorae.  When 
this  occurs  the  urine  will  always  be  found  to  be  distinctly  acid, 
and  if  it  be  left  standing  for  some  time  in  a  cool  place,  the  acid- 
ity will  be  found  to  increase,  owing  to  the  presence  of  a  peculiar 
fungus  which  sets  up  acid  fermoitation.  This  is  said  to  depend  on 
the  formation  of  lactic  and  acetic  acids,  and  crystals  of  uric  acid, 
amorphous  sodium  urate,  and  crystals  of  lime  oxalate  are  depos- 
ited. 

After  a  certain  time  (which  is  shorter  when  the  urine  is  not 
very  acid  and  is  exposed  to  a  warm  atmosphere)  the  development 
of  bacteria  occurs  in  it,  and  causes  the  urea  to  unite  with  water, 
and  to  change  in  the  manner  already  mentioned  (p.  398)  into 
ammonium  carbonate.  This  gradually  neutralizes  the  acidity, 
and  tiually  renders  the  urine  alkaline.  At  the  same  time  an 
amorphous  precipitate  of  lime  phosphate  appears,  and  crystals  of 
ammouio-raagnesium  phosphate  and  of  ammonium  urate  are  pro- 
duced. 

Urinary  Calculi. 

Various  ingredients  of  the  urine,  which  are  difficult  of  solution, 
sometimes  become  massed  together  as  concretions,  particularly  if 
there  exist  any  small  foreign  body  in  the  bladder  which  by  act- 
ing as  a  nucleus  lays  the  foundation  of  a  stone.  Sometimes  small 
concretions  are  formed  in  the  tubes  or  pelvic  recesses  of  the  kidney, 
and,  when  these  make  their  way  into  the  bladder,  they  com- 
monly grow  larger  and  larger.  The  structure  and  composition  of 
a  calculus  often  give  the  history  of  its  own  transit  from  the  kidney, 
and  also  of  various  changes  in  the  metabolism  of  the  individual, 


ORIGIN    OF   UREA.  405 

for  successive  layers  of  different  substances  are  generally  found 
in  a  stone  that  has  attained  any  great  size.  The  chief  materials 
found  in  calculi  are — uric  acid,  ammonium  urate,  calcium  oxa- 
late and  carbonate,  amraonio-raagnesium  phosphate,  etc. 

Source  of  Urea,  etc. 

The  question  as  to  whether  the  chief  materials  of  the  urine  pre- 
exist in  the  blood  and  are  therefore  merely  removed  by  the  kid- 
ney, or  are  manufactured  by  the  special  powers  of  the  renal  cell, 
has  been  widely  discussed,  and  though  the  great  weight  of  evidence 
is  in  favor  of  the  former  view,  some  of  the  experimental  results  on 
the  subject  are  rather  conflicting. 

The  following  are  the  more  important  points  in  the  argument : 

1.  The  blood  does  normally  contain  most  of  the  important  sub- 
stances found  in  the  urine;  so  they  need  not  necessarily  be  made 
in  the  kidney. 

2.  The  blood  in  the  vessel  leading  to  the  kidney — the  renal 
artery — is  said  to  contain  more  urea  than  the  vessel  leading  from 
it — the  renal  vein — so  that  the  blood  appears  to  lose  urea  in  pass- 
ing through  the  kidney. 

3.  If  the  ureters  be  tied  and  the  elimination  be  thus  prevented, 
urea  accumulates  in  the  blood.  This  can  hardly  be  made  by  the 
kidney,  because — 

4.  If  the  renal  arteries  be  tied  so  that  no  blood  goes  to  the  kid- 
neys to  effect  the  elaboration  of  urea  in  those  organs,  then  the  same 
accumulation  results,  showing  that  the  kidneys  are  certainly  not 
the  only  organs  where  urea  is  made. 

5.  Extirpation  of  the  kidneys  also  gives  rise  to  a  great  increase 
of  the  urea  in  the  blood.  The  amount  of  urea  in  the  blood 
after  nephrotomy  is  said  to  increase  steadily  with  the  time  which 
elapses  after  the  operation,  and  the  amount  accumulated  corre- 
sponds to  the  amount  that  would  normally  have  been  excreted  in 
the  same  time,  had  the  animal  not  been  operated  upon. 

6.  Lastly.  In  some  diseases  which  greatly  interfere  with,  or 
quite  suppress  the  secretion  of  the  kidneys,  an  accumulation  in  the 
blood  of  certain  poisonous  or  injurious  materials  takes  place,  and 


406  MANUAL   OF   PMYSIOLOCJY. 

gives  rise  to  the  gravest  symptoms  called  ursemic  p-)isoning,  which 
closely  coincide  with  those  observed  in  experimental  annihilation 
of  the  renal  function. 

From  the  foregoing  it  would  appear  to  be  satisfactorily  settled 
that  the  urea,  which  is  by  far  the  most  important  ingredient  of 
the  secretion  of  the  kidney,  is  probably  made  elsewhere  and  not 
in  that  organ,  whose  duty  seems  to  be  chiefly  to  remove  it  from 
the  blood.  This  is  most  probably  also  true  of  all  the  other  organic 
constituents  of  the  urine.  The  question  then  arises  :  Where  is  the 
urea  formed? 

AVe  naturally  turn  for  an  answer  to  the  most  wide-spread  and 
most  actively  changing  nitrogenous  tissue,  namely  muscle.  Here, 
however,  we  find  no  response,  for  neither  does  muscle  contain 
much  urea,  nor  does  any  very  active  muscular  work  perceptibly 
increase  the  general  urea  elimination.  In  muscle,  however,  a 
material  closely  allied  to  and  readily  convertible  into  urea,  namely 
kreatin,  occurs,  and  it  has  been  suggested  that  this  substance  is 
changed  into  urea  in  the  kidney.  This  cannot  explain  the  origin 
of  all  the  urea  which  appears  in  the  urine,  for,as  already  remarked, 
the  urea  excretion  does  not  correspond  with  the  muscle  meta- 
bolism. 

Without  for  one  moment  doubting  that  some,  probably  a  con- 
siderable quantity  of  urea  comes  from  muscle,  which  forms  so 
large  a  part  of  our  bodies,  we  conclude  that  there  must  be  and 
assuredly  are  many  other  sources  of  urea,  as  there  are  many  other 
parts  or  organs  where  nitrogenous  textures  are  undergoing  chem- 
ical changes  and  gradual  waste. 

One  source  of  urea — the  liver,  is  specially  worthy  of  note,  since 
it  helps  to  explain  the  striking  relation  between  the  amount  of 
albuminous  food  and  the  quantity  of  urea  eliminated,  the  latter 
following  immediately  and  running  parallel  with  the  former. 
There  can  be  no  doubt  that  most  people  consume  much  more  al- 
buminous food  than  is  necessary  for  the  adequate  nutrition  and 
preservation  of  the  nitrogenous  tissues,  and  therefore  must  have 
a  surplus  of  nitrogenous  material  in  their  bodies.  It  may  be  re- 
membered, as  was  pointed  out  in  the  chapter  on  digestion,  that  in 
all  parts  of  the  alimentary  tract  there  is  a  limit  to  the  absorption 


NERVOUS    MECHANISM    OF   URINARY   SECRETION.         4U7 

of  peptones,  aud  that  in  the  small  intestine  when  delay  in  absorp- 
tion occurs  the  decomposition  of  peptones  results,  because  in  pro- 
longed pancreatic  digestion  these  peptones  are  changed  into  leuciu 
(CgHj^NO,,)  aud  tyrosin  (CgHjiNOj)  and  as  such  pass  into  the 
portal  circulation  to  be  borne  to  the  liver.  In  the  liver  it  is 
highly  probable  that  these  bodies  are  converted  into  urea,  for, 
when  they  are  introduced  into  the  intestinal  tract,  they  are  ab- 
sorbed and  an  excess  of  urea  appears 'in  the  urine.  Thus  the  ex- 
cessive part  of  the  proteid  food,  before  it  really  enters  the  system, 
is  broken  up  in  the  intestine  into  bodies  which,  notwithstanding 
the  chemical  difficulty  of  explaining  the  process,  may  be  regarded 
as  a  step  towards  the  formation  of  urea. 

Nervous  Mechanism  of  the  Urinary  Secretion. 

With  regard  to  the  influence  exerted  by  the  nervous  system  on 
the  renal  secretion,  we  have  but  little  satisfactory  information, 
although  there  can  be  no  doubt  that  here,  as  in  other  glands,  the 
process  is  under  the  control  of  the  nerves.  Many  of  the  circum- 
stances which  cause  greater  activity  of  secretion,  such  as  taking 
large  quantities  of  water,  etc.,  have  no  effect  on  the  general  blood 
pressure,  so  that,  if  the  increased  flow  be  brought  about  by  the 
vaso-motor  mechanisms,  it  must  be  by  means  of  nervous  channels 
altering  the  blood  flow  in  the  special  arteries  of  the  glands.  We 
know  further  that  emotional  conditions,  such  as  hysteria,  exist  in 
which  an  unaccountably  great  quantity  of  urine  of  very  low  spe- 
cific gravity  is  evacuated. 

With  regard  to  the  effects  of  the  vaso-motor  nerves,  we  know 
that  section  of  all  the  nervous  twigs  going  to  the  kidneys  causes 
great  congestion  and  an  immense  increase  in  the  secretion,  which 
commonly  contains  albumin.  This  no  doubt  depends  on  the  sud- 
den rise  in  pressure  in  the  glomeruli,  owing  to  the  dilatation  of 
the  arterioles.  If  the  splanchnics,  in  which  the  renal  vaso-motor 
nerves  run,  be  cut,  a  great  quantity  of  urine  is  produced  from  the 
same  cause — vaso-motor  paralysis — but,  on  account  of  the  large 
area  of  vessels  injured,  the  general  blood-pressure  falls,  and  there- 
fore the  effect  is  not  so  much  marked.  If  the  peripheral  end  of 
the  cut  nerves  be  stimulated,  the  secretion  is  diminished,  and, 


408  MANUAL   OF    PUYSIOLOOV. 

owing  to  spasm  of  tlie  renal  arterioles  and  fall  of  blood-pressure 
in  the  glomerular  capillaries,  may  be  brought  to  a  standstill. 
Section  of  the  spinal  cord  at  the  7th  cervical  vertebra  stops. the 
flow,  because  it  so  reduces  the  general  blood-pressure  that  the 
pressure  in  the  renal  vessels  falls  below  that  necessary  for  the  fil- 
tration of  the  urine. 

Passage  of  the  Urine  to  the  Bladder. 

The  pressure  exerted  by  the  blood  in  the  glomerular  capillaries 
is  quite  sufficient  to  make  the  urine  flow  from  the  pelvis  of  the 
kidneys  into  the  bladder,  because  when  the  ureters  are  tied  they 
become  distended  above  the  ligature  by  the  urine  flowing  from 
the  pelvis,  where  a  pressure  may  be  produced  of  some  forty  milli- 
metres of  Hg,  at  which  pressure  the  secretion  stops  and  becomes 
somewhat  changed  in  chemical  composition  (kreatin  appearing  in 
greater  quantity). 

Normally,  however,  the  passage  of  the  urine  along  the  ureters 
is  accomplished  by  the  peristaltic  motion  of  the  ducts,  which  goes 
on  alternately  in  the  two  ureters,  so  that  the  urine  flows  into  the 
bladder  at  diflferent  periods  from  the  right  and  left  kidney. 

The  ureters  have  a  strong  middle  coat  of  smooth  muscle  along 
which  a  wave  of  contraction,  lasting  about  one-third  of  a  second, 
passes  rhythmically  in  about  six  to  ten  seconds  from  the  pelvis  of 
the  kidney  to  the  bladder. 

Having  reached  the  bladder,  the  urine  cannot  return  into  the 
ureters  on  account  of  the  oblique  way  in  which  these  ducts  pass 
through  the  walls  of  the  bladder.  When  the  pressure  in  the 
bladder  increases,  the  opening  of  the  ducts  becomes  closed  and 
acts  as  a  kind  of  valve. 

The  urine,  which  is  continuously  secreted  and  rhythmically 
conveyed  to  the  bladder,  is  only  voided  at  convenient  seasons; 
therefore  special  arrangements  exist  for  its  retention  and  expul- 
sion. 

The  retention  of  urine  in  the  bladder  up  to  a  certain  point 
depends  on  the  elasticity  of  the  parts  concerned,  the  dense  elastic 
tissues  around  its  outlet  being  able  to  resist  the  elastic  force  ex- 
erted by  the  viscera  and  the  walls  of  the  bladder  upon  its  con- 


OUTFLOW   OF   URINE.  409 

teuts.  Thus  wliere  uo  active  muscular  forces  can  possibly  come 
into  play,  as  in  the  case  of  the  dead  subject,  or  in  complete  paral- 
ysis following  destruction  of  the  spinal  cord,  a  considerable 
amount  of  urine  is  retained.  But  when  a  certain  pressure  is 
attained  by  the  gradual  accumulation  of  urine  within  the  blad- 
der, the  elasticity  of  the  sphincter  and  the  other  tissues  around 
the  outlet  is  overcome  by  the  elasticity  of  the  bladder  wall,  and 
the  urine  slowly  dribbles  away. 

In  the  normal  condition  there  are  two  sets  of  muscular  mech- 
anisms which  aid  the  elastic  forces  just  named. 

They  may  be  regarded  as  antagonistic — the  one,  the  sphincter 
muscle,  by  contracting,  strengthens  the  elastic  power  of  the  tissues 
around  the  urethra  which  retains  the  urine;  the  other  formed  by 
the  muscle-coat  of  the  bladder,  called  the  detrusor  iirince,  as  we 
shall  see  presently,  is  the  chief  agent  in  actively  expelling  the 
urine.  When  these  muscles  are  in  good  working  order,  much 
more  urine  can  be  conveniently  retained  than  the  elasticity  of 
the  tissues  about  the  urethra  would  permit  of  If  the  spinal  cord 
be  destroyed  the  bladder  can  only  retain  about  one-third  the 
quantity  of  urine  it  conveniently  holds  when  the  cord  is  intact. 
We  must,  then,  suppose  that  the  sphincter  muscle  acts  more  pow- 
erfully when  the  elastic  forces  are  equalized.  The  accumulation 
of  urine  after  a  certain  time  gives  the  sensation  known  as  a  full 
bladder,  but  this  feeling  is  not  necessarily  accompanied  by  any 
immediate  call  to  make  water,  though  it  soon  produces  a  desire 
in  that  direction.  We  suppose,  then,  that  the  stimulus  given  to 
the  sensory  nerves  by  filling  the  bladder  causes  reflexly  a  con- 
striction of  the  sphincter  muscle,  so  that,  in  proportion  as  the 
pressure  within  the  bladder  increases,  the  resistance  to  its  outflow 
is  also  augmented.  This  does  not  imply  any  automatic  action 
of  the  sphincter  vesicae,  but  merely  a  constant  reflex  excitation 
of  that  muscle,  which  secures  its  contraction  and  the  retention 
of  a  considerable  amount  of  urine  without  the  intervention  of 
voluntary  influences  or  attention. 

Micturition,  or  the  expulsion  of  the  urine,  does  not  normally 
ever  depend  on  elastic  forces  alone,  as  in  the  case  mentioned  of 
paralytic  incontinence,  when  the  urine  commences  to  dribble  away 
as  goon  as  a  certain  pressure  is  attained  within  the  bladder. 


410  MANUAI,   OF   PHYSIOLOGY. 

When  the  bladder  is  full,  the  elastic  forces  tending  to  expel  its 
contents  increase,  and,  as  we  have  seen,  the  resistance  is  propor- 
tionately augmented.  Under  ordinary  circumstances,  then,  there 
is  a  con)bat  going  on  between  the  expelling  and  retaining  powers 
(neither  the  muscle  in  the  wall  of  the  bladder  nor  voluntary 
effort,  however,  coming  into  action),  in  which  the  retaining  forces 
are  just  able  to  overcome  the  expelling  elastic  pressure.  If  the 
urine  be  retained  for  a  considerable  time,  a  moment  arrives  when 
the  reflex  stimulation  of  the  sphincter  no  longer  suffices  to  keep 
back  the  fluid,  and  the  voluntary  contraction  of  the  neighboring 
muscles  has  to  be  called  to  the  aid  of  the  sphincter.  Under  these 
circumstances,  if  a  drop  of  urine  make  its  way  into  the  sensitive 
urethra  matters  are  greatly  altered.  Now,  even  voluntary  eflfort 
does  not  suffice  to  keep  back  tbe  stream,  and  an  imperative  call 
is  made  upon  the  local  mechanisms  to  empty  the  bladder.  This 
is  accomplished  by  the  contraction  of  the  muscular  coat  of  the 
bladder,  which  is  excited  reflexly  by  the  stimulus  starting  from 
the  mucous  membrane  lining  the  urethra.  The  evacuation  of  the 
bladder  is  then  accomplished  quite  independently  of  the  will  by 
a  reflex  act,  which  may  even  be  unconscious. 

When  the  urine  once  commences  to  flow  it  continues  until  the 
bladder  is  quite  empty,  the  last  drops  of  urine  being  expelled 
from  the  urethra  by  rhythmical  spasms  of  the  muscles  around 
the  bulbous  portion  of  that  canal.  The  sequence  of  events  will 
then  be — stimulation  of  the  mucous  membrane  of  the  urethra  by 
escape  of  urine,  contraction  of  the  detrusor  urinse;  relaxation 
of  the  sphincter  ;  rhythmical  contraction  of  the  ejaculator  urinse, 
and  finally  a  voluntary  twitch  of  the  levator  ani  and  neighboring 
muscles. 

This  sequence  of  events  may  go  on  in  sleep,  as  a  result  of  slight 
local  excitations,  frequently  in  children,  when  probably  the 
sphincter  is  more  readily  fatigued. 

Under  ordinary  favorable  circumstances,  however,  we  mictu- 
rate voluntarily,  and  the  bladder  is  never  allowed  to  become  so 
over-distended  that  the  reflex  contraction  of  the  sphincter  is  in- 
sufficient to  retain  the  urine.  Almost  at  any  time  we  can  call 
forth  the  reflex  act  just  described  by  increasing  the  pressure  on 


NERVOUS    MECHANISM    OF    MICTURITION. 


411 


the  bladder  by  voluntary  contraction  of  the  abdominal  rauscles; 
the  diaphragm  being  depressed  and  fixed,  the  muscles  of  expira- 
tion are  put  into  action,  and  the  contraction  of  the  sphincter 
muscle  being  at  the  same  time  probably  checked  by  the  will,  the 
power  of  retention  is  overcome. 

The  moment  the  balance  of  power  is  thus  turned  in  favor  of 


Diagram  of  the  Nervous  Mechanism  of  Micturition. — b.  Bladder,  m. 
Abdominal  muscles,  c.  Cerebral  centres.  R.  Represents  impulses  which 
pass  from  the  bladder  to  the  centre  in  the  spinal  coid,  whence  tonic  impulses 
are  reflected  and  pass  along  T  to  sphincter,  which  retains  the  urine.  When 
the  bladder  is  distended,  impulses  pass  to  the  brain  by  1,  and  when  we  ivitl, 
the  tonus  of  the  spinal  centre  stimulating  the  sphincter  is  checked,  and  the 
abdomin-il  muscles  are  madfe  by  2  to  force  some  urine  into  neck  of  bladder, 
whence  impulses  pass  by  3  to  inhibit  the  sphincter  centre  and  excite  the 
detrusor  through  4 

the  expelling  agencies  and  a  drop  of  urine  reaches  the  urethra, 
the  excitation  thus  produced  brings  about  the  complete  evacua- 
tion of  the  bladder  without  furtlier  voluntary  effort. 

The  nervous  mechanism  that  controls  the  act  of  micturition 
consists  essentially  of  ganglionic  centres  which  are  situated  in  the 


412  MANUAL   or    PHYSIOLOGY. 

lumbar  enlargement  of  the  spinal  cord,  and  of  two  f-cts  of  nerve 
channels  passing  to  and  from  this  centre.  The  centres  may  be 
said  to  be  composed  of  functionally  dislinct  parts — a  retaining 
and  evacuaVuHj  [)art.  The  retaining  centre  causes  the  sphincter 
muscle  to  contract.  The  evacuating  centre  can  excite  the  de- 
trusor to  action  while  the  sphincter  is  relaxed  by  the  inhibition 
of  its  exciting  centre.  One  set  of  nerve  channels  communicates 
between  these  centres  and  the  urinary  organs,  and  the  other  be- 
tween the  cord  centres  and  the  cerebral  hemispheres.  That  which 
connects  the  special  lumbar  centres  with  the  bladder,  contains 
motor  '(efferent)  fibres  of  two  kinds,  going  to  the  antagonistic 
muscles,  the  sphincter  vesicic,  and  the  detrusor  urina;  respectively, 
and  sensory  {aflterent)  fibres  of  different  kinds;  those  going  from 
the  bladder  to  the  nerve  cells  in  the  cord  which  stimulate  them 
and  cause  the  sphincter  to  remain  tonically  contracted,  and  pass- 
ing from  the  mucous  membrane  of  the  urinary  passages  to  these 
ganglionic  cells  in  the  cord  are  two  sets ;  one  of  which  excites 
the  contractions  of  the  detrusor  urintc  and  the  other  inhibits  the 
tonic  action  of  the  retaining  centre. 

The  action  of  the  ganglionic  cells  that  stimulate  the  sphincter 
muscle  can,  to  a  certain  extent,  be  either  aided  or  checked  by 
means  of  cerebral  influences,  so  that  two  kinds  of  fibres  —  a 
stimulating  and  an  inhibitory  one — must  pass  from  the  hemi- 
spheres to  the  micturating  centre  in  the  cord. 

Those  cells  which  govern  the  motions  of  the  detrusor  seem  to 
be  least  under  voluntary  control,  and  are  probably  only  stimulated 
to  action  under  normal  circumstances  by  the  impulses  arising  from 
the  urinary  passages,  and  hence  are  simply  reflex  centres. 

The  effect  of  certain  emotions  on  the  act  of  micturition  seems 
to  show  that  those  ganglion  cells  in  the  cord  which  cause  the 
bladder  to  contract  are  connected  with  the  higher  centres.  Thus, 
extreme  terror  (in  a  dog  at  least)  often  causes  a  forcible  expul- 
sion of  urine,  and  great  anxiety  or  impatience  seems  in  man  often 
to  have  a  checking'  influence,  causing  great  delay  in  initiating 
micturition. 


.      CHAPTER  XXIII. 
NUTRITION. 

We  cau  DOW  compare  the  incomiugs  aud  outgoings  of  the 
economy,  and  should  now  be  in  a  position  to  see  what  light  can 
be  thrown  by  this  comparison  upon  the  actual  changes  which 
take  place  in  the  textures  of  the  body. 

We  have  seen  that  the  income  is  made  up  of  substances  belong- 
ing to  the  same  groups  of  materials  as  are  found  in  the  body,  viz., 
albumins,  fats,  carbo-hydrates,  salts,  and  water,  introduced  by  the 
alimentary  canal,  aud  oxygen,  which  is  admitted  by  the  respira- 
tory apparatus ;  whilst  the  outgoings  consist  of  urea  from  the 
kidneys,  carbonic  acid  from  the  lungs,  certain  excrement  from  the 
intestine  and  other  raucous  passages,  sweat,  sebaceous  secretion, 
epidermal  scales,  from  the  skin  ;  together  with  a  quantity  of  water 
from  all  these  ways  of  exit.  The  milk,  ova,  and  semen  may  be 
here  omitted,  being  regarded  as  exceptional  losses. 

In  order  that  the  body  may  be  kept  in  its  normal  condition,  it 
is  necessary,  that  the  income  should  at  least  be  equal  to  the  out- 
goings of  all  kinds,  and,  except  where  growth  is  going  on  rapidly, 
an  income  equal  to  the  expenditure  ought  not  only  to  suffice,  but 
be  the  most  satisfactory  amount  of  supply  for  the  economy. 

We  know  that  animals  can  live  for  some  considerable  time 
without  food,  in  which  case,  though  there  be  no  income,  a  certain 
expenditure  is  necessary  to  sustain  life,  and  therefore  the  outgoings 
continue.  We  ought  thus  to  be  able  to  arrive  in  a  very  simple 
manner  at  the  minimal  expenditure  necessary  for  the  sustenation 
of  the  body.  We  shall  find,  however,  that  (1)  an  income  equal 
to  this  minimal  expenditure  (starvation)  does  not  at  all  suffice  to 
keep  up  the  body-weight,  aud  that  (2)  a  considerable  margin 
over  and  above  this  minimum  is  necessary  in  order  to  establish 
the    nutritive  equilibrium  ;  (?>)   further,  that  the  proportion   of 


414  MANUAL   OF    PHYSIOLOGY. 

material  eliminated  and  stored  up  in  the  body  respectively  varies 
as  the  income  is  increased;  (4)  and  finally,  that  the  quality  of 
the  food — i.e.,  the  proportion  of  each  group  of  food-stuff  present 
in  the  diet — has  an  important  influence  ou  the  quantity  required 
to  establish  the  equilibrium,  or  that  best  suited  to  cause  increase 
of  weight  or  to  fatten. 

It  will  be  convenient  to  consider  the  following  different  cases  in 
succession  : 

1.  No  income,  except  oxygen,  i.e.,  starvation. 

2.  An  income  only  equal  to  the  expenditure  found  during  starv- 
ation. 

3.  Perfect  establishment  of  nutritive  equilibrium. 

4.  Excessive  consumption. 

Tissue  Changes. 

As  is  well  known,  a  deprivation  of  oxygen — by  the  respiratory 
function  being  stopped — almost  immediately  puts  an  end  to  the 
tissue  changes  necessary  for  life,  so  that  the  oxygen-income  can- 
not be  interfered  with  without  instant  death  ensuing.  Moreover, 
it  has  been  found  that  a  small  supply  of  water  makes  the  investi- 
gation of  the  various  tissue  changes  more  reliable,  by  facilitating 
them  and  prolonging  life.  We,  therefore,  commonly  speak  of  a 
total  abstinence  from  solids  as  starvation. 

When  deprived  of  food,  those  tissues  upon  whose  activity  life 
depends  must  feed  upon  the  materials  stored  up  in  some  part  of 
the  system.  The  first  questions  to  discuss  are  how  much  the  body 
loses  daily  in  weight  during  the  time  that  it  is  thus  feeding  on 
itself,  and  how  far  the  different  individual  tissues  contribute  to 
this  loss. 

The  general  loss  of  weight  is  directly  estimated  by  weighing  the 
animal,  and  the  loss  of  the  individual  tissues  is  calculated  by  a 
careful  analysis  of  the  various  excreta,  by  which  the  exact  amount 
of  nitrogen,  carbonic  acid,  etc.,  are  ascertained  :  the  nitrogen  cor- 
responds to  the  loss  of  muscle;  and  the  carbon  (after  excluding 
that  portion  which  is  the  outcom.e  of  muscle  change,  which  may 
be  calculated  from  the  nitrogen )  corresponds  to  the  fats  oxidized. 

It  has  been  found  that  a  starving;  animal  loses  weight  at  first 


TISSUE   CHANGES    DURING   STARVATION.  415 

rapidly,  and  subsequently  moi'e  slowly  ;  and  the  reason  for  this 
differenceis  that  duringthe  first  three  or  four  days  the  benefit  of  the 
food  last  eaten  continues,  and  the  waste  materials  are  eliminated 
in  proportionately  large  quantity.  AVhen  the  influence  of  the 
food  taken  prior  to  the  commencement  of  starvation  has  ceased, 
the  daily  amount  of  materials  eliminated  remains  nearly  constant, 
and  the  body-weight  diminishes  slowly  until  the  animal's  death. 

Adult  animals  commonly  live  until  they  have  lost  about  half 
of  their  normal  body-weight.  Young  animals  die  when  they  have 
lost  about  20  per  cent,  of  their  weight. 

Roughly  speaking,  we  may  take  the  body  of  a  man  to  be  made 
up  of  the  following  proportions  of  the  more  important  textures: 

Muscles, 50  per  cent. 

Skin  and  fat,     .  • 25         " 

Viscera, 12         " 

Skeleton, 13         " 

Seeing  that  the  muscle  tissue  contributes  such  a  large  propor- 
tion to  the  body-weight,  we  cannot  be  surprised  that  in  starvation 
the  greatest  absolute  loss  occurs  in  this  tissue;  except  in  the  case  of 
excessively  fat  animals.  Next  comes  adipose  tissue,  which  almost 
entirely  disappears,  the  absolute  loss  from  it  varyingiu  proportion 
to  the  fatness  of  the  animal  at  the  beginning  of  the  investigation. 
The  spleen  and  liver  lose  more  than  half  their  weight,  and  the 
amount  of  blood  is  greatly  reduced.  The  smallness  of  the  loss 
that  occurs  in  the  great  nervous  centres  is  very  striking.  They 
seem  to  feed  on  the  other  tissues. 

The  following  table  gives  the  approximate  percentaffe  of  loss 
which  takes  place  in  each  individual  tissue  during  starvation  : 

Fat, 97.0  per  cent. 

Muscle,   .         .■ 30.2         " 

Liver, 56.6         " 

Spleen, 63.1         " 

r.lood, 17.6 

Nerve  centres, 0.  " 

With  regard  to  the  portals  by  which  the  various  materials  make 
their  escape,  it  has  been  found  that  practically  all   the   nitro- 


416 


MANUAL  OF  PHYSIOLOGY. 


gen  passes  off'  with  the  urine,  and  about  nine-tenths  of  the  carbon 
escapes  by  the  lungs  as  CO.^,  the  remaining  one-tenth  passing  off' 
by  the  intestine  and  kidneys.  Three-fourths  of  the  water  is  found 
in  the  urine,  and  one-fourth  goes  off"  from  tlie  skin  and  lungs. 

The  following  table  shows  the  items  of  the  general  loss,  and  the 
amount  per  cent,  which  passes  out  by  the  chief  channels  of  exit: 


Total  Elimination. 

Via  Kidneys. 

Lungs  and 
Skin. 

Excrement. 

995.34  grm. 

20.5.90  " 
:?0.81  " 
10.03    " 

70.2  per  cent. 
6.4   "      " 
100.0    "      " 
97      "      " 

26.1  per  cent. 
92.0    "       " 

3.7  per  cent. 

1.9    "       " 

2.4    "       " 

Nitrogen 

8alt.s 

As  the  loss  of  weight  of  an  animal's  body  during  starvation 
is  at  first  rapid  and  then  more  gradual,  so  also  the  amount  of 
material  eliminated  is  found  to  diminish  much  more  slowly  after 
the  first  few  days.  This  is  well  seen  from  the  nitrogenous  elimi- 
nation. For  the  first  four  days  the  fall  in  the  amount  of  urea 
excreted  is  very  rapid ;  it  then  decreases  almost  constantly  until 
the  death  of  the  animal,  only  slightly  decreasing  in  proportion 
as  the  animal  slowly  decreases  in  weight.  This  has  led  to  the 
conclusion  that  the  amount  of  nitrogenous  material  eliminated 
during  ordinary  circumstances,  with  a  full  diet,  comes  partly 
from  used-up  nitrogenous  tissues,  and  partly  from  nitrogenous 
materials  which  have  never  really  entered  into  the  composition  of 
the  tissues,  but  rather  are  present  as  surplus  or  floating  nitrogenous 
pabulum.  Hence,  two  kinds  of  proteid  are  supposed  to  exist  in 
the  body,  viz.,  (1)  that  forming  part  of  the  tissues,  and  (2)  that  cir- 
culating as  a  ready  supply  for  the  nutritive  demands  of  the  tissues. 

In  the  second  case  mentioned,  namely,  where  an  amount  of  food 
is  supplied  which  is  just  equal  to  the  expenditure  which  was  found 
to  take  place  during  starvation,  one  might  suppose  that  the  diet, 
though  minimal,  would  yet  suffice  to  preserve  the  normal  body- 
weight.  However,  practice  shows  this  to  be  far  from  what  actually 
occurs. 

An  animal,  fed  on  diet  equal  in  quantity  to  the  outgoings  dur- 
ing starvation,  continues  to  lose  weight,  and  the  quantity  of  ni- 


FOOD    REQUIREMENTS.  417 

trogenous  substance  eliminated  (urea)  is  greatly  in  excess  of  the 
low  standard  found  during  complete  abstinence  from  food.  From 
this  it  would  appear  that  even  when  supplied  with  an  amount  of 
nitrogenous  material  equal  to  that  drawn  from  the  tissues  during 
starvation,  an  animal  still  takes  a  further  supply  from  its  own 
textures.  Thus  the  body  subsists  on  the  scanty  allowance  of  nu- 
triment it  borrows  from  the  tissues,  during  starvation,  only  so 
long  as  there  is  absolutely  no  food  income,  and  the  moment  any 
food  is  supplied  an  increased  expenditure  is  set  up,  the  income  is 
exceeded,  and  a  deficit  occurs,  which  is  best  seen  in  the  nitrogen 
balance. 

It  follows,  then,  that  feeding  an  animal  on  an  amount  of  food- 
stuffs exactly  corresponding  to  the  quantity  of  nutriment  ab- 
stracted from  its  own  textures  during  total  abstinence  is  only  a 
slower  form  of  starvation. 

With  regard  to  nitrogenous  substances,  it  has  been  proved  that 
nearly  three  times  as  much  as  the  amount  eliminated  during  starv- 
ation is  required  to  establish  an  equilibrium  between  the  income 
and  expenditure  of  those  special  substances,  and  that  any  less 
than  this  leads  to  a  distinct  nitrogenous  deficit. 

The  third  case  mentioned  in  a  previous  paragraph  (viz., in  which 
the  nutritive  equilibrium  is  exactly  maintained,  so  that  the  body- 
weight  remains  unaltered,  the  gain  and  loss  being  equal)  is  the 
most  imjjortant  one  for  us  to  determine,  since  its  final  settlement 
would  enable  us  to  fix  the  most  beneficial  standard  of  diet.  Un- 
fortunately, this  case  is  also  the  most  difficult  upon  which  to  come 
to  a  satisfactory  conclusion,  for  the  following  reasons: 

1.  The  elaborate  nature  of  the  conditions  imposed  during  the 
experiment  makes  it  difficult  to  carry  on  the  investigation  with 
scientific  accuracy. 

2.  Even  when  the  amounts  of  gain  and  loss  exactly  correspond 
we  cannot  say  that  we  have  the  best  dietary  ;  because  some  of  the 
income  may  be  quite  useless,  and  pass  through  the  economy  with- 
out having  any  function  to  perform  in  it,  and  yet  appear  in  the 
output  so  as  to  give  an  accurate  balance. 

35 


418  MANUAL   OF   PHYSIOLOGY. 

3.  We  have  just  seen  that  the  relative  amounts  of  outgoings 
and  of  malerial  laid  l>y  as  ntore  are  altered  and  regulated  by  the 
quantity  of  income.  And  we  find  that  the  quality  of  the  income, 
i.e.,  the  relative  proportions  of  the  various  food-?tuffs,  has  a  ma- 
terial influence  on  the  quantities  of  material  laid  by  and  elimi- 
nated respectively.  We  must,  therefore,  consider  the  efficacy  of 
each  of  the  groups  of  the  food-stuffs  when  employed  alone  and 
mixed  in  different  proportions. 

4.  Different  animals  seem  to  have  different  powers  of  assiraila- 
tion  ;  and  under  various  circumstances  the  requirements  and 
assimilative  power  of  the  same  animal  may  vary. 

An  animal  fed  upon  a  purely  meat  diet  requires  a  great  amount 
of  it  to  sustain  its  body-weight.  It  has  been  found  that  from  -^jj 
to  2'-  of  the  body-weight  in  lean  meat  daily  is  necessary  to  keep 
an  animal  alive  without  either  losing  or  gaining  weight.  If  more 
than  this  amount  be  suj)plied  the  animal  increases  in  weight,  and 
as  its  weight  increases  a  greater  amount  of  meat  is  required  to 
keep  it  up  to  the  new  standard.  So  that  to  produce  a  progressive 
increase  of  weight  with  a  purely  meat  diet,  it  is  necessary  to  keep 
on  increasing  the  quantity  of  meat  given.  The  reason  of  this  is 
found  in  the  fact  that  albuminous  diet  causes  an  increase  in  the 
changes  occurring  in  the  nitrogenous  tissues. 

If  an  animal  which  is  in  extremely  poor  condition  be  given  an 
ad  libitum  supply  of  lean  meat,  only  a  limited  portion  of  the  albu- 
minous substance  is  retained  in  the  tissues.  By  far  the  larger 
proportion  of  the  nitrogenous  food  is  found  given  off  and  is  repre- 
sented in  the  urine  by  urea,  and  a  comparatively  small  proportion 
is  stored  up.  If  this  large  supply  of  meat  diet  be  continued  for 
some  time,  less  and  less  of  the  albuminous  material  is  stored,  more 
and  more  being  eliminated  as  urea,  until  finally  the  urea  excreted 
justs  corresponds  to  the  albuminous  materials  in  the  ingesta. 
When  only  meat  is  given,  it  must  be  supplied  in  large  quantities 
to  maintain  the  balance  of  the  nitrogenous  income  and  expenditure 
which  is  spoken  of  as  nitrogenous  equilibrium.  Upon  the  occur- 
rence of  a  change  in  the  amount  of  nitrogenous  ingesta,  this  nitro- 
genous equilibrium  varies,  and  it  takes  some  time  to  become  rees- 
tablished, because  a  decrease  in  the  meat  diet  is  accompanied  by 


FOOD    REQUIREMENTS.  419 

a  decrease  iu  the  weight  of  the  animal,  and  an  increase  causes  it 
to  put  on  flesh.  For  each  new  body-weight  there  is  a  new  nitro- 
genous equilibrium,  which  is  only  attained  after  the  disturbed 
relation  between  the  nitrogenous  ingesta  and  excreta  has  been 
readjusted. 

The  increase  of  weight  which  follows  a  liberal  meat  diet  depends 
in  a  great  measure  on  fat  being  stored  up  in  the  body.  Much 
more  of  this  material  is  made  than  could  come  from  the  fat  taken 
with  the  meat ;  hence,  we  must  conclude  that  it  is  made  from  the 
albuminous  parts  of  the  meat. 

The  effect  of  a  diet  without  any  albuminous  food  is  that  the 
animal  dies  of  starvation  nearly  as  soon  as  if  deprived  of  all  forms 
of  food,  with  the  exception  that  the  weight  of  the  body  is  much 
less  reduced  at  the  time  of  death. 

The  addition  of  fats  and  sugars  to  meat  diet  allows  a  consider- 
able reduction  to  be  made  iu  the  supply  of  meat,  and  both  the 
body-weight  and  nitrogenous  tissue  change  can  be  kept  in  equi- 
librium on  a  smaller  amount  of  food.  It  has  been  estimated  that 
the  nitrogenous  tissue  change  is  reduced  7  per  cent,  by  the  addi- 
tion of  fat,  and  10  per  cent,  by  the  addition  of  carbohydrate  food 
to  the  meat  diet ;  therefore  less  meat  is  wanted  to  make  up  nitro- 
genous tissues.  Further,  fats  and  sugars,  which  obviously  cannot 
of  themselves  form  au  adequate  diet,  since  they  contain  uo  nitro- 
gen, seem  to  have  the  power  of  accomplishing  some  end  in  the 
economy  which,  in  their  absence,  requires  a  considerable  expen- 
diture of  nitrogenous  materials  to  bring  about.  Fats  and  sugars, 
then,  supply  to  the  body  readily  oxidizable  materials,  and  thus 
shield  the  albuminous  tissues  from  oxidation,  as  well  as  reduce 
absolutely  the  nitrogenous  metabolism. 

It  would  further  appear  from  the  experience  gained  from  the 
stall-feeding  of  animals  that  a  good  supply  of  carbohydrates,  to- 
gether with  a  limited  quantity  of  nitrogenous  food,  is  admirably 
adapted  to  produce  fat.  Since  much  more  fat  has  been  found  to  be 
produced  in  pigs  than  could  be  accounted  for  by  the  albuminous 
and  fatty  constituents  of  their  diet,  we  must  suppose  that  from 
their  carbohydrated  food  fat  can  be  manufactured  iu  their  body. 

Much  of  the  difficulty  found  in  reconciling  the  opinions  of  dif- 


420  MANUAL   OF   PHYSIOLOGY. 

ferent  authors  upon  this  point  can  be  removed,  and  a  general  idea 
of  the  manufacture  of  fats  from  various  food-stufl's  can  be  gained, 
by  bearing  in  mind  the  assimilative  and  secretive  functions  of  the 
protoplasm.  There  can  be  no  doubt  whatever  that  the  active 
protoplasm  of  many  parts  and  organs  if  properly  nourished  can 
manufacture  fat.  As  examples,  we  may  take  the  liver  and  mam- 
mary cells,  and  those  connective  tissue  cells  which  have  no  great 
nutritive  duty  to  attend  to.  This  fat  production  by  the  proto- 
plasm may  be  regarded  as  a  secretion  of  fat,  though  only  in  one 
of  the  examples  given  does  it  appear  externally  as  a  definite 
secretion — milk.  We  cannot  scrutinize  the  chemical  methods  by' 
which  this  change  is  brought  about  in  protoplasm,  any  more  than 
those  which  give  rise  to  any  other  secretion.  We  know  that  pro- 
toplasm uses  as  pabulum,  albumin,  fat,  and  carbohydrate,  and 
we  have  no  reason  to  doubt  that  the  proportion  of  these  materials 
found  to  form  the  most  nutritious  diet  for  the  body  generally,  is 
also  the  proportion  in  which  protoplasm  can  best  make  use  of 
them.  Probably  such  cells  as  part  with  a  material  containing 
nitrogen — such  as  muciu-yieldiug  gland  cells — require  a  greater 
proportion  of  pabulum  containing  nitrogen  (albumin)  for  their 
perfect  function.  Probably  those  cells  which  produce  a  large 
quantity  of  non-nitrogenous  material  do  not  require  more  nitro- 
gen than  is  necessary  for  their  perfect  reintegration  as  nitrogenous 
bodies.  But  for  their  active  function,  i.e.,  the  manufacture  of 
their  secretion,  they  only  require  a  pabulum  w'hich  contains  the 
same  chemical  elements  as  are  to  be  found  in  the  output.  In  the 
case  of  fat-formation,  then,  a  supply  of  fat  or  carbohydrate  ought 
to  suffice  if  accompanied  by  a  small  amount  of  albuminous  sub- 
stance. If  these  non-nitrogenous  substances  be  withheld,  the 
protoplasm  can  no  doubt  obtain  the  quantity  of  carbon,  hydro- 
gen, and  oxygen  requisite  to  manufacture  fat  from  albumin,  and 
thus  a  large  amount  of  nitrogen  will  be  wasted. 

There  is  nothing  in  the  foregoing  statement  that  is  not  in  ac- 
cord with  the  results  of  practice  and  experiment. 

Fat  cannot  be  produced  without  nitrogen  in  the  diet,  because 
the  fat-manufacturing  j)rotoplasm  cannot  live  without  nitrogen, 
which  is  absolutely  necessary  for  its  own  assijnilative  reintegra- 


ULTIMATE    USES    OF   FOOD-STUFFS.  421 

tiou.  A  good  supply  of  nitrogenous  food  aids  in  fattening,  since 
it  gives  vigor  to  all  the  protoplasmic  metabolisms,  and  among 
them  fat-formation. 

The  albuminoid  substance,  gelatin,  which  is  an  important  item 
in  the  food  we  ordinarily  make  use  of,  is  able  to  effect  a  saving 
in  the  albuminous  food-stufFs.  Although  it  contains  a  sufficiently 
large  proportion  of  nitrogen,  it  cannot  satisfactorily  replace  albu- 
min in  the  food.  Indeed,  in  spite  of  the  great  similarity  in  its 
chemical  composition  to  albuminous  bodies,  it  is  hardly  a  better 
substitute  for  proteids  than  fat  or  carbohydrate ;  and,  although 
an  animal  uses  up  less  of  its  tissue  nitrogen  on  a  diet  of  gelatin 
and  fat  than  when  it  is  fed  on  fat  alone,  it  soon  dies,  as  if  its  diet 
contained  no  nitrogenous  substance. 

The  last  case  we  have  to  consider  is  that  in  which  the  supply 
of  food-material  is  in  excess  of  the  requirements  of  the  economy. 
This  is  certainly  the  commonest  case  in  man. 

Much  of  the  surplus  food  never  really  enters  the  system  but  is 
conveyed  away  with  the  fseces. 

In  speaking  of  pancreatic  digestion,  reference  has  been  made 
to  the  possible  destiny  of  excess  of  nitrogenous  food.  In  the  in- 
testine, some  of  it  is  decomposed  into  leucin  and  tyrosin,  which 
are  absorbed  into  the  intestinal  bloodvessels.  In  the  body  these 
substances  undergo  further  changes,  which  probably  take  place 
in  the  liver.  As  a  result  of  the  absorption  of  leucin,  a  larger 
quantity  of  urea  appears  in  the  urine,  and  hence  the  leucin  formed 
in  the  intestine  by  prolonged  pancreatic  digestion  is  a  source  of 
urea.  (See  pp.  161,  407.)  This  view  is  supported  by  the  almost 
immediate  increase  in  the  quantity  of  urea  eliminated  when 
albuminous  food  is  taken  in  large  quantity. 

From  the  fact  that  a  considerable  amount  of  fat  may  be  stored 
up  by  an  animal  supplied  with  a  liberal  diet  of  lean  meat,  we 
must  conclude  that  part  at  least  of  the  surplus  albumin  goes  to 
form  fat.  It  has  been  suggested  that  after  sufficient  albumin  has 
been  absorbed  for  the  nutritive  requirements  of  the  nitrogenous 
tissues,  the  rest  is  split  up  into  two  parts,  one  of  which  is  imme- 
diately prepared  for  elimination  as  urea  by  the  liver,  and  the 


422  MANUAL   OF   PHYSIOLOGY. 

other  undergoes  changes,  probably  in  the  same  organ,  which  re- 
sult in  its  being  converted  into  fat. 

It  would  further  seem  probable,  from  the  manner  in  which  the 
urea  excretion  changes  during  starvation,  that,  as  before  men- 
tioned, the  absorbed  albumin  exists  in  the  economy  in  two  forms: 
one  in  which  it  has  been  actually  assimilated  by  the  nitrogenous 
tissues  and  forms  part  of  them,  and  hence  is  called  organ-albumin  ; 
the  other,  which  is  merely  in  solution  in  the  fluids  of  the  body, 
being  in  stock,  but  not  yet  absolutely  assimilated,  and  hence 
called  circulating  albumin.  The  latter  passes  away  during  the 
first  few  days  of  starvation,  being  probably  broken  up  to  form  urea, 
and  a  material  which  serves  the  turn  of  non-nitrogenous  food. 
The  organ-albumin  only  appears  to  be  used  for  the  formation  of 
urea  after  the  circulating  albumin  has  completely  disappeared. 

From  the  foregoing  it  will  be  gathered  that  we  cannot  say  what 
are  the  exact  destinies  of  the  various  food-stuffs  in  the  body. 
Proteids  are  not  exclusively  utilized  in  the  reintegration  of  pro- 
teid  tissues,  as  an  excess  gives  rise  to  a  deposit  of  fat.  Carbohy- 
drates are  not  turned  iuto  glycogen  in  the  tissues  simply  to  replace 
the  carbohydrates  used,  but  as  will  be  shown  when  speaking  of 
muscle  metabolism,  they  are  intimately  related  to  the  chemical 
changes  which  take  place  during  the  activity  of  that  tissue.  If 
fats  are  chiefly  devoted  to  the  restitution  of  the  fat  of  the  body, 
they  certainly  are  not  the  only  kind  of  Ibod  from  which  fat  can 
be  made. 

We  may  say,  then,  that  all  food-stuffs  are  destined  to  feed  the 
living  protoplasm,  whether  it  be  in  the  form  of  gland  cells,  the 
cells  of  the  connective  tissues,  or  muscle-plasma,  so  that  all  the 
food-stuffs  that  are  really  assimilated,  contribute  to  the  mainte- 
nance of  protoplasm  and  subserve  to  its  various  functions.  Be- 
sides nourishing  itself  and  keeping  itself  up  to  a  certain  standard 
composition,  protoplasm,  or  rather  the  various  protoplasmata, 
can  make  the  various  chemical  materials  we  find  in  the  body. 
Some  produce  fat,  some  animal  starch  (glycogen),  and  others 
manufacture  the  various  substances  we  find  in  the  secretions  ;  while 
yet  another  group  is  devoted  to  setting  free  and  utilizing  the 
energy  of  the  various  chemical  associations. 


ULTIMATE   USES   OF   FOOD-STUFFS.  423 

But  all  the  food  we  eat  is  not  assimilated ;  indeed,  the  destiny 
of  the  numerous  ingredients  of  our  complex  dietaries  is  not  easily 
traced.  Of  food-stuffs  proper  the  following  classification  may  be 
made,  showing  that  even  the  same  stuff  may  meet  with  a  different 
fate  under  different  circumstances  : 

1.  Stuffs  whic'h  never  enter  the  economy  (fseces). 

2.  Materials  absorbed  and  arriving  at  the  blood  are  at  once 

carried  to  certain  portals  of  excretion  (excess  of  salts). 

3.  Substances  which  are  broken  up  in  the  intestine  to  facili- 

tate their  elimination  (excess  of  proteid). 

4.  Substances  absorbed  and  carried  along  by  the  fluids,  but 

not  really  united  to  the  tissues  (circulating  albumin). 

5.  Materials  which,  after  their  absorption,  are  really  assimi- 

lated by  the  protoplasm  of  the  tissues  (a  certain  amount 
of  all  food-stuffs). 

6.  Substances  which,  after  their  assimilation  by  the  proto- 

plasm, reappear  in  their  original  form,  and  are  stored 
up  (fats). 

The  question  of  the  exact  amounts  and  materials  required  to 
form  the  most  economic  and  wholesome  dietary  is  one  of  too 
great  practical  importance  to  receive  adequate  attention  in  this 
manual.  As  a  rule  men,  like  other  animals,  partake  of  food 
largely  in  excess  of  their  physiological  requirements  when  they 
can  get  it.  This  may  be  seen  by  contrasting  one's  own  daily  food 
with  the  amount  which  has  been  found  to  be  adequate  in  the  case 
of  individuals  who  have  not  the  opportunity  of  regulating  their 
own  supplies  of  comestibles. 

An  adult  man  should  be  well  nourished  if  he  be  supplied  with 
the  following  daily  diet : 


Albuminous  foods. 

Fats,  . 

Starch. 

Salts, 

Water, 


100  grms.  or    3.5  oza. 

90      "       "     3.1    " 

300      "       "  10.7    " 

30      "       "     1.0    " 

2800      "      "     5  pints 


As  a  matter  of  fact,  many  persons  do  thrive  on  a  much  less 


424  MANUAL   OF    PHYSIOLOGY. 

quantity  of  proteid  than  that  given  in  this  table,  but  in  their  ca-e 
the  fats  and  starches  should  be  proportionately  increased. 

Such  a  dietary  could  be  obtained  from  many  comestibles  alone, 
and  hence  the  taste  of  the  individual  may  be  exercised  in  select- 
ing his  food  without  much  departing  from  such  a  standard.  In- 
dividual taste  commonly  selects  foods  with  too  much  proteid — 
i.e.,  an  excess  of  nitrogen — whilst  the  cheapness  of  vegetable 
products  dictates  their  use  in  greater  abundance  as  food. 

Compare  Chap.  V.,  p.  93,  where  the  quantity  of  the  different 
food-stuffs  in  some  of  our  common  articles  of  diet  is  given. 


CHAPTER  XXIV. 

ANIMAL  HEAT. 

Part  of  the  work  done  or  energy  set  free  by  the  chemical 
changes  in  the  animal  tissues  appears  as  heat  which  is  devoted  to 
keeping  the  body  warm  ;  for  the  bodies  of  most  animals  are  con- 
siderably warmer  than  their  surroundings.  Warm-blooded  ani- 
mals are  those  which  habitually  preserve  an  even  temperature 
independent  of  the  changes  which  take  place  in  that  of  the  medium 
in  which  they  live ;  and  as  the  term  warm-blooded  implies,  their 
temperature  is,  as  a  rule,  higher  than  the  surrounding  air  or 
water.  Cold-blooded  animals,  on  the  other  hand,  are  those  whose 
tempet-ature  is  considerably  affected  by,  or  more  or  less  closely 
follows,  that  of  the  medium  surrounding  them. 

The  blood  of  all  mammalia  has  pretty  much  the  same  tempera- 
ture as  that  of  man,  about  37.5°  C,  and  probably  varies  under 
similar  circumstances.  But  birds,  the  other  class  of  warm-blooded 
animals,  have  a  temperature  about  4°  or  6°  C  higher  than  that 
of  mammals. 

The  blood  of  those  animals  whose  temperature  follows  the 
changes  that  occur  around  them,  is  generally  from  1°  to  5°  C. 
higher  than  the  medium  in  which  they  live.  They  produce  some 
heat,  though  it  be  in  small  quantity,  and  since  they  have  no  special 
plan  for  its  regulation,  it  does  not  remain  at  a  certain  standard. 
Wherever  active  oxidization  takes  place,  heat  is  produced  ;  so 
even  in  invertebrate  animals  an  elevation  of  temperature  occurs ; 
this  can  be  easily  ascertained  when  they  exist  in  masses,  as  bees, 
an  active  hive  sometimes  reaching  a  temperature  of  35°  C 

Instead  of  the  term  "  warm-blooded,"  it  is  more  accurate  to 
apply  to  animals  whose  temperature  remains  uniformly  even,  and 
independent  of  their  surroundings,  the  term  "  Homceothermic" 

36 


426  MANUAL   OF   PHYSIOLOGY. 

(constant  temperature),  and  to  animals  with  temperatures  varying 
with  their  surroundings  "  Foikilothermic"  (or  changing  tempera- 
ture), instead  of  the  words  warm-  and  cold-blooded. 

Measurement  of  Temperature. 

On  account  of  the  slight  degrees  of  variation  that  occur  in  the 
temperature  of  man,  all  the  changes  taking  place  can  be  measured 
with  a  thermometer  having  a  short  scale  of  some  20  degrees,  each 
degree  of  which  occupies  considerable  length  on  the  instrument, 
so  that  very  slight  variations  may  be  easily  appreciated.  Such 
thermometers,  with  au  arrangement  for  self-registering  the  maxi- 
mum height  attained  by  the  column  of  mercury,  are  in  daily  use 
for  clinical  observation,  for  the  temperature  of  the  body  is  now  a 
naost  important  aid  to  diagnosis  and  prognosis  in  a  large  class  of 
diseases. 

As  heat  is  constantly  being  lost  at  the  surface  of  the  body,  the 
skin  is  colder  than  the  deeper  parts,  and  in  order  to  avoid  varia- 
tions caused  by  this  surface  loss — which  depends  in  a  measure  on 
the  temperature  of  the  air — special  arrangements  are  necessary 
to  prevent  the  thermometer  being  too  much  influenced  by  it. 
The  instrument  may  be  brought  into  close  proximity  to  the  deeper 
parts  by  being  introduced  into  one  of  the  mucous  passages,  where 
it  is  surrounded  by  vascular  tissue.  In  animals  the  rectum  is  the 
most  convenient  part  for  the  application  of  the  thermometer,  but 
in  clinical  practice  it  is  usually  placed  under  the  tongue,  or  in 
the  arm-pit,  the  bulb  being  held  so  that  on  all  sides  it  is  in  con- 
tact with  the  skin  and  protected  from  the  cool  air. 

The  variations  at  different  parts  of  the  body  are  but  slight,  and 
the  average  normal  temperature  in  man  is  found  to  be  about  SyC. 

Normal  Variations  in  Temperature. 

The  normal  temperature  undergoes  certain  variations,  some  of 
which  are  :  (1.)  Regular  and  periodical,  depending  upon  the  time 
of  day,  the  ingestion  of  food,  and  the  age  of  the  individual.  (2.) 
Accidental  circumstances,  such  as  mental  or  bodily  exertion. 

a.  The  temperature  is  highest  between  4  and  5  p.m.  and  lowesS 
between  2  and  4  a.m.,  the  transition  being  gradual.     This  diurnal 


NOEMAL   VAEIATIONS  IN   TEMPERATURE.  427 

variation,  which  normally  does  not  much  exceed  1°  C,  is  much 
exaggerated  in  hectic  fever. 

b.  The  temperature  rises  after  a  hearty  meal  and  falls  during 
fasting.  During  starvation  the  temperature  sinks  gradually  until 
the  death  of  the  individual. 

c.  The  temperature  is  highest  at  birth,  and  falls  about  1°  C. 
between  that  and  the  age  of  50  years, — in  extreme  old  age  it  is 
said  that  it  again  rises. 

d.  Muscular  exertion,  which  gives  the  individual  the  sensation 
of  great  warmth,  only  changes  the  temperature  of  the  blood  about 
.5°  C.  The  very  high  temperature  which  accompanies  the  disease 
Tetanus,  where  all  the  muscles  are  thrown  into  a  state  of  spasm, 
probably  depends  more  on  pathological  changes  than  on  muscular 
action. 

e.  Mental  exertion  is  also  said  to  cause  a  rise  of  temperature. 
/.  Slight  differences  in  the  heat  of  the  blood  may  be  brought 

about  by  variations  in  the  surrounding  temperature.  The  abnor- 
mally high  temperature  of  fever  is  much  more  easily  affected  by 
changes  in  the  rate  of  removal  of  the  heat  from  the  body,  than  is 
the  normal  temperature,  and  hence  the  therapeutic  value  of  cold 
applications  in  this  class  of  diseases. 

The  temperature  of  different  parts  of  the  body  varies  in  a  slight 
degree,  and  depends  upon  the  following  circumstances :  1.  The 
amount  of  blood  flowing  through  them ;  for  the  blood  is  the 
great  carrier  of  warmth  from  one  part  to  another,  supplying 
heat  where  it  is  lost  by  exposure,  etc.,  and  it  conveys  material 
to  those  parts  where  the  heat  is  generated.  2.  The  amount  of 
heat  produced  in  a  given  part,  i.e.,  the  activity  of  its  tissue 
change.  3.  The  amount  of  heat  lost,  which  depends  on  (a)  the 
extent  of  surface;  (b)  the  external  temperature;  (c)  the  power 
of  conduction  of,  and  the  capacity  for  heat  of  the  surrounding 
medium. 

From  this  it  is  obvious  that  the  deeper  parts  of  the  body,  where 
active  chemical  change  takes  place  and  which  are  protected  from 
exposure,  must  be  warmer  than  the  exterior,  which  is  constantly 
losing  its  heat  to  the  air.  The  blood  then  which  flows  through 
the  surface  vessels  is  cooled,  and  that  which  flows  through  the 


428  MANUAL   OF    PHYSIOLOGY. 

deeper  vascular  viscera  is  warmed.  Thus  the  skin  is  usually  about 
37°  C,  while  the  mouth  beneath  the  tongue  is  about  37.5°  C, 
and  the  rectum  about  38°  C.  Accordingly,  then,  as  the  blood  has 
recently  passed  through  a  part  of  the  body  where  it  has  had  an 
opportunity  of  losing  or  gaining  heat,  its  temperature  varies,  but 
only  within  narrow  limits.  The  mean  temperature  of  the  blood  is 
higher  than  that  of  any  other  tissue.  The  blood  in  the  hepatic 
capillaries  is  the  warmest  in  the  body.  This  reaches  40.73°  in  the 
dog,  or  nearly  two  degrees  higher  than  that  in  the  aorta  of  that 
animal.  The  cool  blood  from  the  extremities  and  head  mingling 
in  the  right  side  of  the  heart  with  the  unusually  warm  blood  from 
the  liver  keeps  the  blood  going  to  the  lungs  at  the  standard  tem- 
perature. The  blood  in  the  left  side  of  the  heart  is  a  little  cooler 
than  the  right,  probably  because  the  latter  lies  on  the  warm  liver, 
as  is  proved  by  the  substitution  of  a  cold  object  for  this  organ, 
when  the  temperature  is  reversed,  and  the  blood  on  the  right  side 
becomes  colder  than  the  left.  It  is  not  because  the  blood  is  cooled 
going  through  the  lungs,  for  the  heat  used  in  warming  the  respired 
air  is  given  off  by  the  nose  and  other  air  passages  and  not  by  the 
alveoli  of  the  lungs. 

Mode  of  Production  of  Animal  Heat. 

It  has  already  been  indicated  that  the  general  effect  of  the  tissue 
change  of  the  body  is  a  kind  of  combustion  in  the  tissues  of  certain 
substances  obtained  from  the  vegetable  kingdom,  viz.  proteid,  fat, 
carbohydrate,  etc.  The  combustible  substances  are  capable  of 
being  burned  in  the  open  air,  or  made  to  unite  with  oxygen  so  as 
to  produce  a  certain  amount  of  heat,  being  thus  converted  into  CO, 
and  HjO.  In  the  body  the  oxidation  goes  on  in  a  gradual  or 
modified  way,  and  the  end  products  of  the  process  can  be  recog- 
nized as  CO2  eliminated  from  the  lungs,  and  as  water  and  urea 
got  rid  of  by  the  kidneys.  The  general  tendency  of  the  chemical 
changes  in  the  tissues  is  such  as  will  set  free  energy  in  the  form 
of  heat. 

The  amount  of  heat  that  any  substance  is  capable  of  giving  off 
corresponds  to  the  amount  of  energy  required  for  the  formation 
from  COj  and  HjO,  etc.,  of  the  compounds  contained  in  it,  and 


HEAT-INCOME.  429 

this  correspondence  remains  whether  the  dissociation  takes  place 
rapidly  or  slowly.  The  substances  we  make  use  of  as  food  have 
thus  a  certain  heat  value  which  depends  upon  their  chemical 
composition. 

The  high  temperature  which  homoeotherraic  animals  can  keep 
up  in  spite  of  the  cold  of  the  atmosphere  in  which  they  live  is 
readily  accounted  for  by  the  chemical  change  which  is  constantly 
occurring  in  the  tissue  of  their  bodies. 

The  amount  of  heat  produced  in  any  part  must,  then,  depend 
upon  the  activity  of  its  tissue  change,  for  we  find  that  the  tem- 
perature varies  with  the  elimination  of  COj,  and  urea,  which  give 
a  fair  estimate  of  the  chemical  changes  of  the  tissues. 

1.  The  diurnal  changes  in  temperature  are  accompanied  by  an 
afternoon  increase  and  a  morning  decrease  of  COj  and  urea. 

2.  The  tissue  change  giving  rise  to  CO2  decreases  in  a  fasting 
animal,  as  does  also  the  production  of  heat. 

3.  More  CO^  is  eliminated  after  meals,  when  the  temperature 
also  rises. 

4.  The  activity  of  various  organs,  such  as  the  muscles  and 
glands,  is  associated  with  a  local  increase  of  temperature. 

Income  and  Expenditure  of  Heat. 

As  repeatedly  stated,  the  chemical  changes  which  give  rise  to 
the  heat  cause  a  certain  waste  of  the  tissues,  which  have  again 
to  be  renewed  by  the  assimilation  of  various  nutrient  materials. 
The  food  is  thus  really  the  fuel  of  the  animal  body,  and  the  pe- 
culiarity of  this  form  of  combustion  is  that  the  tissues  assimilate 
or  convert  into  their  own  substance  the  fuel,  and  then  themselves 
undergo  a  kind  of  partial  combustion,  by  means  of  which  they 
perform  their  several  functions,  amongst  others  heat-production. 

As  already  mentioned,  heat  is  produced  most  abundantly  in 
those  tissues  which  undergo  most  active  chemical  changes,  hence 
the  protoplasmic  cells  of  glands,  and  the  contractile  substance  of 
muscle  must  be  looked  upon  as  the  chief  agents  in  setting  heat 
free. 

The  possible  heat-income  depends  on  the  amount  of  nutrient 
matter  assimilated.     As  each  kind  of  food  has  a  certain  heat 


430  MANUAL  OF   PHYSIOLOGY. 

value,  i.e.,  the  number  of  heat-units  its  combustion  will  produce, 
we  ought  to  be  able  to  estimate  the  amount  of  heat  produced  by 
ascertaining  this  value  and  subtracting  the  calorific  value  of  the 
various  substances  given  off  by  the  body.  Since  practically  the 
temperature  of  the  body  remains  the  same,  the  amount  of  heat  lost 
during  a  given  time  should  correspond  to  the  income,  estimated 
from  the  number  of  heat-units  of  the  food.  So  far,  however,  at- 
tempts to  make  the  calculated  heat-income  correspond  with  the 
expenditure  have  not  been  productive  of  satisfactory  results,  the 
estimated  calorific  value  of  the  food  being  hardly  equal  to  the 
heat  calculated  to  be  given  off  by  the  body.  We  must  remem- 
ber that  it  is  not  the  proteid,  fat  and  starch  of  the  body,  that  we 
burn,  but  the  living  tissues  formed  by  the  assimilation  of  these 
substances.  We  do  not  know  what  chemical  changes  go  on  in 
the  steps  of  tissue  formation,  and  therefore  we  cannot  say  exactly 
what  combinations  are  submitted  to  the  combustion  which  gives 
us  a  high  heat-value. 

Since  the  activity  of  muscle  and  gland  tissue  is  constantly  un- 
dergoing variations  in  intensity,  the  amount  of  chemical  change 
differs  at  different  times,  so  that  the  amount  of  heat  produced  must 
also  vary.  We  know  that  the  heat  set  free  by  any  organ,  such 
as  a  gland  or  a  muscle,  increases  in  proportion  to  the  increase  of 
its  functional  activity,  but  we  cannot  say  that  the  calorific  activity 
can  vary  independently  of  other  circumstances.  Without  such 
a  special  calorific  function  of  some  tissues,  such  as  muscle,  the 
actual  net  heat-income  must  vary  with  circumstances  which  are 
accidental  and  therefore  irregular. 

Since  we  know  that  the  nervous  system  controls  the  tissue  ac- 
tivities which  are  accompanied  by  the  setting  free  of  heat,  we  can 
see  how  the  nerve  centres  can  materially  influence  the  heat  pro- 
duction of  the  body;  thus,  the  more  active  are  the  muscles, 
glands,  etc.,  which  are  under  the  control  of  nerves,  the  greater 
the  amount  of  heat  produced  in  a  given  time.  That  the  nervous 
system  can  cause  in  any  tissue  a  chemical  change,  giving  rise  to 
a  greater  production  of  heat,  without  any  other  display  of  func- 
tional activity,  we  do  not  know,  but  many  facts  seem  to  point  to 
such  a  possibility. 


HEAT-INCOME.  431 

The  effect  of  nerve  influeuce  on  the  production  of  heat  is 
greatly  complicated  also  by  the  enormous  power  exercised  by  the 
nerves  over  the  blood  supply  through  the  means  of  the  vaso- 
motor mechanisms,  for  the  temperature  of  any  given  part  is  so 
intimately  related  to  the  amount  of  blood  flowing  through  it  that 
the  former  has  been  commonly  accepted  as  an  adequate  measure 
of  the  latter. 

For  the  present,  therefore,  we  are  not  in  a  position  to  speak 
with  decision  of  nerves  with  a  purely  thermic  action. 

The  expenditure  of  the  heat  may  be  classed  under  the  follow- 
ing headings : 

1.  In  warming  ingesta :  As  a  rule  all  the  food  and  drink  we 
make  use  of,  as  well  as  the  oxygen  we  breathe,  are  colder  than  the 
body,  and  before  they  pass  out  they  are  raised  to  the  body  tem- 
perature. 

2.  Radiation  and  Conduction  :  From  the  surface  of  the  body 
a  quantity  of  heat  is  being  expended  in  warming  the  surround- 
ing medium,  which  is  habitually  colder  than  our  bodies.  The 
colder  the  medium  the  greater  its  capacity  for  heat,  and  the  more 
quickly  it  comes  in  contact  with  new  portions  of  the  surface  the 
more  warmth  it  robs  us  of  Water  or  damp  air  takes  up  much 
more  heat  from  our  surface  than  dry  air  of  the  same  temperature, 
and  the  quantity  of  heat  lost  is  still  further  increased  if  the  me- 
dium be  in  motion,  so  that  the  relatively  colder  fluid  is  constantly 
renewed. 

3.  Evaporation:  (a.)  From  the  air-passages:  a  quantity  of 
water  passes  into  the  vaporous  state  and  saturates  the  tidal  air, 
and  this  change  of  condition,  from  liquid  to  that  of  vapor,  ab- 
sorbs much  heat,  (b.)  From  the  skin :  surface  evaporation  is 
always  going  on,  even  when  no  moistui-e  is  perceptible  on  the 
skin,  and  much  fluid  of  which  we  are  not  sensible  is  lost  in  this 
way.  The  quantity  of  heat  lost  by  evaporation  from  the  skin  will 
depend  on  the  temperature  and  the  degree  of  moisture  of  the  air 
in  proportion  to  that  of  the  surface  of  the  body. 

As  has  been  said,  the  exact  income  of  heat  is  uncertain  and 
variable,  because  the  data  upon  which  the  absolute  amount  can 


432  MANUAL   OF   PHYSIOLOGY. 

be  calculated  are  uot  scieutifically  free  from  error.  According 
to  the  most  careful  estimates  au  adult  weigliing  82  kilo,  produces 
2,700,000  units  of  heat  in  the  twenty-four  hours,  which  are  ex- 
pended in  the  following  way: 

In  warming  ingesta,         ....  70,1;j7  units  of  heat. 

In  warming  tidal  air 140,0G4             " 

By  the  evaporation  of  G5G  grm.  water 

from  the  air  passages,           .         .         .  o97,53G             " 

By  surface  loss, 2,092,243            " 

From  this  it  appears  that  more  than  three-quarters  of  our  heat 
is  lost  by  the  skin  (77.5  per  cent.)  ;  by  pulmonary  evaporation, 
14.7  per  cent.  ;  in  heating  the  air  breathed,  5.2  per  cent. ;  in 
heating  ingesta,  2.6  per  cent. 

Maintenance  of  Uniform  Temperature. 

In  order  that  the  vital  processes  of  man  aud  the  other  homoeo- 
thermic  animals  should  go  on  in  a  normal  manner,  it  is  necessary 
that  their  mean  temperature  remain  nearly  the  same,  and  we 
have  seen  that  under  ordinary  circumstances  it  varies  only  about 
one  degree  below  or  above  the  standard,  37°  C,  notwithstanding 
the  changes  taking  place  in  the  temperature  around  us.  Thus 
we  can  live  in  any  climate,  however  cold  or  warm,  and  so  long  as 
our  body  temperature  remains  unaltered  we  suffer  no  immediate 
injury. 

There  is  a  limit,  however,  to  this  power  of  maintaining  a  uni- 
form standard  temperature.  If  a  mammal  be  kept  for  some  time 
in  a  moist  medium,  where  evaporation  cannot  take  place,  at  a  tem- 
perature but  little  higher  than  its  body,  say  over  45°  C,  its  tem- 
perature soon  begins  to  rise,  and  it  dies  with  the  signs  of  dyspnoea 
and  convulsions  (probably  from  the  nervous  centres  being  affected) 
when  its  temperature  arrives  at  43°-45°.  If  placed  in  water  at 
freezing  point  an  animal  loses  its  heat  quickly,  and  when  its  body 
temperature  has  fallen  to  about  20°  C.  it  dies  in  a  condition  re- 
sembling somnolence,  the  circulation  and  respiration  gradually 
failing. 

Since  a  variation  of  more  than  one  or  two  degrees  in  the  temr 


MAINTENANCE  OP  UNIFORM  TEMPERATURE.     433 

perature  of  our  bodies  interferes  with  the  vital  activities  of  the 
coDtrolliug  tissue  in  the  nervous  centres,  it  is  of  course  of  the 
utmost  importance  that  adequate  means  for  the  nice  regulation 
of  the  mean  temperature  of  our  bodies  should  exist. 

The  temperature  of  an  animal's  body  must  depend  on  the  re- 
lations existing  between  the  amount  of  heat  generated  in  the  tis- 
sues and  organs  and  the  amount  allowed  to  escape  at  the  surface, 
and  these  must  closely  correspond,  in  order  that  the  heat  of  the 
body  remain  uniform.  Both  these  factors  are  found  to  be  very 
variable.  Every  increase  in  the  activity  of  the  muscles,  liver, 
etc.,  causes  a  greater  production  of  heat,  while  a  fall  in  external 
temperature  or  increase  in  the  moisture  of  the  air  causes  a  greater 
escape  of  heat  from  the  surface. 

The  maintenance  of  uniform  temperature  may  then  be  accom- 
plished by  (1)  variations  in  the  heat  income,  so  arranged  as  to 
make  up  for  the  irregularities  of  expenditure,  or  (2)  variations 
in  the  loss  to  compensate  for  the  differences  of  heat  generated. 
Since  the  temperature  and  moisture  of  our  surroundings  are  con- 
stantly varying  between  tolerably  wide  limits,  the  amount  of  heat 
given  off  by  our  bodies  must  vary  greatly  at  different  times.  In 
cold,  damp  weather  a  great  quantity  of  heat  is  lost  in  comparison 
with  that  which  escapes  from  the  body  when  the  air  is  dry  and 
warm.  If  the  heat  generated  had  to  make  up  for  the  changes  in 
the  heat  lost,  one  would  expect  to  find  a  correspondingly  great 
difference  in  the  amount  of  heat  generated  at  different  times  of 
the  year,  and  no  doubt  we  have  some  evidence  in  the  keener  ap- 
petite and  consequent  use  of  more  fuel,  and  the  natural  tendency 
to  active  muscular  exertion  during  cold  weather,  to  show  that  a 
greater  amount  of  combustion  does  take  place  in  winter  than  in 
warm  summer  weather.  If  the  preservation  of  a  uniform  body 
temperature  depended  upon  the  variations  in  the  amount  of  in- 
come exactly  following  those  of  the  expenditure,  we  should  find 
it  impossible  to  set  our  muscular  or  glandular  tissue  in  action 
except  when  the  external  temperature  were  such  as  would  enable 
us  easily  to  get  rid  of  the  increased  heat  following  their  activity. 
It  certainly  would  appear  that  the  general  tissue  combustion,  as 
measured  by  the  amount  of  CO^  given  off,  does  increase  when  we 


434  MANUAL   OF   PHYSIOLOGY. 

are  placed  in  colder  surroundings — such  as  a  cold  bath  ;  still,  as 
will  presently  appear,  it  is  probable  that  the  variations  in  heat- 
incorae  have  but  little  regulating  influence  on  the  body  tempera- 
ture, and  if  they  have  any  we  are  certainly  ignorant  of  the  man- 
ner in  which  such  influence  is  carried  out. 

On  the  other  hand,  we  know  that  the  amount  of  heat-expenditure 
may  be  varied  by  mechanisms  which  are  almost  self-regulating. 
It  has  already  been  stated  that  the  great  majority  of  the  heat  is 
lost  by  the  parts  in  contact  with  the  air,  namely,  the  skin  and  air 
passages.  In  these  places  the  warm  blood  is  exposed  to  the  cool 
air,  and  therefore  loses  much  of  its  heat  by  radiation,  conduction, 
and  evaporation.  It  is  obvious  that  the  greater  the  quantity  of 
blood  thus  exposed  for  cooling,  the  greater  will  be  the  amount  of 
heat  lost  in  a  given  time  by  the  body  as  a  whole. 

If  we  review  the  circumstances  which  tend  to  interfere  with  the 
uniformity  of  the  temperature  of  the  body,  we  shall  see  that  each 
one  is  accompanied  by  certain  physiological  actions  which  tend 
to  compensate  for  the  disturbing  influences. 

The  chief  common  events  tending  to  make  our  temperature  ex- 
ceed or  fall  short  of  its  normal  standard  may  be  enumerated  as 
follows,  and  the  explanation  of  their  modes  of  compensation  will 
at  the  same  time  be  given  : 

Compensation  for  Internal  Variations. 

A  casual  increase  in  the  heat  income  may  be  induced  by  any 
increased  chemical  activity  in  the  tissues,  notably  the  action  of 
the  muscles  and  large  glands.  The  moment  this  increased  heat 
is  communicated  to  the  blood,  the  warm  blood  brings  about  the 
following  results  (partly  through  the  stimulation  of  certain  nerve 
centres) :  (a.)  An  acceleration  of  respiratory  movement,  which 
increases  the  amount  of  cold  air  to  be  warmed  and  saturated  with 
moisture  by  the  air  passages,  and  thus  facilitates  the  escape  of  the 
surplus  caloric,  (b.)  Relaxation  of  the  cutaneous  arterioles,  so 
that  a  greater  quantity  of  blood  is  exposed  to  the  cooling  influence 
of  the  air.  (c.)  Greater  rapidity  of  the  heart-beat,  by  which  a 
greater  quantity  of  blood  is  supplied  to  the  air  passages  and  to 


HEAT    REGULATION.  435 

the  surface  vessels,  (d.)  And  commonly  an  increase  in  the 
amount  of  sweat  poured  out  on  the  surface,  affording  opportunity 
for  greater  surface  evaporation.  As  an  example  of  these  points 
may  be  mentioned  active  muscular  exercise,  which  daily  expe- 
rience shows  us  is  always  accompanied  by  quick  breathing,  rapid 
heart's  action,  and  a  moist  skin.  The  increased  production  of 
heat  in  fever  gives  rise  to  the  same  results,  with  the  exception  of 
the  secretion  of  the  sweat,  the  want  of  the  secretion  (probably 
owing  to  the  toxic  inhibition  of  the  special  nerve  mechanisms  of 
the  glands)  is  a  deficiency  in  the  heat-regulating  arrangements, 
which  has  much  to  do  with  the  abnormally  high  temperature  of 
the  disease. 

When  a  lesser  quantity  of  heat  is  produced,  owing  to  inactivity 
of  the  heat-producing  tissues,  the  reverse  of  these  events  takes 
place,  namely  the  respiration  and  heart's  action  are  slow,  the  skin 
is  pale  and  dry,  so  that  little  heat  can  escape. 

Compensation  for  External  Variations  of  Temperature. 

AVhen  the  temperature  of  the  air  rises  much  above  the  average, 
the  escape  of  heat  is  correspondingly  hindered ;  and  when  the 
general  body  temperature  begins  to  rise  by  this  retention  of  calo- 
ric, we  have  the  sequence  of  events  detailed  in  the  last  paragraph  . 
as  being  caused  by  excessive  production  of  heat.  But  before  the 
blood  can  become  warmer  by  the  influence  of  the  increased  exter- 
nal temperature,  the  warm  air,  by  stimulating  the  skin,  brings 
about  certain  changes  independent  of  the  body  temperature  which 
satisfactorily  check  the  tendency  to  an  abnormal  rise.  This  can 
be  shown  by  the  local  application  of  external  heat,  by  means  of 
which  (a)  a  rush  of  blood  to  the  skin,  and  (6)  copious  sweat  secre- 
tion may  be  induced  in  a  part.  This  is  brought  about  by  impulses 
sent  directly  from  the  skin  to  the  centres  regulating  the  vasomo- 
tor and  secretory  mechanisms,  and  thus  causing  vascular  dilata- 
tion and  secretive  activity.  If  only  a  part  be  warmed,  only  a 
local  effort  is  made  to  cool  that  part,  and  this  has  but  little  influ- 
ence on  the  general  body  temperature. 

When,  however,  the  atmosphere  becomes  very  warm,  all  the 
cutaneous  vessels  dilate  simultaneously,  and  the  escape  of  heat  is 


436  MANUAL   OF   PHYSIOLOGY. 

greatly  increased  ;  while,  at  the  same  time,  so  much  blood  beiug 
occupied  in  circulating  through  the  skin,  the  deeper — heat-pro- 
ducing— tissues  are  supplied  with  less  blood,  and  therefore  gener- 
ate a  lesser  quantity  of  heat.  Thus  a  marked  rise  in  the  external 
temperature,  which  at  first  sight  would  seem  to  impede  the  escape 
of  heat  from  the  body,  really  facilitates  it,  by  causing,  through 
the  vascular  and  glandular  nerve  mechanisms  of  the  skin,  a  greater 
exposure  of  the  blood  to  the  cooler  air,  and  a  greater  quantity  of 
moisture  to  be  evaporated  from  the  warm  skin.  When  the  tem- 
perature of  the  air  reaches  that  of  the  body,  then  the  only  way 
of  disposing  of  the  heat  generated  in  the  body  is  by  evaporation, 
for  radiation  and  conduction  become  impossible.  In  animals  like 
man,  whose  cutaneous  moisture  is  so  great,  external  heat  seldom 
causes  marked  change  in  the  rate  of  breathing,  but  in  animals 
whose  cutaneous  secretion  is  limited,  external  heat  distinctly  affects 
their  respiratory  movements,  as  may  be  seen  by  the  panting  of  a 
dog  on  a  very  warm  day,  even  when  the  animal  is  at  rest. 

Almost  more  important  than  facilitating  the  escape  of  heat  in 
very  warm  weather,  are  the  arrangements  for  preventing  its  loss 
when  the  surroundings  ax'e  unusually  cold.  In  this  case  the  cold 
acting  as  a  stimulus  to  the  vaso-constrictor  nerve  agencies  of  the 
skin  causes  the  blood  to  retire  from  the  surface  and  fill  the  deeper 
organs,  where  more  heat  is  produced.  This  bloodless  skin  and 
the  underlying  fat  then  act  as  a  non-conducting  layer  or  boundary 
protecting  the  warm  blood  from  the  cooling  exposure.  At  the 
same  time  the  secretion  of  the  sweat  is  checked  by  a  special  nerve 
mechanism.  Here,  too,  the  cold  air,  which  would  soon  rob  the 
moist  surface  of  its  caloric,  checks  the  secretion  and  thereby  nul- 
lifies its  effects  in  this  direction,  and  enables  the  body  to  remain 
at  the  normal  standard  temperature. 

The  chief  factors  that  regulate  the  body  temperature  belong 
then  to  the  expenditure  department,  and  may  be  said  to  be — (a) 
variation  in  the  quantity  of  blood  exposed  to  be  cooled,  and  (6) 
variation  in  the  quantity  of  moisture  exposed  for  evaporation. 
These  regulators  have  to  compensate  not  only  for  differences  of 
external  temperature,  but  also  for  great  fluctuations  in  the  amount 
of  heat  produced  in  the  tissues. 

The  regulating  power  of  the  skin,  etc.,  appears  to  be  adequate 


HEAT    REGULATION.  437 

for  the  perfect  maintenance  of  uniform  temperature  only  within 
certain  limits.  When  the  limits  are  passed  by  the  rise  or  fall  of 
the  surrounding  medium,  the  preservation,  for  any  greath  length 
of  time,  of  a  perfectly  uniform  body  temperature  becomes  impos- 
sible. These  limits  vary  very  much  in  different  animals,  many 
of  which  have  special  coverings  protecting  them  from  external 
influences,  and  thus  retaining  their  warmth  for  all  their  lifetime 
in  a  temperature  seldom  above  0°  C  In  man  the  limits  vary 
much,  different  individuals  being  differently  affected  according  to 
many  circumstances,  e.g.,  in  both  extremes  of  age  the  limits  are 
narrowed.  It  would  appear  that  for  about  10°  C.  above  and 
below  the  body  temperature  our  skin-regulating  mechanisms  are 
adequate,  but  beyond  these  limits  external  changes  affect  our 
general  temperature,  and  if  continued  become  injurious.  Of 
course  by  imitating  with  clothing  the  natural  protection  with 
which  some  animals  are  endowed  we  can  aid  the  normal  regulating 
factors,  and  bear  much  greater  extremes  of  temperature  with  safety 
or  even  comfort. 

It  surprises  many  people  to  hear  that  their  bodies  are  always 
at  the  same  temperature,  no  matter  how  hot  or  cold  they  feel, 
but,  practically,  this  is  the  case,  for  our  sensations  of  being  hot  or 
cold  mean  simply  this;  when  we  feel  hot  our  cuti^neous  vessels 
are  full  of  warm  blood,  and  this  communicates  to  the  cutaneous 
nerve  terminals — the  sensory  nerves — the  sensation  of  general 
warmth.  On  the  other  hand,  when  the  cutaneous  vessels  are 
empty,  the  sensory  nerves  are  directly  affected  by  the  cold  of  the 
external  air.  Since  the  full  or  empty  state  of  the  vessels  of  the  skin 
depends  generally  on  the  heat  or  cold  of  the  air,  we  commonly 
speak  of  its  being  cold  and  ourselves  being  cold  as  synonymous 
terms.  But  we  can  make  ourselves  warm  by  violent  exercise  even 
on  a  frosty  day,  because  we  generate  so  much  heat  by  muscular 
action  that  the  cutaneous  vessels  have  to  be  dilated  in  order  to 
get  rid  of  the  surplus,  and  thereby  regulate  our  body  temperature, 
and  thus  we  have  the  sensation  of  being  warm.  Our  feelings 
when  we  say  we  are  warm  or  cold  simply  depend  upon  our  cuta- 
neous vessels  being  full  or  empty  of  warm  blood. 

The  local  appreciation  of  differences  of  temperature  will  be  dis- 
cussed under  the  sense  of  Touch. 


CHAPTER  XXV. 

CONTRACTILE  TISSUES. 

In  the  lower  forms  of  organisms  the  motions  executed  by  proto- 
plasm suffice  for  all  their  requirements.  Thus  the  amoeba  man- 
ages to  pass  through  its  entire  lifetime  with  no  other  kind  of 
motion  at  its  disposal  but  the  flowing  circulation  and  the  bud- 
ding out  of  its  soft  protoplasm.  A  vast  number  of  minute  organ- 
isms depend  wholly  upon  the  protoplasmic  stream  and  the  twitch- 
ing of  cilia  for  their  digestive  and  progressive  movements.  Before 
we  leave  the  class  of  animals  which  never  pass  beyond  the  uni- 
cellular stage,  we  find,  however,  examples  in  which  a  portion  of 
their  protoplasm  is  specially  adapted  to  the  performance  of  sud- 
den and  rapid  motions.  The  protoplasm  so  modified  in  function 
deserves  the  name  of  contractile  material.  Thus,  though  the 
protoplasm  which  lies  within  the  stalk  of  the  bell  animalcule  is 
morphologically  undifferentiated,  it  can  contract  with  such  rapid- 
ity that  the  eye  cannot  follow  the  motion. 

As  we  ascend  in  the  scale  of  animal  life,  the  necessity  of  having 
motions  of  varied  rapidity  and  duration  at  the  command  of  the 
animal  becomes  more  and  more  urgent,  and  so  we  find  not  only 
one,  but  several  kinds  of  tissue  specially  adapted  for  carrying  out 
motions  of  different  rate  and  duration. 

As  a  general  rule  the  more  rapid  the  contraction  it  performs 
the  more  the  tissue  differs  from  the  original  type  of  protoplasm  ; 
and  the  slow^er  and  more  persistent  the' contraction,  the  more  the 
tissue  elements  resemble  protoplasmic  cells.  Thus,  in  the  minute 
bloodvessels,  as  we  have  seen,  a  very  prolonged  form  of  contrac- 
tion, only  varied  by  partial  relaxations,  is  the  rule,  and  gives  rise 
to  the  tone  of  the  arterioles,  and  the  contractile  elements  differ 
but  little  from  ordinary  protoplasmic  cells.  The  intestinal  move- 
ments are  rapid  compared  with  those  of  the  arterial  muscles,  and 


HISTOLOGY  OF   MUSCLE. 


439 


in  thera  we  find  a  thin,  elongated  form  of  muscle  cell.     In  the 
heart  a  forcible  and  quick  contraction  takes  place,  which,  how- 
ever, is   slow   when    compared  with  the 
sudden  jerk  of  a  single  spasm  of  a  skeletal  •  ^"-  ^''^ 

muscle,  and  we  find  its  texture  is  different, 
being  a  form  intermediate  between  the 
slow-contracting  smooth  muscle  and  the 
quick-contracting  striated  skeletal  muscle. 

By  borrowing  examples  from  the  lower 
animals,  this  parallelism  of  structural  dif- 
ferentiation and  increase  of  functional 
energy  can  be  more  perfectly  demon- 
strated, and  we  can  make  out  a  gradual 
scale  of  increasingly  rapid  motion  cor- 
responding with  greater  complexity  of 
structure. 

The  contractile  tissues  of  the  human 
body  show  many  varieties  both  of  func- 
tional and  structural  differentiation. 

Histology  of  Muscle. 

The  term  muscle  includes  the  textures 
in  which  the  protoplasm  is  specially  dif- 
ferentiated for  purposes  of  contraction. 

The  muscle  tissues  of  the  higher  ani- 
mals may  be  divided  into  two  classes  ;  1, 
nonstriated  or  smooth,  and  2,  striated,  in 
which  again  there  are  some  slight  varia- 
tions. 

The  unstriated  muscle  tissue  is  that  in 
which  the  elements  are  most  like  contrac- 
tile protoplasmic  cells,  and  have  so  far 
retained  the  typical  form  as  to  be  easily 
recognizable  as  cells  when  separated  one 
from  the  other.  These  cells  are  more  or 
less  elongated,  flattefied,  homogeneous  ele- 
ments with  a  single,  long,  rod-like  nucleus 
and  no  cell  wall.   They  are  tightly  cemented  together  by  a  tough 


Muscle  cells,  showing 
difTerent  condition  of  the 
protoplasm  of  the  cell 
and  nucleus. 


440 


MANUAL   OF   PHYSIOLOGY. 


elastic  substance,  so  that  tlieir  tapering  extremities  fit  closely 
together  and  form  commonly  a  dense  mass  or  sheet.  Sometimes 
they  branch  more  or  less  regularly,  and  then  are  arranged  in 
networks. 

These  cells  vary  greatly  in  size  as  well  as  in  the  relation  of 
their  length  to  their  width,  iii  some  places  deserving  the  name 
fibres,  or  fibre-cells,  and  in  others  being  only  elongated  cells. 

The  striated  muscle  tissue  is  that  which  is  found  in  the  volun- 
tary skeletal  muscles  and  in  the  tissue  of  the  heart,  and  therefore 

forms  the  large  proportion  of  the  ani- 
mal, and  is  known  as  the  flesh.  The 
flesh  can  by  judicious  dissection  easily 
be  divided  into  single  parts  called 
muscles,  each  of  which  contains  many 
other  tissues  and  is  so  attached  as  to 
carry  on  certain  movements,  and  may, 
therefore,  be  regarded  as  an  organ. 

Such  a  muscle  is  inclosed  in  a  sheath 
of  connective  tissue,  from  which  sheet- 
like partitions  or  septa  pass  into  the 
mass  of  the  muscle  and  divide  it  into 
bundles  of  fibres,  which  they  inclose. 
These  septa  also  act  as  the  bed  in 
which  the  vessels  and  nerves  lie. 

The  bundles  of  fibres  of  skeletal 
muscle  vary  much  in  size,  giving  a 
coarse  or  fine  grain  in  different 
muscles ;  they  are  composed  of  a 
greater  or  less  number  of  fibres,  which 
lying  side  by  side  run  parallel  one  to  the  other.  The  single  fibres 
of  striated  muscle  vary  in  length,  sometimes  reaching  4-5  cm. 
(2  inches),  but  being  on  an  average  much  shorter,  so  that  they 
only  extend  the  entire  length  of  a  muscle  in  the  case  of  very 
short  muscles.  In  long  muscles  their  tapering  points  are  made 
to  correspond  with  those  of  other  fibres  to  which  they  are  firmly 
attached.  The  soft  fibres  are  pressed  by  juxtaposition  into  pris- 
matic forms,  so  that  in  a  fresh  condition  they  appear  polygonal 


Short  striated  cells  of  the 
heart  muscle,  separated  one 
showing  the  truncated  (a), 
or  divided  (c)  ends,  and 
branches  (6). 


STRUCTURE  OF   MUSCLE. 


441 


in  transverse  section.  When  freed 
they  become  cylindrical  and  the 
transverse  striation  of  the  contrac- 
tile substance  appears  regular,  and 
is  easily  recognized. 

Each  fibre  consists  of  a  delicate 
case  of  thin,  elastic,  homogeneous 
membrane,  forming  a  sheath  called 
sarcolemma,  within  which  the  es- 
sential contractile  substance  is  in- 
closed. The  soft  contractile  sub- 
stance completely  fills  and  distends 
the  elastic  sarcolemma,  so  that  when 
the  latter  is  broken  its  contents 
bulge  out  or  escape.  After  death, 
particularly  if  preserved  in  weak 
acid  (HCl),  the  striation  becomes 
more  marked,  and  the  dead  and 
now  rigid  contractile  substance  can 
be  easily  broken  up  into  transverse 
plates  or  disks. 

Besides  the  transverse  striation  a 
longitudinal  marking  can  be  seen  in 
the  muscle  fibre  which  indicates  the 
subdivision  of  the  contractile  sub- 
stance into  thin  threads  called 
primitive  fibrillse.  Each  primitive 
fibril  shows  a  transverse  marking, 
corresponding  with  the  transverse 
striation,  which  divides  the  fibrils 
into  short  blocks  called  sarcous,  or 
muscle  elements.  These  markings, 
as  well  as  the  transverse  striations  of 
the  muscle  fibre  in  general,  depend 
on  different  parts  of  the  contractile 
substance  having  different  powers 

37 


from  all  pressure  or  traction 
Fig.  179. 


Two  fibres  of  striated  muscle, 
in  which  the  contractile  sub- 
stance (m)  has  been  ruptured 
and  separated  from  the  sarco- 
lemma (a)  and  (s) ;  (p)  space 
under  sarcolemma.      (Kanvier.) 


442  MANUAL  OF  PHYSIOLOGY. 

of  refraction,  which  give  tlic  appearance  of  dark  and  light 
bands. 

In  the  muscle  fibre  are  found  long  granular  masses  like  proto- 
plasm ;  these  are  the  nuclei  of  the  contractile  substance.  They 
must  not  be  confounded  with  the  nuclei  of  the  sarcolemma,  which 
are  much  more  numerous  along  the  edge  of  the  fibre,  or  with  the 
other  short  nuclei  seen  in  such  numbers  between  the  fibres,  which 
indicate  the  position  of  the  capillary  vessels. 

It  is  stated  that  each  striated  muscle  fibre  has  a  nerve  fibre 
passing  directly  into  it,  but  the  exact  details  of  the  mode  of  union 
in  mammalia  are  not  yet  satisfactorily  made  out. 

Properties  of  Muscle  in  the  Passive  State. 

Consistence. — The  contractile  substance  of  muscle  is  so  soft  as 
to  deserve  rather  the  name  fluid  than  solid  ;  it  will  not  drop  as  a 
liquid,  but  its  separated  parts  will  flow  together  again  like  a  half- 
melted  jelly.  In  this  respect  it  resembles  the  protoplasm  of  ele- 
mentary organisms,  the  buds  from  which  are  so  soft  that  they  can 
unite  around  foreign  bodies,  and  yet  have  sufficient  consistence  to 
distinguish  them  from  fluid. 

Chemical  composition. — The  chemical  composition  of  the  con- 
tractile substance  of  muscle  in  the  living  state  is  not  accurately 
known.  The  death  of  the  tissue  is  accompanied  by  certain 
changes  of  a  chemical  nature  which  give  rise  to  a  kind  of  coagu- 
lation, resulting  in  the  formation  of  two  substances,  viz.,  muscle 
serum  and  muscle  clot  or  myosin.  This  coagulation  can  be  post- 
poned almost  indefinitely  in  the  contractile  substance  of  the 
muscles  of  cold-blooded  animals,  by  keeping  the  muscle  after  its 
removal  at  about  5°  C.  In  this  way  a  pale  yellow,  opalescent, 
alkaline  juice  may  be  pressed  out  of  the  muscle  and  separated  on 
a  cold  filter.  This  substance  turns  to  a  jelly  at  freezing  point, 
and  on  being  allowed  to  come  to  the  ordinary  temperature  of  the 
room  it  passes  through  the  stages  of  coagulation  seen  in  the 
contractile  substance  of  dead  muscle,  and  gives  the  same  fluid 
serum  and  clot  of  myosin.  Since  a  frog's  muscle  can  be  frozen 
and  thawed  without  the  tissue  being  killed,  it  is  supposed  that 


CHEMISTEY   OF   MUSCLE.  443 

the  thick  juice  is  really  the  contractile  substance,  and  it  has  been 
called  viuscle  j^lasma. 

The  coagulation  of  muscle  plasma  reminds  us  in  many  ways  of 
the  clotting  of  the  blood  plasma,  but  the  muscle  clot,  or  myosin, 
which  is  gelatinous  and  not  in  threads  like  fibrin,  is  a  globulin,  and 
is  soluble  in  ten  per  cent,  solution  of  salt.  It  is  readily  changed 
into  syntoniu  or  acid  albumin,  and  forms  the  preponderant  albu- 
minous substance  of  muscle. 

The  serum  of  dead  muscle  has  a  distinctly  acid  reaction,  and 
contains  three  distinct  albuminous  bodies  coagulating  at  different 
temperatures,  one  of  which  is  serum-albumin,and  another  a  derived 
albumin,  potassium-albumin.  The  serum  of  muscle  also  contains : 
(1.)  Kreatin,  kreatinin,  xanthin,  etc.  (2.)  Haemoglobin.  (3.) 
Grape  sugar,  muscle  sugar,  or  inosit,  and  glycogen.  (4.)  Sarco- 
lalic  acid  made  from  the  inosit  by  fermentation.  (5.)  Carbonic 
acid.  (6.)  Potassium  salts ;  and  (7.)  75  per  cent,  of  water.  Traces 
of  pepsin  and  other  ferments  have  also  been  found. 

Chemical  change. — In  the  state  of  rest  a  certain  amount  of 
chemical  change  constantly  goes  on,  by  which  oxygen  is  taken 
from  the  haemoglobin  of  the  blood  in  the  capillaries,  and  carbonic 
acid  is  given  up  to  the  blood.  These  changes  seem  necessary  for 
the  nutrition,  and  therefore  the  preservation  of  the  life  and  active 
powers  of  the  tissue,  because  if  a  muscle  after  removal  be  placed 
in  an  atmosphere  free  from  oxygen,  it  soon  loses  its  chief  vital 
character,  viz.,  its  irritability. 

Elasticity. — Striated  muscle  is  easily  stretched,  and,  if  the  ex- 
tension be  not  carried  too  far,  recovers  very  completely  its  original 
length.  We  say  then  that  the  elasticity  of  muscle  is  small  or 
weak,  but  very  perfect.  When  the  muscle  is  stretched  to  a  given 
extent  by  a  weight — say  of  one  gramme — if  another  gramme  be 
then  added  it  will  not  stretch  the  muscle  so  much  as  the  first 
did ;  and  so  on  if  repeated  gramme  weights  be  added  one  after 
the  other,  each  succeeding  gramme  will  cause  less  extension  of 
the  muscle  than  the  previous  one ;  so  that  the  more  a  muscle  is 
stretched  the  more  force  is  required  to  stretch  it  to  the  given  ex- 
tent, or,  in  other  words,  the  elastic  force  of  muscle  increases  with 
its  extension. 


444 


MANUAL   OF   PHYSIOLOGY. 


If  a  tracing  be  drawn  showing  the  extending  effect  of  a  series 
of  equal  weights  attached  to  a  fresh  muscle,  it  will  be  found  that 
a  great  difference  exists  between  it  and  a  similar  record  drawn  by 
inorganic  bodies  or  an  elastic  band  of  rubber. 

When  a  weight  is  applied  to  a  muscle,  it  does  not  immediately 
stretch  to  the  full  extent  the  weight  is  capable  of  effecting,  but  a 
certain  time,  which  varies  with  circumstances,  is  allowed  for  its 
complete  extension.  The  rate  of  extension  is  at  first  rapid,  then 
slower,  until  it  ceases.  As  a  muscle  loses  its  powers  of  contrac- 
tion from  fatigue,  it  becomes  more  easily  extended.     Dead  muscle 

Fig.  180. 


1.  Shows  graphically  the  amount  of  extension  caused  by  equal  weight 
increments  applied  to  a  steel  spring.  2.  Shows  graphically  the  amount  of 
extension  caused  by  equal  weight  increments  applied  to  an  india-rubber 
band.  3.  The  same  applied  to  a  frog's  muscle.  Showing  the  decreasing 
increments  of  extension,  the  gradual  continuing  stretching,  and  the  failure 
to  return  to  the  abscissa  when  the  weight  is  removed. 


has  a  greater  but  less  perfect  elasticity  than  living,  i.e.,  it  requires 
greater  force  to  stretch  it,  but  does  not  return  so  perfectly  to  its 
former  shape.  The  importance  of  the  elastic  property  of  muscle 
in  the  movements  of  the  body  is  noteworthy.  The  muscles  are 
always  in  some  degree  on  the  stretch  (as  can  be  seen  in  a  frac- 
tured patella,  the  fragments  of  which  remain  far  apart  and  cause 
the  surgeon  much  anxiety),  and  brace  the  bones  together  like  a 


ELECTRIC   PHENOMENA   OF   MUSCLE. 


445 


series  of  springs,  the  various  skeletal  muscles  being  so  arranged 
as  to  stretch  others  by  their  contraction.  When  one  muscle — for 
example,  the  biceps — contracts,  it  finds  an  elastic  antagonist 
already  tense;  this  it  has  to  stretch  as  it  shortens.  The  triceps 
thus  acts  as  a  weak  spring,  opposing  the  biceps,  and  it  gently  returns 
to  its  natural  length  when  the  contraction  of  the  biceps  ceases. 
By  their  mere  elasticity  the  muscles  are  kept  tense  and  ready  for 
action,  and  have  to  act  against  a  gentle  spring-like  resistance,  so 
that  the  motions  are  even,  and  there  is  no  jerking,  as  would  occur 


Fig.  18L 


■''jJ^Mjju^..,^^ 


Non-polarizable  Electrodes.  The  glass  tubes  (a  o)  contain  sulphate  of 
zinc  solution  (z.  s),  into  which  well  amalgamated  zinc  rods  dip.  The  lower 
extremity  is  plugged  with  china  clay  (c/t.  c),  which  protrudes  at  (c'')  the  point. 
The  tubes  can  be  moved  in  the  holders  (A  h),  so  as  to  be  brought  accurately 
into  contact  with  the  muscle.     (Foster.) 

if  the  attachments  of  the  inactive  muscles  were  allowed  to  become 
slack. 

Electric  Phenomena. — In  a  living  muscle  electric  currents  may 
be  detected,  having  a  definite  direction,  and  certain  relations  to 
the  vitality  of  the  tissue.  As  they  seem  to  be  invariably  present 
in  a  passive  muscle,  they  have  been  called  natural  muscle- currents. 

They  are  generally  studied  in  the  muscles  of  cold-blooded  ani- 
mals after  removal  from  the  body.  The  muscle  is  spoken  of  as  if 
it  were  a  cylinder  with  longitudinal  and  transverse  surfaces  cor- 
responding to  its  natural  surface  and  its  cut  extremities.     In  such 


446  MANUAIi   OF   PHYSIOLOGY. 

a  block  of  frog's  muscle  the  measurement  of  the  electric  currents 
requires  considerable  care,  because  they  are  so  difficult  to  detect 
that  a  most  sensitive  galvanometer  must  be  used  ;  and  such  an 
instrument  can  easily  be  disturbed  by  currents  due  to  bringing 
metal  electrodes  into  contact  with  the  moist  saline  tissues.  Spe- 
cially constructed  electrodes  must  be  used  to  avoid  these  currents  of 
polarization  taking  place  in  the  terminals  touching  the  muscle. 
These  are  called  non-polarizable  electrodes,  and  may  be  made  on 
the  following  plan  :  Some  innocuous  material  moistened  in  saline 
solution  (.65  per  cent.)  is  brought  into  direct  contact  with  the  mus- 
cle, and,  by  means  of  saturated  solution  of  zinc  sulphate,  into  elec- 
trical connection  with  amalgamated  zinc  terminals  from  the  gal- 
vanometer. Thus  the  muscle  is  not  injured,  and  the  zinc  solution 
prevents  the  metal  terminals  from  producing  adventitious  currents. 
Small  glass  tubes  drawn  to  a  point,  the  opening  of  which  is 
plugged  with  moist  china  clay,  make  a  suitable  receptacle  for  the 
zinc  solution,  or,  instead  of  the  china  clay,  a  camel-hair  brush 
set  in  plaster  of  Paris  may  be  used  to  keep  the  zinc  solution  in 
the  tube,  and  the  hair  moistened  in  salt  solution  forms  a  suitable 
point  of  contact  with  the  muscle.  If  a  pair  of  such  electrodes  be 
applied  to  the  middle  of  the  longitudinal  surface  at  (e)  (Fig.  182), 
and  of  the  transverse  surface  at  (p)  respectively,  and  then  be 
brought  into  connection  with  a  delicate  galvanometer,  it  is  found 
that  a  current  passes  through  the  galvanometer  from  the  longitu- 
dinal to  the  transverse  surface.  A  current  in  this  direction  can 
be  detected  in  any  piece  of  muscle,  no  matter  how  much  it  be 
divided  longitudinally,  and  probably  would  be  found  in  a  single 
fibre  had  we  the  means  of  examining  it.  The  nearer  to  the  centre 
of  the  longitudinal  and  transverse  sections  the  electrodes  are 
placed,  the  stronger  will  be  the  current  received  by  them.  If 
both  the  electrodes  be  placed  on  the  longitudinal  or  on  the  trans- 
verse surfaces,  a  current  will  pass  through  the  galvanometer  from 
that  electrode  nearer  the  middle  of  the  longitudinal  section  (called 
the  equator  of  the  muscle  cylinder)  to  the  electrode  nearer  the 
centre  of  the  transverse  section  (pole  of  muscle  cylinder).  If  the 
electrodes  be  placed  equidistant  from  the  poles  or  from  the  equator, 
no  current  can  be  detected. 


NATURAL  MUSCLE-CURRENT. 


447 


The  central  part  of  the  longitudinal  surface  of  a  piece  of  muscle 
is  then  positive,  compared  with  the  central  part  of  the  extremities 
or  transverse  sections.  And  between  these  parts,  the  equator  and 
poles  of  the  muscle  cylinder,  where  the  difference  is  most  marked, 
are  various  gradations,  so  that  any  point  near  the  equator  is  posi- 
tive when  compared  with  one  near  the  poles. 

There  is,  then,  a  current  passing  through  the  substance  of  the 

Fia.  182. 


Diagram  to  illustrate  the  currents  in  muscle. — (e)  Equator,  corresponds 
to  the  centre  of  the  muscle  ;  (jy)  Polar  regions  of  cylinder,  representing  the 
extremities  of  the  muscle.  The  arrowheads  show  the  direction  of  the  sur- 
face currents,  and  the  thickness  of  lines  indicates  the  strength  of  the  currents. 
(After  Fick.) 

piece  of  muscle  from  both  the  transverse  sections  or  extremities 
of  the  muscle  block  to  the  middle  of  the  longitudinal  surface, 
whether  it  be  a  cut  surface  (longitudinal  section). or  the  natural 
surface  of  the  muscle.  This  is  called  the  muscle-current,  or  some- 
times natural  muscle-current. 

If  the  cylinder  in  the  accompanying  figure  be  taken  to  repre- 
sent a  block  of  muscle,  e  would  correspond  to  the  equator,  and  p 
to  the  poles,  and  the  arrowheads  show  the  direction  of  the  cur- 


448  MANUAL   OF   PHYSIOLOGY. 

rents  passing  through  the  galvanometer,  the  thickness  of  the  lines 
indicating  their  force.  The  clotted  lines  o  are  connected  with 
points  where  the  electro-motive  force  is  equal,  and  therefore  no 
current  exists. 

The  electro-motive  force  of  the  muscle-current  in  a  frog's  gra- 
cilis has  been  estimated  to  be  about  .05-08  of  a  Daniell  cell.  It 
gradually  diminishes  as  the  muscle  loses  its  vital  properties,  and 
is  also  reduced  by  fatigue.  The  electro-motive  force  rises  with 
the  temperature  from  5°  C.  until  a  maximum  is  reached  at  about 
the  body  temperature  of  mammals. 

These  muscle-currents  are  very  weak  if  the  uninjured  muscle 
be  examined  in  situ,  the  tendon  being  used  as  the  transverse  sec- 
tion ;  they  soon  become  more  marked  after  the  exposure  of  the 
muscle,  and  if  the  tendon  be  injured  they  appear  at  once  in  almost 
full  force.  In  animals  quite  inactive  from  cold  the  muscles  nat- 
urally are  but  slowly  altered  by  exposure,  etc.,  and  the  muscle- 
currents  do  not  appear  for  a  considerable  time,  which  is  shortened 
on  elevating  the  temperature.  It  has,  therefore,  been  supposed 
that  in  the  perfectly  normal  state  of  a  living  animal  there  are 
no  muscle-currents  so  long  as  the  muscle  remains  in  the  passive 
state. 

Active  State  of  Muscle. 

A  muscle  is  capable  of  changing  from  the  passive  elongated 
condition,  the  properties  of  which  have  just  been  described,  into 
a  state  of  contraction  or  activity.  Besides  the  change  in  form, 
obvious  in  the  contracted  state  of  the  muscle,  its  chemical,  elastic, 
electric,  and  thermic  properties  are  altered.  The  capability  of 
passing  into  this  active  condition  is  spoken  of  as  the  irritability 
of  muscle.  This  is  directly  dependent  upon  its  chemical  condi- 
tion, and  therefore  related  to  its  nutrition  and  to  the  amount  of 
activity  recently  exerted,  which,  it  will  hereafter  appear,  changes 
its  chemical  state. 

Under  ordinary  circumstances,  during  life,  the  muscles  change 
from  the  passive  state  into  that  of  contraction  in  response  to  cer- 
tain impulses  communicated  to  them  by  nerves,  which  carry  im- 
pressions from  the  brain  or  spinal  cord  to  the  skeletal  muscles. 


ACTIVE   STATE   OF   MUSCLE.  449 

The  influence  of  the  will  is,  then,  the  common  stimulus  which 
excites  most  skeletal  muscles  to  action.  But  we  find  that  there 
are  many  other  iufluences  which,  when  applied  to  a  muscle,  can 
also  bring  about  the  same  change.  These  iufluences  are  culled 
stimuli. 

We  commonly  utilize  a  nerve  belonging  to  a  muscle  in  order 
to  throw  it  into  the  contracted  state,  but  the  great  majority  of 
stimuli  can  bring  about  the  change  wheu  applied  to  the  muscle 
directly.  Since  the  nerves  branch  in  the  substance  of  the  muscle, 
and  are  distributed  to  the  individual  fibres,  it  might,  as  has  been 
argued,  be  the  stimulation  of  the  terminal  nerve  ramifications 
that  brings  about  the  contraction,  even  when  the  stimulus  is  ap- 
plied to  the  muscle  directly,  for  the  nerves,  of  course,  would  be 
affected  by  the  stimulus  applied  to  the  muscle.  That  muscles 
can  be  stimulated  without  the  intervention  of  nerves  is  satisfac- 
torily proved  by  the  following  facts:  1.  Some  parts  of  muscles, 
such  as  the  lower  end  of  the  sartorius,  and  many  muscular  struc- 
tures which  have  no  nerve  terminals  in  them,  respond  energeti- 
cally to  all  kinds  of  muscle-stimuli.  2.  There  are  some  substances 
which  act  as  stimuli  when  applied  directly  to  muscle,  but  have 
no  such  effect  wheu  applied  to  nerves,  viz.,  ammonia.  3.  For 
some  time  after  the  nerve  has  ceased  to  react,  on  account  of  its 
dying  after  removal  from  the  body,  the  attached  muscle  will  be 
found  quite  irritable  if  directly  stimulated.  4.  The  arrow-poison, 
Curara,  has  the  extraordinary  effect  of  paralyzing  the  nerve  ter- 
minals, so  that  the  strongest  stimulation  of  the  nerve  calls  forth 
no  muscle  contraction.  If  the  muscles  in  an  animal  under  the 
influence  of  this  poison  be  stimulated  directly,  they  respond  with 
a  contraction. 

This  separation  of  muscles  from  nerves  would  appear  rather 
artificial,  and  antagonistic  to  the  teachings  of  the  development 
of  these  tissues,  both  in  the  ascending  scale  of  the  animal  king- 
dom and  in  the  individual. 

Muscle-Stimuli. 
The  circumstances  which  call  forth  muscle  contraction  may  be 

enumerated  thus : 

38 


450  MANUAL   OF   PHYSIOLOGY. 

1.  Mechanical  Slimulation . — Any  sudden  blow,  pinch,  etc.,  of 
a  living  muscle  causes  a  momentary  contraction,  which  rapidly 
passes  off"  when  the  irritation  is  removed. 

2.  Thermic  Stiv}ulatio7i. — If  a  frog's  muscle  be  warmed  to  over 
30°  C.  it  will  begin  to  contract,  and  before  it  reaches  40"  C.  the 
muscle  will  pass  into  a  condition  known  as  heat  rigor,  which  will 
be  mentioned  presently.  If  the  temperature  of  a  muscle  be  re- 
duced by  0°  C,  it  shortens  before  it  becomes  frozen. 

3.  Chemical  Stimulaiioyi. — A  number  of  chemical  compounds 
also  act  as  stimuli  when  they  are  applied  to  the  transverse  section 
of  a  divided  muscle.  Among  these  may  be  named — (1)  the  min- 
eral acids  (HCl,  .1  per  cent.)  and  many  organic  acids;  (2)  salts 
of  iron,  zinc,  silver,  copper,  and  lead;  (3)  the  neutral  salts  of  the 
alkalies  of  a  certain  strength  ;  (4)  weak  glycerin  and  weak  lactic 
acid,  which  only  excite  nerves  when  concentrated  ;  (5j  bile  also 
is  said  to  stimulate  muscle  in  much  weaker  solutions  than  it  will 
nerve  fibres. 

4.  Electric  Stiinulatioti. — Electricity  is  the  most  convenient  form 
of  stimulation,  because  we  can  accurately  regulate  the  force  of 
the  stimulus.  The  occurrence  of  any  variation  in  the  intensity 
of  an  electric  current  passing  through  a  muscle  causes  it  to  con- 
tract. The  sudden  increase  or  decrease  in  the  strength  of  a  cur- 
rent acts  as  a  stimulus,  but  a  current  of  exactly  even  intensity 
may  be  made  to  pass  through  a  muscle  without  exciting  any  con- 
traction. The  common  method  employed  is  that  of  opening  or 
closing  a  circuit  of  which  the  muscle  forms  a  part,  so  as  to  make 
or  break  the  current;  and  thus  a  variation  of  intensity,  equal  to 
the  entire  strength  of  the  current  used,  takes  place  in  the  muscle, 
and  acts  as  a  stimulus. 

The  irritability  of  muscle  substance  is  not  so  great  as  that  of 
the  motor  nerves,  that  is  to  say,  a  slight  stimulus  will  make  the 
muscle  contract  when  applied  to  its  nerve,  while  the  same  stimulus 
will  have  no  effect  if  applied  to  the  muscle  directly.  In  experi- 
menting on  the  contraction  of  muscle,  as  already  stated,  the  in- 
tervention of  the  nerve  is  commonly  used,  the  stimulus,  by  means 
of  an  electric  current  applied  to  the  nerve,  being  more  conve- 
niently and  completely  distributed  to  the  muscle  than  when  ap- 
plied directly. 


MUSCLE-STIMULI. 


451 


The  current  of  a  battery  may  be  used  to  stimulate  a  muscle,  but 
an  induced  current  is  more  commonly  employed  on  account  of  the 
greater  efficacy  of  its  action.  The  instrument  in  ordinary  use  in 
physiological  laboratories  is  Du  Bois-Reymond's  inductorium,  in 
which  the  strength  of  the  stimulus  can  be  reduced  by  removal 


Du  Bois-Keymond's  Inductorium  with  Magnetic  Interrupter. — c.  Primary 
coil  through  which  the  primary,  inducing,  current  passes,  on  its  way  through 
the  electro-magnet  (6).  i.  Secondary  coil,  which  can  be  moved  nearer  to 
or  further  from  the  primary  coil  (c),  thereby  allowing  a  stronger  or  weaker 
current  to  be  induced  in  it.  This  induced  current  is  the  stimulating  one. 
b.  Electro-magnet,  which,  on  receiving  the  current,  breaks  the  contact  in 
the  circuit  of  the  primary  coil  by  pulling  down  the  iron  hammer  (A),  and 
separating  the  spring  from  the  screw  of  e.  When  it  brings  the  spring  in 
contact  with  the  point  of  the  pillar  (a),  it  also  demagnetizes  itself  by  "  short- 
circuiting  "  the  battery.  When  tetanus  is  to  be  produced,  the  wires  from 
the  battery  are  to  be  connected  with  g  and  d.  When  a  single  contraction 
is  required,  the  magnetic  interrupter  is  cut  out  by  shifting  the  wire  from  a 
to  the  binding  screw  to  the  right  of/. 


of  the  secondary  coil,  and  which  is  supplied  with  a  magnetic  in- 
terrupter, by  means  of  which  repeated  stimuli  may  be  given. 
(See  Fig.  183.) 


452  MANUAL   OF    PHYSIOLOGY. 


Changes  occurring  in  Muscle  on  its  entering  the 
Active  State. 

Changes  in  Strudnre. — The  examiuation  of  muscle  with  the 
microscope  during  its  contractiou  is  atteuded  with  considerable 
difficulty,  and  in  the  higher  animals  has  not  led  to  satisfactory 
results.  In  the  muscles  of  insects,  where  the  differentiation  of 
the  contractile  substance  is  more  complicated,  certain  changes 
can  be  observed.  The  fibres,  and  even  the  fibriilse  within  them, 
can  easily  enough  be  seen  to  undergo  changes  in  form  correspond- 
ing to  those  of  the  entire  muscle,  namely,  increase  in  thickness 
and  diminution  in  length.  A  change  in  the  position  and  relative 
size  of  the  singly  and  doubly  refracting  portions  of  the  muscle 
element  has  been  described,  and  some  authors  state  that  the  lat- 
ter increases  at  the  expense  of  the  former  after  an  intermediate 
period  in  which  the  two  substances  seem  fused  together. 

Chemical  Changes. — During  the  contracted  condition  the  chem- 
ical changes  which  go  on  in  passive  muscle  are  intensified,  and 
certain  new  chemical  decompositions  arise,  of  which,  however, 
not  much  is  known. 

Active  muscle  takes  up  more  oxygen  than  muscle  at  rest,  as  is 
shown  by  the  facts  that,  during  active  muscular  exercise,  more 
oxygen  enters  the  body  by  respiration,  and  the  blood  leaving  active 
muscles  is  poorer  in  oxygen  than  when  the  same  muscles  are 
passive.  This  absorption  of  oxygen  cannot  be  detected  in  a 
muscle  cut  out  of  the  body,  nor  is  any  supply  of  oxygen  neces- 
sary for  a  contraction  of  such  a  muscle,  since  a  frog's  muscle  will 
contract  in  an  atmosphere  containing  no  oxygen.  From  this  it 
would  appear  that  a  certain  ready  store  of  oxygen  must  exist  in 
some  chemical  constituent  of  the  muscle  substance;  and  it  is  pos- 
sible that  some  chemical  compound,  which  is  con-*tantly  renewed 
by  the  blood  existing  in  the  muscle,  is  its  normal  source  of  oxygen, 
and  not  the  oxyhsemoglobin  of  the  blood. 

The  amount  of  CO.^  given  off  by  a  muscle  increases  in  its  state 
of  a*  tivity,  as  may  be  seen  by  the  greater  elimination  from  the 
lungs  during  active  muscular  exercise,  and  by  the  fact  that  the 


CHEMICAL   CHANGES   DURING   CONTRACTION.  453 

venous  blood  of  a  limb,  when  the  muscles  are  contracted,  con- 
tains more  COj  than  when  they  are  relaxed.  The  increase  of  CO^ 
can  also  be  detected  in  a  muscle  removed  from  the  body  and  kept 
in  a  state  of  contraction.  Moreover,  this  increase  in  the  forma- 
tion of  COj  in  a  muscle  takes  place  whether  there  is  a  new  sup- 
ply of  oxygen  given  to  it  or  not,  and  the  quantity  of  CO.^  given 
off  always  greatly  exceeds  the  quantity  of  oxygen  that  is  used 
up.  So  that  it  is  not  exclusively,  if  at  all,  from  the  newly-sup- 
plied oxygen  that  the  CO.^  is  produced. 

Muscle  tissue,  when  passive,  is  neutral  or  faintly  alkaline; 
during  contraction,  however,  it  becomes  distinctly  acid.  The  lit- 
mus which  it  changes  from  blue  to  red  is  permanently  altered, 
and  we  can,  therefore,  conclude  that  CO.^  is  not  the  only  acid  that 
makes  its  appearance-  The  other  acid  is  sarcolaotlc  acid,  which 
is  constantly  present  in  muscle  after  prolonged  contriction,  and 
varies  in  amount  in  proportion  to  the  degree  of  activity  the 
muf^cle  has  undergone.  It  therefore  varies  directly  with  the  CO,, 
which  would  seem  to  suggest  a  relationship  between  the  origin  of 
the  two  acids. 

The  amount  of  glycogen  and  grape  sugar  is  said  to  diminish  in 
muscle  during  its  activity,  and  it  is  stated  that  sarcolactic  acid 
can  be  produced  from  these  carbohydrates  by  the  action  of  cer- 
tain ferments. 

Active  muscle  contains  more  substances  than  can  be  extracted 
by  alcohol,  and  less  that  are  soluble  in  water  than  passive  muscle. 

The  chemical  changes  which  take  place  during  muscle  contrac- 
tion are  probably  the  result  of  a  decomposition  of  some  carbo- 
hydrates, in  which  the  albuminous  substances  do  not  take  any  part 
that  requires  their  own  destruction.  This  seems  supported  by  the 
fact  that  the  increased  gas  exchange  in  muscle  during  active  ex- 
ercise can  be  recognized  in  a  corresponding  change  in  the  gas 
exchange  in  pulmonary  respiration  ;  and,  moreover,  there  seems 
no  relation  between  muscular  labor  and  the  amount  of  nitro- 
genous waste,  as  estimated  by  the  urea  elimination,  which  one 
would  expect  if  muscular  activities  were  the  outcome  of  a  de- 
composition of  the  nitrogenous  (albuminous)  parts  of  the  muscle 
substance. 


454  MANUAL   OF   PHYSIOLOGY. 

The  chemical  changes  which  are  commonly  said  to  take  place 
in  muscle  during  its  contraction  are: 

1.  The  contractile  substance,  which  is  normally  neutral  or 
faintly  alkaline,  becomes  acid  in  reaction,  owing  to  the  formation 
of  sarcolactic  acid. 

2.  More  oxygen  is  taken  up  from  the  blood  than  in  the  muscle 
at  rest.  This  using  up  of  oxygen  occurs  also  in  the  isolated 
muscle,  and  its  amount  appears  to  be  independent  of  the  blood 
supply. 

3.  The  extractives  soluble  in  water  decrease,  those  soluble  in 
alcohol  increase. 

4.  A  greater  amount  of  CO^  is  given  off,  both  in  the  isolated 
muscle  as  well  as  in  the  muscles  in  the  body,  and  the  change  in 
the  quantity  of  CO^  has  no  exact  relation  to  that  of  the  oxygen 
used. 

5.  A  diminution  is  said  to  occur  in  the  contained  glycogen,  and 
certainly  prolonged  inactivity  causes  an  increase  in  the  amount 
of  glycogen. 

6.  A  peculiar  muscle-sugar  makes  its  appearance. 

Change  in  Elasticity. — The  elasticity  of  a  muscle  during  its 
state  of  contraction  is  less  than  when  it  is  in  the  passive  state. 
That  is  to  say,  that  a  given  weight  will  extend  the  same  muscle 
more  if  attached  to  it  while  contracted  (as  in  tetanus)  than  when 
it  is  relaxed.  The  contracted  muscle  is  then  more  extensible. 
If,  then,  a  weight  which  is  just  over  the  maximum  load  the  mus- 
cle can  lift,  be  hung  from  it  and  the  muscle  then  stimulated,  it 
should  become  extended,  because  the  change  to  the  active  state 
lessens  its  elastic  power,  while  it  cannot  contract,  being  over- 
weighted. 

Electrical  Changes. — If  a  muscle,  in  connection  with  a  galvano- 
meter, so  as  to  show  the  natural  current,  be  stimulated  by  means 
of  the  nerves,  a  marked  change  occurs  in  the  current.  The  gal- 
vanometric  needle  swings  towards  zero,  showing  that  the  current 
is  weakened  or  destroyed.  This  is  called  the  negative  variation 
of  the  muscle  current  which  initiates  the  change  to  the  active 
condition.  When  the  muscle  receives  but  a  momentary  stimulus 
so  as  only  to  give  a  single  contraction,  this  negative  variation 


CHANGES   IN   ELECTRICAL   STATE.  455 

takes  place  in  the  current,  but,  owing  to  its  extremely  short  dura- 
tion, the  galvanometric  needle  is  prevented  by  its  inertia  from 
following  the  change.  Only  the  most  sensitive  and  well-regulated 
instruments  show  the  electric  change  of  a  single  contraction,  but 
when  the  muscle  is  kept  contracted  by  a  series  of  rapidly  re- 

FiG.  184, 


Diagram  illustrating  the  arrangement  in  the  Rheoscopic  Frog. — a  = 
stimulating  hmh.  b  =  stimulated  limb.  The  current  from  the  electrodes 
passes  into  nerve  (n)  of  stimulating  limb  (a),  causing  its  gastrocnemius  to 
contract.  Whereupon  the  negative  variation  of  the  natural  current  between 
+  and  — stimulates  the  nerve  (n^),  and  excites  the  muscles  of  b  to  action. 

peated  stimulations  then  the  inertia  of  the  needle  is  readily  over- 
come. The  negative  variation  of  a  single  contraction  can,  how- 
ever, be  easily  shown  on  the  sensitive  animal  tissues.  For  this 
purpose  the  nerve  of  one  nerve-muscle  preparation*  is  placed 
upon  the  surface  of  another  muscle  so  as  to  pass  over  the  middle 
of  the  transverse  and  longitudinal  sections.  Then  the  second 
(stimulating)  muscle  is  made  to  contract,  the  negative  variation 
acts  as  a  stimulus  to  the  nerves  lying  on  it,  and  so  the  first  (stim- 
ulated) muscle  contracts.  Not  only  does  this  show  the  negative 
variation  of  a  single  contraction,  but  it  also  demonstrates  that 

*  By  a  nerve-muscle  preparation  is  meant  a  muscle  of  a  frog  (commonly 
the  ga.strocnemiiis  and  the  half  of  the  femur  to  which  it  is  attached)  and 
its  nerve  which  has  been  carefully  separated  from  other  parts  and  removed 
from  the  body. 


456  MANUAL   OF    PHYSIOLOGY. 

the  continued  (tetanic)  contraction  produced  by  repeated  stimu- 
lation is  associated  with  repeated  negative  variations.  Because 
the  contraction  of  the  stimulated  muscle  whose  nerve  lies  oq  the 
stimulating  muscle  follows  exactly  all  the  variations  of  the  stim- 
ulator, and  is  kept  contracted  as  long  as  the  other  is  contracted, 
and,  as  we  shall  see  presently,  the  continued  contraction  can  only 
be  brought  about  by  a  rapidly  repeated  series  of  stimulations,  so 
that  the  electric  condition  of  the  stimulating  muscle  must  undergo 
a  series  of  variations. 

If  an  isolated  part  of  a  muscle  be  stimulated  the  contraction 
passes  from  that  point  as  a  wave  to  the  remainder  of  the  muscle. 
This  contraction  wave  is  preceded  by  a  wave  of  negative  varia- 
tion, which  passes  along  the  muscle  at  the  rate  of  3  metres  per 
second  (the  same  rate  as  the  contraction  wave,  see  under),  lasting 
at  any  one  point  .003  of  a  second,  so  that  the  negative  variation 
is  over  before  the  contraction  begins,  for  the  muscle  requires  a 
certain  time,  called  the  latent  period,  before  it  commences  to 
contract. 

The  origin  of  the  electric  currents  of  muscle  will  be  discussed 
with  nerve-currents,  to  which  the  reader  is  n  ferred  (p.  504). 

Temperature  Change. — Long  since  it  was  observed  in  the  human 
subject  that  the  temperature  of  muscles  rose  during  their  activity. 
In  frogs'  muscle  a  contraction  lasting  three  minutes  caused  an 
elevation  of  .18°  C.  And  a  single  contraction  is  said  to  produce 
a  rise  varying  from  .001°  to  .005°  C,  according  to  circumstances. 

The  production  of  heat  is  iu  proportion  to  the  tension  of  the 
muscle.  When  the  muscles  are  prevented  from  shortening  a 
greater  amount  of  heat  is  said  to  be  produced. 

The  amount  of  heat  has  also  a  definite  relation  to  the  work  per- 
formed. Up  to  a  certain  point  the  greater  the  load  a  muscle  has 
to  move,  the  greater  the  heat  produced;  when  this  maximum  is 
reached  any  further  increase  of  the  weight  causes  a  falling  off 
in  the  heat  production.  Repeated  single  contractions  are  said 
to  produce  more  heat  than  tetanus  kept  up  for  a  corresponding 
time. 

The  fatigue  which  follows  prolonged  activity  is  accompanied  by 
a  diminution  iu  the  temperature  elevation. 


MUSCLE   CONTRACTION.  457 


Muscle  Contraction. 

Change  in  form. — The  most  obvious  change  a  muscle  undergoes 
in  passing  into  the  active  state  is  its  alteration  in  shape.  It 
becomes  shorter  and  thicker.  The  actual  amount  of  shorten- 
ing varies  according  to  circumstances,  (a)  A  muscle  on  the  stretch 
when  stimulated  will  shorten  more  in  proportion  than  one  whose 
elasticity  is  not  called  into  play  before  contraction,  so  that  a 
weighted  muscle  shortens  more  than  an  unweighted  one  with  the 
same  stimulus,  {b)  The  fresher  and  more  irritable  a  muscle  is, 
the  shorter  it  will  become  in  response  to  a  given  stimulus;  and, 
conversely,  a  muscle  which  has  been  some  time  removed  from  the 
body,  or  is  fatigued  by  prolonged  activity,  will  contract  propor- 
tionately less,  (c)  Within  certain  limits,  the  stronger  the  stimulus 
applied  the  shorter  a  muscle  will  become,  (c?)  A  warm  tempera- 
ture augments  the  amount  of  shortening,  the  amount  of  contrac- 
tion of  frogs'  muscles  increasing  up  to  33°  C.  A  perfectly  active 
frog's  muscle  shortens  to  about  half  its  normal  length.  If  much 
stretched  and  stimulated  with  a  strong  current  it  may  contract 
nearly  to  one-fourth  of  its  length  when  extended.  Muscles  are 
seldom  made  up  of  perfectly  parallel  fibres,  the  directicm  and 
arrangement  varying  much  in  different  muscles.  The  more 
parallel  to  the  long  axis  of  the  muscle  the  fibres  run,  the  more 
will  the  given  muscle  be  able  to  shorten  in  proportion  to  its 
length. 

The  thickness  of  a  muscle  increases  in  proportion  to  its  shorten- 
ing during  contraction,  so  that  there  is  but  little  change  in  bulk. 
It  is  said,  however,  to  diminish  slightly  in  volume,  becoming 
less  than  yuViJ^'^  smaller.  This  can  be  shown  by  making  a  muscle 
contract  in  a  bottle  filled  with  weak  salt  solution  so  as  to  exclude 
all  air  and  to  communicate  with  the  atmosphere  only  by  a  capil- 
lary tube  into  which  the  salt  solution  rises.  The  slightest  decrease 
in  bulk  is  then  shown  by  the  fall  of  the  thin  column  of  fluid  in 
the  tube. 

Since  a  muscle  loses  in  elastic  force  and  gains  but  little  in  den- 
sity during  contraction,  the  hardness  which  is  communicated  to 


468  MANUAL   OF   PHYSIOLOGY. 

the  touch  depends  on  the  difference  of  tension  of  the  semi-fluid 
contractile  substance  within  the  muscle  sheath. 

The  Graphic  Method  of  Recording  Muscle  Contraction. 

In  order  to  study  the  details  of  the  contraction  of  muscle,  the 
graphic  method  of  recording  the  motion  is  applied.  The  curve 
may  be  drawn  on  an  ordinary  cylinder  moving  sufficiently  rapidly. 
Where  accurate  time  measurements  are  required  it  is  better  to 
use  one  of  the  many  special  forms  of  instruments,  called  myographs, 
made  for  the  purpose.  The  principle  of  all  these  instruments  is 
the  same  ;  namely,  an  electric  current,  which  passed  through  the 
nerve  of  a  frog's  muscle  connected  with  the  marking  lever,  is 
broken  by  some  mechanism,  while  the  surface  is  in  motion  ;  the 
exact  moment  of  breaking  the  contact  can  be  accurately  marked 
off  on  the  recording  surface  by  the  lever  which  draws  the  muscle 
curve  before  the  instrument  is  set  in  motion.  The  rate  of  motion 
is  registered  by  a  curve  drawn  by  a  tuning  fork  of  known  rate  of 
vibration. 

In  order  that  the  muscle-nerve  preparation  may  not  be  injured 
by  the  tissues  becoming  too  dry,  it  is  placed  in  a  small  glass 
box,  the  air  of  which  is  kept  moist  by  a  damp  sponge.  This 
mout  chamber  is  used  when  any  living  tissue  is  to  be  protected 
from  drying. 

The  first  myograph  used  was  a  complicated  instrument  devised 
by  Helmholtz ;  in  which  a  small  glass  cylinder  is  made  to  rotate 
rapidly  by  a  heavy  weight,  and  when  a  certain  velocity  of  rotation 
is  attained,  a  tooth  is  thrown  out  by  centrifugal  force,  which 
breaks  the  circuit  of  the  current  passing  through  the  nerve  of  the 
muscle.  The  tendon  is  attached  to  a  balanced  lever,  at  one  end 
of  which  hangs  a  rigid  style  pressed  by  its  own  weight  against 
the  glass  cylinder.  When  the  circuit  is  broken  the  muscle  con- 
tracts, raises  the  lever,  and  makes  the  style  draw  on  the  smoked 
glass  cylinder. 

Fick  introduced  a  flat  recording  surface  moving  by  the  swing 
of  a  pendulum,  by  which  the  abscissa  is  made  a  segment  of  a  circle, 
and  not  a  straight  line,  and  the  rate  varies,  so  that  the  different 
parts  of  the  curve  have  varying  time-values.     The  curves  given 


PHASES   OF   A   SINGLE   CONTRACTION.  459 

in  the  following  wood-cuts  are  drawn  with  the  Pendulum  Myo- 
graph. 

Du  Bois-Reymoud  draws  muscle  curves  on  the  smoked  surface 
of  a  small  glass  plate  contained  in  a  frame,  which  is  shot  by  the 
force  of  a  spiral  spring  along  tense  wires,  and  on  its  way  breaks 
the  contact.  The  trigger  used  for  releasing  the  spring  sets  a 
tuning  fork  at  the  same  time  vibrating. 

Single  Contraction. 

In  response  to  a  single  instantaneous  stimulus,  such  as  the 
making  or  breaking  of  an  electric  current,  a  muscle  gives -a  mo- 
mentary twitch  or  spasm,  commonly  spoken  of  as  a  single  con- 
traction, which  is  of  so  short  duration  that  without  the  graphic 

Fig.  185. 


Curve  drawn  by  a  frog's  gastrocnemius  on  the  Pendulum  Myograph. 
Below  is  seen  the  tuning  fork  record  of  the  time  occupied  by  the  contraction. 
Parallel  to  the  latter  is  the  abscissa.  The  little  vertical  mark  at  the  left 
shows  the  moment  of  stimulation,  and  the  distance  from  this  to  the  begin- 
ning of  the  rise  of  the  curve  gives  the  latent  period,  which  is  followed  by 
the  ascent  and  descent  of  the  lever. 

method  of  recording  the  motion  we  could  not  appreciate  the 
phases  which  are  seen  on  the  curve. 

The  curve  drawn  on  the  recording  surface  of  a  pendulum  myo- 
graph, by  such  a  single  contraction,  is  represented  in  figure  185. 
The  short  vertical  stroke  on  the  abscissa  or  base  line  is  drawn  by 
touching  the  lever  when  the  muscle  is  in  the  uncontracted  state, 
and  indicates  the  time  of  stimulation.  The  upper  curved  line  is 
drawn  by  the  lever  and  during  the  contraction  of  the  muscle. 

In  such  a  curve  the  following  stages  are  to  be  distinguished : 


460  MANUAL   OF   PHYSIOLOGY. 

1.  A  short  period  between  the  moment  of  stimulation  and  that 
at  which  the  lever  begins  to  rise,  during  which  the  muscle  does 
not  move.  This  is  known  as  the  latent  period.  In  the  skeletal 
muscles  of  the  frog  this  period  lasts  about  .01  sec. 

2.  A  period  during  which  the  lever  rises,  at  first  slowly,  then 
more  quickly,  then  again  slowly,  until  it  ceases  to  rise.  This 
stage  has  been  called  the  period  of  i-ising  energy.  It  lasts  about 
.04  sec. 

3.  When  the  highest  point  is  attained  the  lever  commences  to 
fall,  at  first  slowly,  then  more  quickly,  and  at  last  slowly.  There 
is  then  no  pause  at  the  height  of  contraction.  The  stage  of 
relaxing  has  been  called  the  period  o^  falling  energy.  It  is  said 
to  occupy  a  somewhat  longer  time  than  the  second  period,  lasting 
about  .05  sec. 

Thus  we  see  that  a  stimulus  occupying  an  immeasurably  short 
time  sets  up  a  change  in  the  molecular  condition,  which  taking 
nearly  .1  sec.  to  run  its  course,  and  requiring  .01  sec.  before  it 
exhibits  any  change  of  form,  then  in  .04  sec.  attaining  the  maxi- 
mum height  of  contraction,  and  without  waiting  in  the  contracted 
condition,  spends  .05  sec.  in  relaxing. 

The  latent  period  which  appears  in  a  single  contraction  curve 
drawn  by  a  muscle  stimulated  in  the  usual  way,  through  the 
medium  of  a  nerve,  is  not  entirely  occupied  by  preparatory 
changes  going  on  in  the  substance  of  the  muscle,  but  a  certain 
part  of  the  time  recorded  as  latent  period  corresponds  to  the  time 
required  for  the  transmission  of  the  impulse  along  the  nerve. 
This  may  be  shown  by  stimulating  first  the  far  end  of  the  nerve 
and  then  the  muscle  itself.  In  this  case  two  curves  will  be  drawn 
having  different  latent  periods,  that  obtained  by  direct  stimula- 
tion of  the  muscle  being  shorter,  and  representing  the  real  latent 
period  of  the  muscle,  while  the  longer  one  includes  the  time  taken 
by  the  impulse  to  travel  along  the  piece  of  nerve  between  the 
electrodes  and  the  muscle  (see  p.  502). 

The  latent  period  varies  much  in  different  kinds  of  muscle,  in 
the  same  kind  of  muscle  in  different  animals,  and  in  the  same 
individual  muscle  under  different  conditions.  As  a  rule  the  slow- 
contracting  muscles  have  a  longer  latent  period.     Thus  the  non- 


VARIATIONS    IN    THE   SINGLE   CONTRACTION. 


461 


striated  slow-contracting  muscles  found  in  the  hollow  viscera 
have  a  latent  period  of  some  seconds.  The  striated  muscles  of 
coldblooded  animals  have  a  longer  latency  than  the  same  kind 
of  muscle  in  birds  and  mammalia.  The  same  gastrocnemius  of  a 
frog  has  a  shorter  latent  period  when  strongly  stimulated,  or  when 
its  temperature  is  raised,  and  vice  versa. 

The  latent  period  is  considerably  lengthened  by  fatigue.  If 
the  weight  be  so  applied  that  it  does  not  extend  the  muscle  before 
contraction,  but  only  bears  on  it  the  instant  it  commences  to 
shorten,  the  duration  of  the  latent  period  increases  in  proportion 
to  the  weigiit  the  muscle  has  to  lift. 

Fig.  1S6. 


Curves  drawn  by  the  same  ninscle  in  different  stages  of  fatigue. — A,  when 
fresh;  B,  C,  D,  E,  each  immediately  after  the  muscle  had  contracted  200 
times.    Sbowing  that  fatigue  causes  a  low,  long  contraction. 

The  duration  of  the  single  contraction  of  striated  muscle  varies 
in  different  ca.ses  and  under  varying  circumstances.  The  greatest 
difference  is  reached  by  the  muscles  found  in  different  kinds  of 
animals.  The  contraction  of  some  kinds  of  muscle  tissue  (non- 
striated  muscle  of  mollusca,  for  example)  occupies  several  minutes, 
and  reminds  one  of  the  slow  movement  of  protoplasm  ;  while  the 
rapid  action  of  the  muscle  of  the  wing  of  a  horsefly  occurs  330 
times  a  second.  Various  gradations  between  these  extremes  in 
the  rapidity  of  muscle  contraction  may  be  found  in  the  contrac- 
tile tis-^ues  of  different  animals.  The  following  table  gives  the 
rate  of  contraction  of  some  insects'  muscles,  which  may  help  to 
show  the  extent  of  these  variations. 


Horsefly,  . 
Bee, 
Wasp, 
Drag'  n-fl/, 
Butterfly, 


330  contractions  per  second. 
190 

110  "  " 

28  "  " 

9  "  " 


462 


MANUAL   OF    PHYSIOLOGY. 


Among  the  vertebrata  the  duration  of  the  contraction  of  the 
skeletal  muscles  varies  considerably  according  to  the  habits  of  the 
animal.     The  limb  muscles  of  the  tortoise  and  toad  take  a  very 


Fig.  187 


Six  curves  drawn  by  the  same  muscle  wlicn stretched  by  di/yerent  weighs. 
Showing  that  as  the  weight  is  increased  the  latency  becomes  longer  and  the 
contraction  less  in  height  and  duration. 

long  time  to  finish  their  contraction  ;  other  muscles  of  the  same 
animals  act  more  quickly,  but  do  not  attain  the  rapidity  of  con- 
traction of  the  skeletal  muscles  of  warm-blooded  animals. 

Fig.  188. 


Curves  drawn  by  the  same  muscle  at  different  temperatures.  Showing 
that  with  elevation  of  temperature  the  latency  and  the  contraction  become 
shorter.     (The  muscle  had  been  previously  cooled.) 

The  duration  of  a  single  contraction  of  the  same  muscle  is  also 
capable  of  considerable  variation.     It  seems  to  be  lengthened  by 


MAXIMUM   CONTRACTION.  463 

anything  that  leads  to  an  accumulation  of  the  chemical  products 
which  arise  from  muscle  activity.  Hence  fatigue  or  over-stimu- 
lation cause  a  slow  contraction  (Fig.  186). 

Moderate  increase  of  temperature  greatly  shortens  the  time  oc- 
cupied by  the  single  contraction  of  any  given  muscle.  Excessive 
heat  causes  a  state  of  continued  contraction. 

The  reduction  of  temperature  causes  a  muscle  to  contract  more 
slowly,  and,  when  extreme,  the  muscle  remains  contracted  long 
after  the  stimulus  is  removed. 

Wave  of  Contraction. — If  one  extremity  of  a  long  muscle  be 
stimulated  without  the  aid  of  the  nerve  (it  is  best  to  employ  a 

Fig.  189. 


Curves  drawn  by  the  same  muscle  while  being  cooled.     Showing  that  the 
latency  and  the  contraction  become  longer  as  the  temperature  is  reduced. 

muscle  from  a  curarised  animal),  the  contraction  passes  along  the 
muscle  from  the  point  of  stimulation  in  a  wave  which  travels  at 
a  definite  rate  of  3-4  metres  per  sec.  in  a  frog,  and  4-5  metres 
per  sec.  in  a  mammal.  Reduction  of  temperature  and  fading  of 
vital  activity  cause  the  velocity  of  the  wave  to  be  lessened,  until 
finally  the  tissue  ceases  to  conduct ;  then  only  a  local  contraction 
occurs,  severe  stimulus  causing  simply  an  elevation  at  the  point 
of  contact.  This  seems  analogous  to  the  idio-muscular  contrac- 
tion, which  marks  the  seat  of  severe  mechanical  stimulation  after 
the  general  contraction  has  ended. 

Maximum  Contraction. 

The  extent  to  which  a  muscle  will  contract  depends  upon  the 
conditions  in  which  it  is  placed,  and  varies,  as  we  have  seen,  with 
the  load,  its  irritability,  the  temperature,  and  the  force  of  the 


4G4  MANUAL   OF   PHYSIOLOGY. 

stimulus.  A  fre.«h  muscle  then,  at  the  ordinary  temperature  with 
a  medium  load,  will  contract  more  and  more  as  the  intensity  of 
the  current  employed  increases.  There  is  a  limit  to  this  increase, 
and  with  comparatively  weak  stimulation,  an  effect  is  produced 
which  cannot  be  surpassed  by  the  same  muscle,  no  matter  what 
stimulus  be  applied.  This  greatest  contraction  is  the  same  for  all 
medium  stimuli  while  the  muscle  is  fresh,  and  is  called  the  maxi- 
mum contraction,  being  the  greatest  shortening  which  can  be  pro- 
duced by  a  single  instantaneous  stimulus. 

Fig.  190. 


Pendulum  Myograph  Tracings  showing  Summation. — 1.  Curve  of  maxi- 
mum contraction  drawn  by  first  stimulus,  the  exact  time  of  application  of 
which  is  shown  by  the  small  up-stroke  of  the  left  hand  of  the  base  line. 
2.  Maximum  contraction  resulting  from  second  simple  stimulation  given  at 
the  moment  indicated  by  the  other  small  up-stroke.  3.  Curve  drawn  as  the 
result  of  double  stimulation  sent  in  at  an  interval  indicated  by  the  distance 
between  the  up-strokes,  showing  summation  of  stimulus  and  consequent 
increase  in  contraction  over  the  "maximum  contraction." 

Summation. — Each  time  a  muscle  receives  an  induction  shock 
of  medium  strength  it  contracts  to  its  maximum.  If  a  second 
shock  be  given  while  the  muscle  is  in  the  contracted  state,  a  new 
maximum  contraction  is  added  to  the  extent  of  the  contraction 
the  muscle  was  in  at  the  moment  of  the  second  stimulation,  and 
if  stimulated  when  the  lever  is  at  the  apex  of  the  curve  the  sum 
of  the  effect  produced  will  be  equal  to  two  maximum  contractions. 

If  applied  in  the  middle  of  the  period  of  the  ascent  or  descent 
of  the  lever,  a  second  stimulation  gives  rise  to  H  maximum  con- 
tractions, and  so  on,  in  various  parts  of  the  curve,  a  new  maxi- 


TETANUS,  465 

mum  curve  is  produced,  arising  from  the  point  at  which  the  lever 
is  when  the  second  stimulus  is  applied  (Fig.  190). 

During  the  latent  period  a  second  stimulation  produces  less 
marked  effect,  and  is  difficult  to  demonstrate,  but  if  the  second 
stimulus  come  after  an  interval  of  more  than  g^^  sec,  summation 
can  be  appreciated. 

This  summation  of  effect  also  takes  place  when  the  stimulus  is 
iusufficient  to  produce  a  maximum  contraction,  the  succeeding 
weak  stimuli  give  rise  to  the  same  extent  of  contraction  of  the 
already  partially  contracted  muscle,  as  if  it  were  at  its  normal 
length  at  the  time  of  the  second  stimulation.  The  following 
tracings  (Figs.  191-193)  show  the  effects  of  repeated  stimulations 
applied  at  the  various  periods  indicated  by  the  numbers  on  the 
abscissa  line. 

Tetanus. 

If  a  series  of  stimuli  be  applied  one  after  the  other,  at  intervals 
equal  to  about  half  the  duration  of  a  single  contraction,  a  sum- 
mation of  contractions  occurs,  which  results  in  the  accumulation 
of  effect  until  the  muscle  has  shortened  to  about  one-half  of  the 
length  it  attains  during  a  single  contraction,  or  about  one-fourth 
the  normal  length  of  the  relaxed  muscle ;  it  can  then  shorten  no 
more  no  matter  how  the  stimulus  be  increased  in  rate  or  strength. 
Not  having  sufficient  time  between  the  stimuli  it  cannot  relax,  so 
it  remains  contracted  permanently  as  long  as  the  stimulus  is  con- 
tinued, the  various  single  contractions  caused  by  the  repeated 
shock  all  being  fused  together  (Fig.  191)  ;  but  if  the  interval  be 
more  than  half  the  time  occupied  by  a  single  contraction,  then 
the  line  drawn  by  the  lever  will  show  notches  indicating  the 
apices  of  the  fused  single  contractions  (Figs.  192  and  193). 

This  condition  of  continuous  summation  of  contractions  is  called 
tetaniis,  and  in  all  probability  is  not  only  the  commonest  but  the 
only  kind  of  muscular  motion  that  can  be  produced  by  the  action 
of  the  nerves  in  obedience  to  the  will.  All  the  actions  of  our 
skeletal  muscles  are  then  made  up  of  the  fusion  of  many  single 
contractions  into  tetanus,  and  such  motions  as  appear  too  quick 

39 


466 


MANUAL   OF   PHYSIOLOGY. 


for  tetanic  action  are  accomplished  by  the  interposition,  at  a  cer- 
tain moment,  of  the  action  of  an  antagonistic  muscle  which  stops 
the  movement  and  makes  the  act  extremely  rapid. 

With  from  twenty  a  second  to  u{)wards  of  many  hundreds  of 


Fio.  101. 


Curve  of  tetanus  resulting  from  3U  stimulations  per  second,  drawn  _-  .. 
drum  rotating  slowly  compared  with  the  motion  of  the  Pendulum  Hyograph. 
Tlie  stimulation  commences  at  "30,"  and  ceases  just  before  the  lever  begins 
to  fall.  No  trace  of  the  individual  contractions  of  which  the  tetanus  is 
composed  can  be  recognized. 

induced  shocks  one  can  produce  tetanus  in  a  frog's  muscle.  The 
lowest  limit  of  this  range  is  probably  about  the  number  of  im- 
pulses communicated  to  human  muscles  by  their  nerves,  since  the 
tone  produced  by  contracting  muscle  corresponds  to  the  first  over- 

FiG.  192. 


Curve  of  tetanus  composed  of  imperfectly  fused  contractions  resulting 
from  12  stimulations  per  second.  The  serrations  on  the  left  of  the  curve  in- 
dicate the  individual  contractions. 

tone  of  a  primary  note  produced  bv  19.5  vibrations  in  a  second. 
The  number  of  stimuli  required  varies  with  the  rate  of  contrac- 
tion of  the  muscle  employed,  the  quick  contracting  bird's  muscle 
requiring  70  per  second,  while  the  exceptionally  slow-moving 
tortoise  muscle  only  requires  3  per  second.     According  to  some, 


MUSCLE  TONE.  467 

there  is  a  limit  to  the  number  of  stimuli  which  will  cause  tetanus, 
360  per  second  is  named  as  the  maximum  for  a  certain  strength 
of  stimulus;  with  stronger  stimuli,  even  when  more  frequent, 
tetanus  occurs.  It  has  been  shown  that  many  thousand  stimuli 
per  second  can  cause  tetanus  even  with  very  weak  currents.  If 
tetanus  be  kept  up  for  some  seconds,  and  the  stimulation  be  then 
suddenly  stopped,  the  lever  falls  rapidly  for  a  certain  distance, 
but  the  muscle  does  not  quite  return  to  its  normal  length  for  some 
few  seconds.  This  residue  contraction  is  easily  overcome  by  any 
substantial  load.  If  kept  in  a  state  of  tetanus  by  weak  stimula- 
tion, after  some  time  the  muscle  commences  to  relax  from  fatigue. 

Fig.  193. 


Tetanus  produced  by  8  stimulations  per  second.  The  more  perfect  fusion 
of  the  single  contractions  shown  towards  the  end  of  the  curve  depends  on 
the  altered  condition  of  the  muscle. 

at  first  rapidly,  then  more  slowly  ;  this  falling  off  of  the  tetanic 
contraction  may  be  prevented  by  increasing  the  stimulus. 

Muscle  Tone. 

Although  the  tracing  drawn  by  a  lever  attached  to  a  muscle 
in  tetanus  is  straight,  and  does  not  show  any  variation  in  the 
tension  of  the  tetanized  muscle,  some  variation  in  tension  must 
occur,  since  a  low  humming  sound  is  produced  during  contraction. 
This  muscle-tone  can  be  heard  by  applying  the  ear  firmly  over  any 
large  muscle  (biceps)  while  in  tetanus,  or  by  throwing  the  mus- 
cles attached  to  the  Eustachean  tube  into  action,  as  in  swallow- 
ing, or  during  spasm  of  the  muscles  in  mastication. 

The  number  of  vibrations  of  the  muscle-sound  has  been  esti- 
mated to  be  from  18-20  for  the  human  skeletal  muscles.     This 


468  MANUAL   OF   PHYSIOLOGY. 

number  of  vibrations,  however,  does  not  produce  any  audible 
note  ;  hence  it  has  been  supposed  that  the  note  we  hear  is  really 
the  first  overtone,  and  not  the  fundamental  tone.  When  a  muscle 
is  thrown  into  tetanus  by  a  current  interrupted  by  a  tuning  fork, 
a  tone  is  said  to  be  produced  which  corresponds  to  the  number  of 
vibrations  of  the  fork  which  causes  the  interruption  in  the  cur- 
rent, and  thus  regulates  the  number  of  stimulations  which  the 
muscle  receives.  If,  however,  a  contraction  of  the  muscle  be 
brought  about  by  stimulating  the  spinal  cord,  with  the  same 
apparatus  for  making  and  breaking  the  current,  then  the  normal 
muscle-tone  is  produced,  just  as  if  the  contraction  was  the  result 
of  a  nerve  impulse  coming  from  the  brain. 

Irritability  and  Fatigue. 

The  life  of  the  muscle  tissue  of  mammalian  animals  is  closely 
dependent  upon  a  good  supply  of  nutrition,  and  if  its  blood- 
current  be  completely  cut  off  by  any  means  for  a  length  of  time 
it  loses  its  power  of  contracting.  While  the  muscle  remains  in 
the  body,  and  is  therefore  kept  warm  and  moist  by  the  juices  in 
the  tissues,  it  will  live  a  very  considerable  time  without  any  blood 
flowing  through  it,  and  it  at  once  regains  its  contractility  when 
the  blood  stream  is  again  allowed  to  flow  through  its  vessels. 
This  is  seen  when  the  circulation  of  a  limb  is  brought  to  a  stand- 
still by  means  of  a  tourniquet  or  a  tightly  applied  bandage. 
When  removed  from  the  body,  a  mammalian  muscle  soon  ceases 
to  be  irritable  and  dies,  but  its  functional  activity  may  be  renewed 
by  passing  an  artificial  stream  of  arterial  blood  through  its  ves- 
sels, and  an  isolated  muscle  may  thus  be  made  to  contract  re- 
peatedly for  a  considerable  time. 

On  the  other  hand,  the  muscle  of  a  cold-blooded  animal  will 
remain  alive  for  a  long  time — many  hours — if  kept  cool  and 
moist.  When  its  functional  activity  is  about  to  fade,  it  may  be 
revived  by  means  of  an  artificial  stream  of  blood  being  caused 
to  flow  through  its  vessels,  just  as  in  the  case  of  the  mammalian 
muscle. 

Common  experience  teaches  us  that  even  when  well  supplied 
with  blood  our  own  muscles  become  fatigued  after  very  prolonged 


FATIGUE.  469 

exertion,  and  are  incapable  of  further  action.  This  occurs  all  the 
more  rapidly  when  anything  interferes  with  the  flow  of  blood 
through  them,  such  as  when  we  use  our  arms  in  an  elevated  posi- 
tion ;  the  simple  operation  of  driving  in  a  screw  overhead  is  soon 
followed  by  pain  and  fatigue  in  the  muscles  of  the  forearm,  though 
the  same  amount  of  force  could  be  exerted  when  the  arras  are  in 
a  dependent  posture  without  the  least  feeling  of  fatigue. 

The  difficulties  of  experimenting  with  the  muscles  of  mammals 
make  the  frog-muscle  the  common  material  for  investigation,  and 
from  it  we  learn  the  following  facts : 

AVhen  removed  from  the  body  and  deprived  of  its  blood  supply, 
the  muscle  of  a  cold-blooded  animal  slowly  dies  from  want  of  nu- 
trition. However,  if  it  be  placed  under  favorable  circumstances, 
and  allowed  perfect  rest,  it  may  live  twenty-four  hours.  If  it  be 
frequently  excited  to  action,  on  the  other  hand,  it  rapidly  loses  its 
irritability,  becoming  in  fact  fatigued. 

From  a  muscle  removed  from  a  recently  killed  animal,  we  learn, 
moreover,  that  even  without  any  blood  supply  the  muscle-tissue  is 
capable  of  recovering  from  very  well-marked  fatigue,  if  it  be 
allowed  to  rest  for  a  little  time,  so  that  the  muscle  has  in  itself 
the  material  requisite  for  its  recuperation. 

The  first  question  then  is,  what  causes  the  loss  of  irritability 
which  we  call  fatigue  ?  And  the  second  is,  by  what  means  is  the 
muscle  enabled  to  return  to  a  state  of  functional  activity?  We 
know  that  the  mere  life  of  a  tissue  must  be  accompanied  by  certain 
chemical  changes  which  require  (1)  a  supply  of  fresh  material, 
and  (2)  the  removal  of  certain  substances  which  are  the  outcome 
of  the  tissue-change.  In  the  case  of  muscle  this  chemical  inter- 
change is  constantly  but  slowly  going  on  between  the  contractile 
substance  and  the  blood.  When  the  muscle  contracts  much  more 
active,  and  probably  different,  changes  go  on  in  the  contractile 
substance,  more  new  material  being  required,  and  more  effete 
matter  being  produced.  It  is  probable  that  the  accumulation  of 
these  effete  matters  is  the  more  important  cause  of  the  loss  of  irrita- 
bility in  a  muscle,  for  a  frog's  muscle  when  quite  fatigued  may 
be  rendered  active  again  by  washing  out  its  bloodvessels  with  a 
stream  of  salt  solution  of  the  same  density  as  the  serum  (.6  per 


470  MANUAL  OF   PHYSIOLOGY. 

cent  NaCl),  and  thus  removing  the  injurious  "fatigue-stuffs,"  as 
they  have  been  called.  We  know  also  that  a  very  minute  quantity 
of  lactic  acid  injected  into  the  vessels  of  a  muscle  destroys  its  irri- 
tability, and  brings  it  to  a  state  resembling  intense  fatigue.  Of 
the  new  material  required  for  thesustentation  of  muscle  irritability 
we  know  that  oxygen  is  amongst  the  most  important,  though  its 
supply  is  not  absolutely  necessary  for  the  recuperation  of  a  par- 
tially exhausted,  isolated  frog's  muscle. 

The  slow  recovery  of  a  bloodless  muscle  from  fatigue  may  be 
explained  by  supposing  time  to  be  necessary  for  the  reconstruction 
of  new  contractile  material,  and  probably  also  for  a  secondary 
change  to  take  place  in  the  effete  materials  by  which  they  become 
less  injurious. 

When  working  actively,  then  it  is  obvious  that  the  muscles  re- 
quire an  adequate  supply  of  good  arterial  blood  in  order  to  ward 
off  exhaustion  ;  and,  as  already  explained  in  speaking  of  the  vaso- 
motor influences,  a  muscle  does  in  reality  receive  a  much  greater 
supply  of  blood  when  actively  contracting  than  when  in  the  passive 
state. 

The  irritability  of  a  muscle  and  the  rate  at  which  it  becomes 
exhausted  may  be  said  to  depend  upon  : 

1.  The  adequacy  of  its  blood  supply:  the  better  the  supply  of 
new  material  and  the  more  quickly  the  injurious  effete  materials 
are  removed,  the  more  work  a  muscle  can  do  without  becoming 
exhausted. 

2.  Temperature  has  a  marked  effect  on  the  irritability  as  well 
as  form  of  contraction  of  muscles.  Very  low  temperatures — ap- 
proaching zero  C. — diminish  the  irritability  of  a  muscle,  but  do 
not  seem  to  tend  towards  more  rapid  exhaustion.  High  tempera- 
tures— approaching  30°  C. — increase  the  irritability,  and  at  the 
same  time  rapidly  bring  about  fatigue.  At  about  35°  C.  an  iso- 
lated frog's  muscle  begins  to  pass  into  heat  tetanus,  and  soon  loses 
its  irritability  forever. 

3.  Functional  activity  is  accompanied  by  an  increased  blood- 
supply,  and  a  more  perfect  nutrition  of  the  muscles,  and  hence 
use  is  advantageous  for  their  growth  and  power;  while,  on  the 
other  hand,  continued  and  prolonged  inactivity  causes  a  lowering 


RIGOR  MORTIS.  471 

of  the  nutrition  and  loss  of  irritability.  Thus  when  the  nerves 
supplying  the  voluntary  muscles  are  injured,  there  is  considerable 
danger  of  atrophy  and  tissue-degeneration  of  the  muscles ;  the 
contractile  substance  becomes  replaced  by  fat  granules.  This  de- 
generation also  occurs  in  the  stump  when  a  limb  is  amputated, 
the  distal  attachments  of  the  muscles  having  been  cut,  they  atro- 
phy ;  for,  although  their  nervous  supply  is  uuinjured,  they  cannot 
act,  and  after  some  time  muscle  tissue  can  hardly  be  recognized 
in  them. 

Death  Rigor. 

The  death  of  muscle  tissue  is  preceded  by,  and  associated  with, 
a  set  of  changes  which  are  a  kind  of  exaggeration  of  those  observed 
in  its  active  state.  The  most  obvious  phenomenon  is  an  unyielding 
contraction,  which  causes  the  stiffening  of  the  body  after  systemic 
death.  Hence  it  is  called  rigor  mortis.  The  muscles  harden  ; 
lose  their  elasticity,  and  the  tissue  is  torn  if  forcibly  stretched. 
When  isolated,  the  muscle  is  seen  to  be  opaque,  and  its  reaction 
is  found  to  be  distinctly  acid.  A  considerable  quantity  of  heat  is 
developed  during  the  progress  of  the  rigor.  The  electric  currents 
alter  in  direction  and  finally  disappear. 

The  period  at  which  rigor  comes  on,  as  well  as  the  time  it  lasts, 
depend  on  (a)  the  state  of  the  muscles  themselves,  and  (6)  the 
circumstances  under  which  they  are  placed  at  the  time  of  death. 
All  influences  which  tend  to  facilitate  the  approach  of  tissue-death 
also  tend  to  induce  early  and  rapidly-terminating  rigor,  viz.,  (1.) 
Prolonged  activity — as  may  be  shown  in  a  muscle  artificially 
tetanized,  or  may  be  seen  in  an  animal  whose  death  was  preceded 
by  intense  muscular  exertion — causes  rigor  to  appear  almost  im- 
mediately, and  to  terminate  rapidly.  (2.)  Within  certain  limits, 
a  high  temperature  facilitates  the  production  of  rigor  in  dying 
muscles,  and  indeed  a  temperature  not  much  exceeding  that  nor- 
mal to  the  tissue  induces  rigor  immediately.  This  form  of  con- 
traction, which  is  c&Ued  heat-rigor,  is  brought  about  in  mammalian 
muscles  by  a  temperature  of  about  50°  C,  and  in  frogs'  muscles 
below  40°  C.  If,  however,  the  temperature  of  a  muscle  be  sud- 
denly raised  to  the  boiling  point,  it  is  killed,  and  the  chief  phe- 
nomena of  rigor  are  prevented  from  occurring.     (3.)  Freezing 


472  MANUAL   OF    PHYSIOLOGY. 

postpones  the  appearance  of  the  changes  in  the  muscles  upon  which 
rigor  depends.  (4.)  Stretching,  or  any  mechanical  excitation 
which  tends  to  injure  or  hasten  the  death  of  the  tissue,  causes  it 
to  pass  more  rapidly  into  rigor.  (5.)  The  application  of  water 
and  of  a  number  of  chemical  substances  causes  muscles  quickly  to 
pass  into  a  state  of  rigor  similar  in  all  essential  respects  to  that 
which  ordinarily  follows  the  death  of  the  tissue.  (6.)  Any  stop- 
page in  the  blood-current  normally  flowing  through  a  muscle, 
after  some  little  time  makes  it  pass  into  a  state  of  rigidity  like 
rigor  mortis,  but  this  may  be  removed  by  allowing  the  blood  to 
flow  freely  again  through  the  muscle. 

It  is  generally  admitted  that  muscle  rigor  depends  on  the  coag. 
ulation  of  the  muscle  plasma,  giving  rise  to  myosin  and  muscle 
serum.  This  is  in  most  respects  comparable  with  the  coagulation 
of  the  blood,  and  also  seems  to  be  produced  by  the  action  of  some 
ferment,  of  which  several  have  been  made  out  in  dead  muscle  tis- 
sue (compare  the  par.  on  chemistry,  p.  442). 

Most  of  the  phenomena  of  the  process  of  muscle  rigor  remind 
us  of  the  changes  which  were  noted  as  occurring  in  muscle  when 
it  passes  from  the  passive  to  the  active  state.  Thus  the  shortening 
of  the  fibres,  the  evolution  of  heat,  and  the  chemical  changes  may 
be  said  to  be  identical  in  contraction  and  rigor  mortis.  The  elec- 
trical changes  are,  however,  very  transitory,  and  are  followed  by 
complete  loss  of  elasticity  and  irritability.  Opacity  of  the  tissue 
accompanies  its  later  stages. 

Thus,  while  dying,  the  muscle  tissue  may  be  said  to  go  through 
a  series  of  events  analogous  to  those  which  would  occur  in  a  pro- 
longed contraction  without  any  period  of  recuperation.  The  idea 
naturally  has  suggested  itself  to  the  minds  of  physiologists  that 
the  active  state  of  muscle  depends  upon  chemical  changes  which 
are  the  initial  steps  in  the  coagulation  of  the  contractile  substance, 
■when  the  muscle  is  dying.  The  muscle  tissue  is  supposed  to  con- 
tain a  special  proteid  of  extremely  intricate  and  unstable  chemical 
constitution,  which,  like  all  plasmata,  is  constantly  undergoing 
slow  molecular  change,  and  which  if  not  reintegrated  by  constant 
assimilation  would  pass  into  coagulation.  Under  the  influence  of 
stimuli  a  comparatively  sudden  and  intense  molecular  disturbance 


UNSTRIATED   MUSCLE   TISSUE.  473 

is  brought  about,  which  produces  shortening  of  the  fibres  and  the 
same  chemical  changes  as  precede  the  coagulation.  Before  the 
stage  of  coagulation  appears,  however,  a  chemical  rearrangement 
takes  place,  the  result  of  which  is  the  reconstruction  of  the  un- 
stable complex  proteid.  If  nutriment  be  withheld,  or.if  the  stimu- 
lation be  too  powerful,  the  recovery  cannot  take  place,  and  we 
find  the  muscle  passing  from  a  state  of  physiological  contraction 
to  one  of  intense  exhaustion,  and  then  to  coagulation  and  death. 


UNSTRIATED  MUSCLE. 


So  far  reference  has  only  been  made  to  the  skeletal  muscles, 
the  fibres  of  which  are  marked  by  transverse  striations,  and  whose 
single  contraction  is  extremely  rapid  and  short.  The  contractile 
tissues  which  carry  on  the  movements  in  the  various  organs  of  the 
body  are  not  striated  fibres,  but,  as  has  been  already  stated,  con- 
sist of  elongated  flattened  cells  with  rod-shaped  nuclei.  They 
occur  generally  in  the  form  of  sheets  or  layers  forming  coats  for 
the  organs  in  which  they  lie.  Their  single  contraction  is  slow 
and  prolonged,  and  commonly  is  transmitted  from  one  muscle-cell 
to  another  as  a  kind  of  sluggish  wave.  They  are  incapable  of 
passing  into  a  tetanic  state  of  contraction  like  striated  muscles. 

The  slowest  contraction  seems  to  be  that  of  the  little  muscle- 
cells  in  the  walls  of  the  bloodvessels.  These  remain  in  a  state  of 
partial  contraction,  which  undergoes  a  brief  and  partial  rhythmi- 
cal relaxation.  The  most  forcible  aggregate  of  unstriated  muscle 
elements  is  met  with  in  the  uterus.  This  organ,  which  has  very 
exceptional  motor  powers  to  perform,  contracts  in  somewhat  the 
same  way  as  the  muscles  of  the  bloodvessels,  but  more  quickly 
and  with  longer  rhythmical  intervals  of  partial  relaxation.  The 
muscular  wall  of  the  intestine  and  the  iris  are  among  the  most 
rapidly  contracting  smooth  muscles. 

The  chemical  properties  of  the  smooth  muscle  are  much  the 
same  as  those  of  striated  skeletal  muscles,  and  they  pass  into  a 
state  of  rigor,  while  dying,  which  seems  to  depend  on  the  same 
causes  as  the  rigor  mortis  already  described. 

40 


CHAPTER  XXVI. 

THE  APPLICATION  OF  SKELETAL  MUSCLES. 

The  cousideration  of  the  many  varieties  of  muscles,  and  the 
various  modes  in  which  they  are  attached  to  the  bones  that  tliey 
are  destined  to  move,  belongs  to  the  department  of  practical  anat- 
omy, and  needs  no  mention  here.  As  a  general,  but  by  no  means 
universal  rule,  a  muscle  has  one  attachment  which  is  fixed,  com- 
monly spoken  of  as  its  origin,  and  a  second,  called  its  insertion, 
upon  which  it  acts  by  approximating  it  to  the  origin.  Muscles 
mostly  pass  in  a  straight  line  between  their  two  attachments, 
but  sometimes  they  act  round  an  angle  by  sliding  over  a  pulley, 
or  by  means  of  a  small  bone  in  the  tendon,  like  the  patella. 

The  muscles  are  so  attached  that  they  are  always  slightly  on 
the  stretch,  and  thus  at  the  moment  they  begin  to  contract  they 
are  in  an  advantageous  position  to  bring  their  action  to  bear  on 
the  bones  which  they  move.  When  the  contraction  ceases  the 
bones  are  drawn  back  to  their  former  position  without  any  sudden 
jerk  or  jar. 

The  muscles  commonly  act  upon  the  bones  as  levers  by  working 
upon  the  short  arm  of  the  lever,  so  that  more  direct  force  is  re- 
quired on  the  part  of  a  muscle  than  the  weight  of  the  body  moved  ; 
but  from  this  arrangement  considerable  advantages  are  gained, 
viz.,  that  a  small  contraction  of  the  muscle  causes  an  extensive 
excursion  of  the  part  moved,  and  much  greater  rapidity  of  motion 
is  attained. 

All  the  three  orders  of  levers  are  met  with  in  the  movements 
of  the  different  bones  of  the  skeleton  ;  often,  indeed,  all  three 
varieties  are  found  in  the  same  joint,  as  the  elbow,  where  the  sim- 
ple extension  and  flexion  motions  of  the  biceps  and  triceps  muscles 
give  us  good  examples  (Fig.  194). 

The  first  order  of  lever  is  used  when  the  triceps  is  the  power 
and  draws  upon  the  olecranon,  thus  moving  the  hand  and  forearm 


SKELETAL   MOVEMENTS. 


475 


Fig.  194. 


arouud  the  trochlea  which  acts  as  the  fulcrum.  This  is  shown  in 
the  upper  diagram,  in  which  the  hand  is  striking  a  blow  with  a 
dagger. 

The  second  order  comes  into  play 
when  the  hand,  resting  on  a  point  of 
support,  acts  as  the  fulcrum,  and  the 
triceps  pulling  on  the  olecranon  is  the 
power  which  raises  the  humerus  upon 
which  is  fixed  the  body  or  weight 
(middle  diagram). 

The  third  order  may  be  exemplified 
by  the  action  of  the  biceps  in  ordinary 
flexion  of  the  elbow.  Here  the  muscle, 
which  is  the  power,  is  placed  between 
the  fulcrum — represented  by  the  lower 
end  of  the  humerus — and  the  weight 
which  is  carried  by  the  hand  (lower 
diagram). 

The  various  groups  of  muscles,  which 
are  so  arranged  as  to  assist  each  other 
when  acting  together,  are  called  syner- 
getie,  and  those  which  when  contracting 
at  the  same  time  oppose  each  other,  are 
called  antagonistic.  The  same  muscles 
may,  in  different  positions  of  a  joint  or 
in  combination  with  other  different 
muscles,  have  totally  different  actions, 
at  one  time  being  synergetic  and  at 
another  antagonistic.  Thus  the  sterno- 
mastoid  muscle  may,  in  different  positions  of  the  head,  either 
bend  the  cranium  backward  or  forwards,  and  so  cooperate  with 
two  sets  of  muscles  which  are  definitely  antagonistic  to  one 
another. 

Joints. 

The  unions  between  the  bones  of  the  skeleton  are  very  varied 
in  function  and  character.     They  may  be  classified  as : 

1.  Sutures,  in  which  the  bones  are  firmly  united  by  rugged 


w  p 

Diagrams  showing  the 
mode  of  action  of  the  three 
orders  of  levers  (numbered 
from  above  downwards)  il- 
histrated  by  the  action  of 
the  elbow-joint. 


476  MANUAL   OF   PHYSIOLOGY. 

surfaces  without  the  interposition  of  any  cartilage.  They  are 
practically  only  the  lines  of  union  of  differeut  bones,  which  grow 
together  to  form  a  single  bone. 

2.  Symphyses,  in  which  two  bony  substances  are  strongly 
cemented  together  by  ligaments,  and  a  more  or  less  thick  ad- 
herent layer  of  fibro-cartilage,  are  joints  allowing  of  some  move- 
ment, which  is,  however,  very  limited. 

3.  Arthroses,  or  true  movable  joints,  such  as  are  commonly 
met  with  in  the  extremities.  They  are  characterized  by  a  syno- 
vial sac  lining  the  surrounding  ligaments,  and  two  smooth  sur- 
faces of  cartilage  which  cover  over  the  bony  extremities  taking 
part  in  the  articulation,  and  form  what  are  called  the  articular 
surfaces.  The  synovial  sac  is  strengthened  by  a  loose  mem- 
branous covering — the  capsular  ligament  —  which  is  attached 
round  the  edge  of  the  cartilages  next  to  the  periosteum,  which 
here  ceases. 

The  articular  surfaces  are  always  in  exact  and  close  contact, 
being  pressed  together  by  the  following  influences  :  (1.)  The 
elastic  tension  and  tonic  contraction  of  the  surrounding  muscles, 
which  exert  considerable  traction  on  them.  (2.)  The  traction  of 
the  surrounding  ligaments,  which  in  some  cases  holds  the  bones 
firmly  together,  no  matter  what  their  relative  positions  may  be. 
This  can  be  well  seen  in  the  knee-joint,  in  which  a  comparatively 
small  number  of  the  ligaments  suflice  to  keep  the  articular  sur- 
faces in  contact.  (3.)  The  atmospheric  pressure  also  tends  to 
hold  the  bones  in  close  apposition,  as  may  be  seen  in  the  hip- 
joint,  which  is  not  easily  disarticulated,  even  when  all  the  sur- 
rounding structures  and  the  ligaments  have  been  severed. 

The  synovial  joints  may  be  classified  according  to  the  form  of 
their  surfaces,  or  their  mode  of  motion,  as  follows  : 

1.  Flat  articular  surfaces  held  together  by  a  short,  rigid  cap- 
sule, allowing  of  but  very  slight  gliding  movement ;  examples  of 
this  form  of  joint  are  to  be  found  in  the  tarsus  and  the  articular 
processes  of  the  vertebrse. 

2.  Hinge  joints,  in  which  the  surfaces  are  so  adapted  that  only 
one  kind  of  motion  can  take  place.  A  groove-like  cavity  in  one 
bone  fits  closely  and  glides  around  the  axis  of  a  roller  on  the 


SKELETAL    MOVEMENTS. 


477 


other  bone,  while  the  sides  of  the  joint  are  kept  tightly  together 
by  means  of  strong  lateral  ligaments.  Examples  of  this  form  of 
joint  are  to  be  found  between  the  phalanges  of  the  digits  and  at 
the  humero-ulnar  joint. 

3.  The  rotatory  hinge,  or  pivot  joint,  is  that  in  which  a  part 
moves  round  the  axis  of  a  bone,  instead  of  the  axis  of  rotation 
being  at  right  angles  to  both  bones,  forming  the  joint  as  in  an 
ordinary  hinge.  Such  joints  are  seen  at  the  head  of  the  radius 
and  at  the  articulation  between  the  atlas  and  the  odontoid  pro- 
cess of  the  axis. 

4.  A  saddle-shaped  joint  is  a  kind  of  double-hinge,  in  which 
each  of  the  two  articulating  bones  form  a  partial  socket  and 
roller,  and  hence  there  are  two  axes  of  rotation  placed  more  or 
less  at  right  angles  one  to  the  other.  A  good  example  of  this 
kind  of  joint  occurs  between  the  thumb  and  one  of  the  wrist 
bones. 

5.  Spiral  articulations  are  modifications  of  the  hinge,  in  which 
the  surface  of  the  roller  does  not  run  "  true,"  but  becomes  eccen- 
tric, so  that  the  surface  of  the  roller  forms 
really  a  part  of  a  spiral  by  means  of  which 
the  bone  articulating  with  it  is  forced  away 
from  the  central  axis  of  rotation  and  be- 
comes jammed  as  if  stopped  by  a  wedge. 
The  best  example  of  this  is  the  knee.  In 
this  joint  the  axis  of  rotation  is  near  the 
posterior  surfaces  of  the  bones,  and  passes 
transversely  through  the  condyles  of  the 
femur,  the  surfaces  of  which  form  an  arc, 
the  centre  for  which  corresponds  to  the  axis 
of  motion.  In  ordinary  flexion  the  head  of 
the  tibia  moves  on  the  arc  around  the  axis 
so  as  to  partially  relax  the  lateral  ligament 
and  allow  of  some  rotation  on  the  axis  of 
the  tibia.  When  the  head  of  the  tibia  moves 
forwards,  in  extension,  it  becomes  wedged 
against  the  anterior  part  of  the  articular 
surface  of  the  femur,  which  presents  an  eccentric  spiral-like  curve, 
departing  more  and  more  from  the  centre  of  rotation  as  the  ar- 


FiG.  195. 


Diagram  of  the 
action  of  the  knee- 
joint. — w=  articular 
surface  of  femur,  e 
=  tibia  in  position  of 
extension.  f  =  tibia 
in  position  of  flexion, 
c  ^=  centre  of  rota- 
tion. 


478  MANUAL   OF    PHYSIOLOGY. 

ticular  surface  of  the  tibia  proceeds  forwards.  The  effect  of  this 
is,  that  in  exteusion  of  the  leg  the  ligaments  are  made  tense,  and 
the  bones  are  firmly  locked  together.  Owing  to  the  inequality 
between  the  size  of  the  internal  and  external  condyles  the  axis 
of  rotation  is  not  at  right  angles  to  the  axis  of  the  femur,  but  is 
at  such  an  angle  that  extreme  extension  causes  also  a  slight 
amount  of  outward  motion  of  the  leg. 

6.  In  the  ball  and  socket  joints — the  name  of  which  implies 
their  mechanism — the  most  varied  movements  occur.  (Hip  and 
shoulder.) 

Standing. 

In  order  that  an  elongated  rigid  body  may  stand  upright  it  is 
only  necessary  that  a  line  drawn  vertically  through  its  centre  of 
gravity  should  pass  within  its  basis  of  support,  and  if  the  latter 
be  sufficiently  wide  the  object  will  remain  permanently  in  that 
position.  The  human  body  is,  however,  in  the  first  place  not 
rigid,  and  in  the  second  place  the  basis  of  support  is  too  small 
to  insure  a  satisfactory  degree  of  steadiness.  The  act  of  standing 
must,  therefore,  be  accomplished  by  the  action  of  certain  muscles, 
which  are  employed  in  preventing  the  different  joints  from  col- 
lapsing, and  in  so  balancing  the  various  parts  of  the  body  as  to 
keep  the  whole  frame  from  toppling  over. 

In  order  to  economize  muscular  energy  while  standing,  we 
must  lock  as  many  of  our  joints  as  possible,  and  thus  depend 
rather  on  the  passive  ligaments  than  upon  muscle  action  for  the 
rigidity  of  the  body.  With  this  object  we  are  taught  to  place 
the  heels  together,  turn  out  the  toes,  bring  the  legs  parallel  by 
approximating  them,  and  extending  the  knees  to  the  utmost,  to 
straighten  and  to  throw  back  the  trunk  so  as  to  render  tense  the 
anterior  hip  ligaments,  to  direct  the  face  straight  forwards  so  as 
to  balance  the  head  evenly,  and  to  let  the  arms  fall  by  the  sides. 

In  this  position,  as  a  soldier  stands  at  attention,  the  knee  and 
hip-joints  remain  fixed,  without  any  effort  on  the  part  of  the 
muscles,  but  it  is  far  from  being  the  most  comfortable  attitude 
one  can  assume  for  prolonged  standing,  and  hence  the  position 
known  best  by  the  order  "  stand  at  ease  "  is  adopted  if  more  com- 


ERECT  POSTURE.  479 

plete  rest  is  desired.  In  this  position  the  weight  of  the  body  is 
usually  allowed  to  rest  on  one  leg  while  the  other  lightly  touches 
the  ground  to  form  a  kind  of  stay  and  relieve  the  muscles  which 
surround  the  supporting  ankle  from  too  great  an  effort  of  bal- 
ancing. At  the  same  time  the  knee  is  extended  and  the  pelvis 
becomes  somewhat  oblique  so  as  to  bring  it  more  directly  over  the 
head  of  the  femur.  In  ordinary  easy  standing,  the  joints  are  not 
commonly  kept  locked  by  the  tension  of  the  ligamentous  struc- 
ture, but  their  position  is  constantly  being  very  slightly  altered 
so  as  to  vary  the  muscles  employed  in  preserving  the  balance  and 
thus  to  prevent  fatigue. 

The  joints  most  exercised  in  the  erect  posture  are  the  following: 

1.  The  anhle  has  to  support  the  weight  of  the  entire  body, 
while  the  joint  is  neither  flexed  nor  extended  to  its  utmost,  and 
cannot  be  fixed  in  this  position  by  ligamentous  arrangements. 
The  foot,  being  placed  on  the  ground,  resting  on  the  heel  and  the 
balls  of  the  great  and  little  toes,  is  supported  in  an  arch-like  form 
by  strong,  though  elastic,  ligaments,  which  allow  but  little  motion 
in  the  numerous  joints.  The  bones  of  the  leg  can  move  in  the 
freest  way,  backwards  or  forwards,  around  the  articular  surface 
of  the  astragalus,  which  forms  the  roller  of  the  hinge,  any  lateral 
motion  of  which  is  prevented  by  the  malleoli.  The  line  passing 
through  the  centre  of  gravity  of  the  body  generally  falls  slightly 
in  front  of  the  axis  of  rotation  of  the  ankle-joint,  so  that  the 
entire  body  tends  to  falls  forwards  at  the  ankles.  This  tendency 
is  checked  by  the  powerful  calf  muscles,  which,  attached  to  the 
calcaneum  by  means  of  the  strong  tendo  Achillis,  keep  the  parts 
in  such  a  position  that  an  exact  balance  is  nearly  constantly 
kept  up. 

2.  The  knee-joint,  when  completely  extended,  requires  no  mus- 
cular action  to  prevent  it  from  bending,  because  the  line  of  grav- 
ity then  passes  in  front  of  the  axis  of  rotation,  and  the  weight  of 
the  body  tends  to  bend  the  knee  backwards.  This  is  impossible 
on  account  of  the  powerful  ligaments  which  exert  their  traction 
behind  the  axis  of  rotation.  Commonly,  however,  these  ligaments 
are  not  put  on  the  stretch  in  this  way,  but  the  joint  is  held,  by 
muscular  power,  in  such  a  position  that  the  line  of  gravity  passes 


480  MANUAL   OF   PHYSIOLOGY. 

just  through,  or  very  slightly  behind,  the  axis  of  rotation  of  the 
joint,  so  that,  if  anything,  there  is  a  slight  tendency  for  the  knee 
to  bend.  This  is  completely  checked,  and  the  body  balanced  by 
the  powerful  extensor  muscles  of  the  thigh. 

3.  In  the  hip  joints,  which  have  to  support  the  trunk  and  head, 
thelineof  gravity  falls  just  behind  the  line  uniting  the  joints  when 
the  person  is  perfectly  erect,  so  that  here  the  body  has  a  tendency 
to  fall  backwards.  This  is  prevented  by  the  strong  ilio-femoral 
ligament.  When,  however,  the  knee  is  not  straightened  to  the 
full  extent,  so  that  the  line  of  gravity  passes  through  or  a  little 
behind  the  axis  of  rotation  of  that  joint,  then  the  pelvis  is  very 
slightly  flexed  on  the  femora  so  that  the  axis  of  the  joints  lies 
exactly  in  or  a  little  behind  the  line  of  gravity,  and  thus  the  body 
inclines  rather  to  fall  forwards.  This  tendency,  however,  is  pre- 
vented by  the  powerful  glutei  muscles,  which  also  enable  us  to 
regain  the  erect  posture  after  bending  the  trunk  forwards. 

The  motions  of  which  the  pelvis  and  vertebral  column  are  capable 
are  too  slight  to  deserve  attention  here.  The  vertebral  column, 
wedged  in  as  it  is  between  the  two  innominate  bones,  may  be 
taken,  together  with  the  pelvis,  as  forming  a  very  yielding  and 
elastic,  but  practically  jointless,  pillar,  the  upper  part  of  which 
can  alone  be  bent  to  such  an  extent  as  to  require  mention  in  dis- 
cussing the  mechanism  of  station. 

The  individual  joints  between  the  cervical  vertebrce  permit  but 
a  slight  amount  of  movement  when  taken  separately,  but  by 
their  aggregate  motion  they  enable  considerable  extension  and 
flexion  of  the  neck  to  take  place.  These  motions  follow  so  closely, 
and  are  so  inseparably  associated  with  those  of  the  head  on  the 
upper  vertebra,  that  there  is  no  need  to  consider  them  separately 
from  the  latter. 

The  atlaiito- occipital  joints  admit  of  some  little  lateral  move- 
ment, but  that  in  the  antero-posterior  direction  is  much  the  more 
important,  but  even  this  would  be  insignificant  were  it  not  asso- 
ciated with  the  movements  between  the  other  cervical  vertebrae. 

The  cranium  has  then  to  be  balanced  on  the  top  of  a  flexible 
column,  and  rests  immediately  in  a  kind  of  socket,  which  can 
move  as  a  double  hinge  around  two  axes  at  right  angles  one  to 


WALKING    AND    RUNNING.  481 

the  other.  The  vertical  line  from  the  centre  of  gravity  of  the 
cranium  must  vary  with  every  forward,  backward,  or  lateral 
movement  of  the  head  or  neck,  but  in  the  erect  posture  it  passes 
a  little  in  front  of  the  axis  of  rotation  of  the  atlanto-occipital  joint 
and  somewhat  behind  the  curve  of  the  cervical  vertebrae,  so  that 
the  head  may  be  said  to  be  poised  on  the  apex  of  the  vertebral 
column,  with  some  tendency  to  fall  forwards.  There  are  no 
ligamentous  structures  which  can  lock  the  joints  so  as  to  keep 
the  head  in  the  erect  position  ;  therefore  without  the  aid  of  mus- 
cular force  the  head  will  fall  forwards  or  backwards,  according 
to  the  position  it  is  in  when  the  muscles  become  relaxed  as  in 
sleep. 

From  the  foregoing  facts  it  will  be  seen  that  there  exists  a  kind 
of  coordinated  antagonism  at  work  in  ordinary  easy  standing 
which  keeps  the  elastic  pliable  body  upright,  without  the  rigidity 
adopted  when  standing  "at  attention."  The  muscular  action  is 
more  exercised  when  we  are  not  on  steady  ground  and  varied  co- 
ordination becomes  necessary ;  for  instance  when  we  go  on  board 
ship  for  the  first  time.  Station  then  takes  some  little  time  to  be- 
come perfected,  and  requires  new  associations  of  movement.  The 
gastrocnemius  and  soleus  relax  the  ankle  in  a  degree  just  propor- 
tionate to  the  amount  of  flexion  of  the  knee  permitted  by  the 
quadriceps  extensor  cruris,  while  simultaneously  the  great  gluteal 
muscle  allows  the  body  to  incline  forwards  so  as  to  keep  its  cen- 
tre of  gravity  in  the  proper  relation  to  the  basis  of  support. 

Walking  and  Running. 

The  common  act  of  progression  is  accomplished  by  poising  the 
weight  of  the  body  alternately  on  one  leg — called  the  supporting 
limb — and,  with  the  other — the  pendulous  limb — tilting  the  body 
forwards  out  of  equilibrium,  and  then  swinging  the  latter  limb 
forwards  and  placing  it  in  front  so  as  to  prevent  the  body  falling 
forwards.  In  its  turn  this  then  becomes  the  supporting  leg.  The 
swinging  leg  is  described  as  having  two  phases  (1)  active  while 
pushing  off"  from  the  ground,  and  (2)  passive,  while  swinging  for- 
wards like  a  pendulum.  In  starting  one  foot  is  placed  behind 
the  other,  so  that  the  line  of  gravity  lies  between  the  two,  the 


482  MANUAL   OF   PHYSIOLOGY. 

hindmost  linil)  having  the  ankle  and  knee  a  little  bent.  By  sud- 
denly straightening  these  joints  it  gives  a  "push  off"  with  the 
toes  and  propels  the  body  forward  so  as  to  move  it  around  the 
axis  of  motion  of  the  fixed  or  supporting  ankle-joint.  At  the  end 
of  the  swing  the  pendulous  leg  comes  to  the  ground  and  leaves 
the  other  limb  in  the  attitude  ready  for  the  push  off.  Thus  on 
level  ground  walking  is  carried  on  with  but  small  muscular  exer- 
cise ;  but  in  ascending  a  steep  incline  or  going  up  stairs,  the  sup- 
porting limb  has  to  elevate  the  body  at  each  step  by  extending 
the  knee  and  ankle-joints  by  the  thigh  extensors  and  the  calf 
muscles. 

Running  is  distinguished  from  walking  by  the  fact  that,  while 
in  the  latter  both  feet  rest  on  the  ground  for  the  greater  part  of 
each  pace,  in  the  former  the  time  that  either  foot  rests  on  the 
ground  is  reduced  to  a  minimum,  and  in  fact  the  supporting  limb 
disappears.  The  legs  are  kept  in  a  semi-flexed  position  ready  for 
the  push  off  or  spring,  which  is  so  forcibly  carried  out  that  the 
body  is  propelled  through  the  air  without  any  support.  Thus  an 
interval — of  greater  or  less  duration  according  to  the  pace — exists 
during  which  both  the  feet  are  off  the  ground,  as  the  moment 
either  foot  comes  to  the  ground  it  executes  a  new  spring  without 
waiting  for  the  pendulous  swing  described  in  walking. 


CHAPTER  XXVII. 

VOICE  AND  SPEECH. 

The  human  voice  is  produced  by  an  expiratory  blast  of  air 
being  forced  through  the  narrow  opening  at  the  top  of  the  wind- 
pipe called  the  glottis.  This  glottis,  which  lies  in  the  lower 
part  of  the  larynx,  is  bounded  on  each  side  by  the  edges  of  thin, 
elastic,  membranous  folds  that  project  into  the  air-passages. 
These  membranous  folds,  called  the  vocal  cords,  are  set  vibrating 
by  the  current  of  air  from  below,  and  in  turn  communicate  their 
vibrations  as  sound  to  the  air  in  the  air- passages  situated  above 
them. 

Anatomical  Sketch. 

The  vocal  apparatus  is  really  a  musical  instrument  of  the  reed- 
pipe  kind.  If  we  compare  it  with  the  pipe  of  an  organ,  we  find 
all  the  parts  of  the  latter  represented.  The  lungs  within  the 
moving  thorax  act  as  the  bellows.  The  bronchi  and  trachea  are 
the  supply  pipes  and  air  box.  The  vocal  cords  are  the  vibrating 
tongues ;  while  the  larynx,  pharynx,  mouth,  and  nose,  act  as  the 
accessory  or  resonating  pipes.  The  blast  of  air  is  produced  and 
regulated  by  the  respiratory  muscles  ;  and  special  intrinsic  mus- 
cles of  the  larynx  change  the  condition  of  the  vocal  cords  so  as  to 
alter  the  pitch  of  the  notes  produced.  Other  sets  of  muscles,  by 
altering  the  condition  of  the  resonating  pipes,  give  rise  to  many 
modifications  in  the  vocal  tones,  and  thus  produce  what  is  called 
speech. 

The  larynx,  which  may  be  regarded  as  the  special  organ  of 
voice,  is,  in  the  main,  made  up  of  four  cartilages,  viz.,  the  cricoid, 
thyroid,  and  two  arytenoids,  jointed  together  so  as  to  allow  of  con- 
siderable motion.     Of  these  the  inferior,  the  cricoid,  is  attached 


484 


MANUAL   OF   PHYSIOLOGY. 


to  the  trachea  which  it  joins  to  the  others.  It  forms  a  ring,  which 
is  thin  iu  front,  but  deep  and  thick  behind,  owing  to  a  peculiar 
projection  upwards  of  its  posterior  part.  The  thyroid  consists  of 
two  side  wings  so  bent  as  to  form  the  greater  part  of  the  anterior 


Anterior  half  of  a  transverse  section  through  the  larynx  near  its  middle. 
More  is  cut  away  on  the  upper  part  of  the  right  side. — 1.  Upper  division  of 
the  laryngeal  cavity ;  2.  Central  portion  ;  3.  Lower  portion  continued  into  4, 
trachea;  e,  epiglottis;  e',  its  cushion;  t,  thyroid  cartilage  seen  in  section, 
vl,  true  vocal  cord  at  the  riraa  glottidis;  s,  ventricle  of  larynx;  «'',  saccule. 
(A,  Thomson.) 


and  lateral  boundaries  of  the  voice  box,  and  can  be  felt  easily  in 
the  front  of  the  throat.  It  is  articulated  to  the  sides  of  the  cricoid 
by  its  two  inferior  and  posterior  extremities,  so  that  the  upper 
part  of  the  cricoid  cartilage  can  move  backwards  and  forwards. 


ANATOMY   OF   THE    LARYNX. 


485 


The  arytenoid  cartilages  are  little  three-sided 
pyramidal  masses  placed  on  the  upper  surface 
of  the  posterior  part  of  the  cricoid,  to  which 
they  are  attached  by  a  loose  joint.  They  are 
so  placed  that  one  surface  looks  inwards,  the 
second  backwards,  and  the  third  forwards  and 
outwards,  while  the  inferior  surface  rides  on  the 
cricoid.  One  point,  therefore,  looks  forwards, 
and  to  it  is  attached  the  vocal  cord  on  each 
side,  hence  it  has  been  called  the  vocal  process. 
The  apex,  which  looks  outwards  and  back- 
wards, gives  attachment  to  some  of  the  in- 
trinsic muscles,  and  hence  has  been  called  the 
muscular  process. 

The  thyroid  cartilage  is  connected  with 
the  cricoid  below,  and  to  the  hyoid  bone 
above  by  ligaments  and  tough  membranes, 
which  hold  the  parts  together,  fill  in  the 
intervals,  and  complete  the  skeleton  of  the 
larynx. 

The  vocal  cords  are  composed  of  small 
strands  of  elastic  tissue,  which  are  stretched 
between  the  anterior  processes  of  the  arytenoid 
cartilages  and  the  inferior  part  of  the  thyroid, 
where  they  are  attached  side  by  side  to  the 
posterior  surface  of  the  angle  formed  by  the 
junction  of  the  two  lateral  parts  or  alse  of  the 
thyroid.  The  mucous  membrane  which  lines 
the  larynx  is  thin,  and  closely  adherent  over 
the  vocal  cords.*  The  surface  of  the  laryngeal 
cavity  is  smooth  and  even,  the  lining  membrane 
passing  over  the  cartilages  and  muscles  so  as 
to  obliterate  all  ridges  except  the  vocal  cords 
and  two  others,  less  sharply  defined,  called  the 
false  vocal  cords,  which  lie  parallel  to  and 
above  the  true  vibrating  cords.  Between 
these  is  the  cavity  known  as  the  ventricle  of 
the  larynx. 


Three  diagrams 
taken  from  laryngo- 
scopic  views  of  the 
superior  aperture  of 
the  larynx,  showing 
the  position  of  the 
vocal  cords  and  the 
arytenoid  cartilages 
supposed  to  be  seen 
in  transverse  sec- 
tion during  differ- 
ent actions  of  the 
larynx. — k.' .  Vocal 
chink  as  in  singing. 
b'  In  easy  quiet  in- 
halation of  air.  c' . 
In  forced  inspira- 
tion. 


486  manual  of  physiology. 

Mechanism  of  Vocalization. 

Taking  the  thyroid  cartilage  as  the  fixed  base,  the  cricoid  and 
arytenoid  cartilages  undergo  movements  which  bring  about  two 
distinct  sets  of  changes  in  the  glottis  and  its  elastic  edges,  namely, 
(1)  widening  and  narrowing  the  opening;  (2j  stretching  and  re- 
laxing of  the  vocal  cords.  During  ordinary  respiration  the  glottis 
remains  about  half  open,  the  muscles  being  in  a  state  of  relaxa- 
tion (B').  During  forced  inspiration  the  glottis  is  widely  dilated 
by  muscular  action  (C).  If  an  irritating  gas  be  inspired,  the 
glottis  is  tightly  closed  by  a  spasmodic  action  of  certain  muscles, 
so  that  the  true  vocal  cords  act  as  a  kind  of  valve. 

During  vocalization  the  glottis  is  formed  into  a  narrow  chink 
with  parallel  sides  (A'),  while  the  cords  are  made  more  or  less 
tense,  according  to  the  pitch  of  the  note  to  be  produced  ;  both 
these  changes  being  brought  about  by  muscular  action. 

The  opening  of  the  chink  of  the  glottis  is  accomplished  chiefly 
by  a  muscle  called  the  posterior  crico-arytenoid,  which  passes  from 
the  posterior  surface  of  the  cricoid  cartilage  to  the  outer  and  poste- 
rior angle  of  the  arytenoids.  By  pulling  the  latter  point  down- 
wards and  backwards  it  separates  the  arytenoid  cartilages, 
particularly  at  their  anterior  extremity,  where  the  cords  are  at- 
tached. In  this  action  they  are  aided  by  a  small  muscle  con- 
necting the  posterior  surfaces  of  the  arytenoid,  namely,  the 
posterior  arytenoid,  which  tends,  when  the  two  arytenoid  cartilages 
are  held  apart,  to  rotate  them  so  that  the  vocal  processes  are 
separated. 

The  narrowing  of  the  glottis  is  executed  by  the  lateral  crico- 
arytenoids which  run  upwards  and  backwards  from  the  antero- 
lateral aspect  of  the  cricoid  to  the  muscular  processes  of  the  ary- 
tenoid cartilages.  They  pull  the  muscular  processes  forwards, 
and  thus  rotate  the  arytenoid  cartilages  so  as  to  approximate  the 
vocal  processes  to  one  another,  while  any  tendency  towards  pull- 
ing apart  the  bodies  of  the  cartilages,  owing  to  the  downward 
direction  of  the  muscle,  is  overcome  by  the  posterior  arytenoid 
muscle  and  those  muscular  bauds  which  pass  from  the  posterior 
surface  of  the  arytenoid  cartilages  to  the  epiglottis  and  the  upper 


INTRINSIC    MUSCLES   OF   THE   LARYNX. 


487 


part  of  the  thyroid  cartilage,  the  external  thyro-arytenoid,  aud 
the  thyro-ary-epiglottic  muscles  (Heule).  The  other  fibres,  which 
pass  directly  from  the  arytenoid  to  the  thyroid  cartilages — inter- 
nal and  external  thyro-arytenoid  muscles — in  the  same  direction 
as  the  vocal  cords,  complete  the  closure  by  helping  to  press  to- 
gether the  vocal  processes  and  by  approximating  the  cords  them- 
selves. In  spasmodic  closure  of  the  glottis,  all  these  latter  muscles 
act  violently  together,  and  have  been  grouped  by  Henle  as  the 


Fig.  198. 


Fig.  199. 


Cr.\ 

I 

Fig.  198. — Diagram  of  the  side  view  of  the  larynx  showing  the  position 
of  the  vocal  cords  (v).  (Huxley.) — ^r.  Arytenoid  cartilage.  Hy.  llyoid 
bone.  Th.  Thyroid  cartilage.  Cr.  Cricoid  cartilage.  Tr.  Trachea.  G.th. 
Crico-thyroid  muscle.     TLA.  Thyro-arytenoid  muscle.     Ep.  Epiglottis. 

Fig.  199. — Diagram  of  the  opening  of  the  larynx  from  above.  (Huxley.) 
— Th.  Thyroid  cartilage.  Cr.  Cricoid  cartilage.  Ary.  Superior  extremities 
of  the  arytenoid  cartilages.  V.  Vocal  cords.  Th.A.  Thyro-arytenoid 
muscles.  C.a.l.  Lateral  crico-arytenoid  muscle.  C.a.p.  Posterior  crico- 
arytenoid muscle.     Ar.p.  Posterior  arytenoid  muscle. 

constrictor  of  the  glottis.  Relaxation  of  the  vocal  cords  accom- 
panies voluntary  closure  of  the  glottis,  as  in  holding  the  breath, 
when  the  false  vocal  cords  are  said  to  have  a  valvular  action. 
But  the  muscular  fibres,  which  run  from  the  arytenoid  cartilages 
to  the  thyroid,  nearly  parallel  to  the  true  vocal  cords,  are  those 
concerned  in  the  act  of  relaxation  when  the  cords  are  active. 
They  pull  forwards  the  arytenoid  cartilages,  and  at  the  same  time 


488  MANUAL  OF   PHYSIOLOGY. 

draw  the  upper  part  of  the  cricoid  slightly  forwards.  Moreover, 
these  muscles  have  au  all-important  action  in  adapting  the  edges 
of  the  cords  and  the  neighboring  surfaces  to  the  exact  shape  most 
advantageous  to  their  vibration. 

The  stretching  of  the  vocal  cords  is  caused  by  the  contraction 
of  one  muscle,  the  crico-thyroid,  which,  on  the  outer  side  of  the 
larynx,  passes  downwards  and  forwards  from  the  lower  part  of 
the  thyroid  to  the  anterior  part  of  the  cricoid  cartilage.  It  thus 
pulls  the  anterior  part  of  the  cricoid  cartilage  upwards,  causing 
it  to  rotate  round  an  axis  passing  through  its  thyroid  joints.  The 
upper  part  of  the  cricoid,  which  carries  the  arytenoids,  is  thus  re- 
moved from  the  anterior  attachment  of  the  vocal  cords,  and  the 
membranes  are  put  on  the  stretch. 

The  requirements  necessary  for  the  production  of  voice  are  the 
following  : 

1.  Perfect  elasticity  and  clearness  of  edge  of  the  vocal  cords, 
and  freedom  from  all  surface  irregularity,  such  as  would  be  caused 
by  thick  mucus  adhering  to  them,  or  any  abnormality. 

2.  The  cords  must  be  very  accurately  adjusted,  closely  approxi- 
mated together,  and  kept  parallel,  almost  touching  each  other 
evenly  throughout  their  entire  length. 

3.  The  cords  must  be  held  in  a  certain  degree  of  tension,  or  the 
vibration  does  not  produce  any  vocal  tone,  but  simply  a  raucous 
noise. 

4.  The  air  must  be  propelled  through  the  glottis  by  a  forced 
expiration.  The  normal  expiratory  current  is  too  gentle  to  give 
the  necessary  vibration.  After  the  operation  of  tracheotomy,  the 
air  escapes  through  the  abnormal  opening,  and  sufficient  pressure 
cannot  be  brought  to  bear  on  the  cords,  so  no  vocal  sound  can  be 
produced,  and  the  person  speaks  in  a  whisper,  unless  the  exit  of 
air  through  the  tracheotomy  tube  is  prevented  by  placing  the  fin- 
ger temporarily  upon  the  opening. 

Properties  of  the  Human  Voice. 

In  the  voice  we  can  recognize  the  same  properties  as  are  noted 

in  other  kinds  of  sound.     These  are  quality,  pitch  and  intensity. 

(1.)  The  quality  of  vocal  sounds  is  almost  endless  in  variety, 


VOCAL  QUALITIES.  489 

as  is  shown  by  the  vocal  capabilities  of  different  individuals. 
The  quality  of  tone  of  any  musical  sound  depends  upon  the  rela- 
tive power  of  the  fundamental  note,  and  of  the  overtones  that 
accompany  it.  The  less  disturbed  the  fundamental  notes  are  by 
overtones,  the  clearer  and  better  is  the  voice.  This  difference  in 
quality  of  the  human  voice  depends  upon  the  perfectness  of  the 
elasticity,  the  relation  of  thickness  to  length,  surface  smoothness, 
and  other  physical  conditions  of  the  cords  themselves,  and  the 
exactitude  with  which  the  muscles  can  adapt  the  surfaces.  For 
singing  well,  much  more  is  necessary  than  good  quality  of  tone, 
which  is  common  enough.  The  muscles  of  the  larynx,  thorax, 
and  mouth  must  be  all  educated  to  an  extraordinarily  high  degree. 

(2.)  The  pitch  of  the  notes  produced  in  the  larynx  depends 
upon — first,  the  absolute  length  of  the  vocal  cords.  This  varies 
with  age,  particularly  in  males,  whose  vocal  organs  undergo  rapid 
growth  at  puberty,  when  the  voice  is  said  to  crack.  The  vocal 
cords  of  women  have  been  found  by  measurement  to  be  about 
one-third  shorter  than  those  of  men,  and  people  with  tenor  voices 
have  shorter  cords  than  basses  or  baritones.  Secondly,  on  the 
tension  of  the  cords :  the  tighter  the  vocal  cords  are  drawn  by 
the  crico-thyroid  muscles,  the  higher  the  notes  produced ;  and  the 
well-known  singer  Garcia  believed  he  observed  with  the  laryn- 
goscope the  vocal  processes  so  tightly  pressed  together  as  to  im- 
pede the  vibration  of  the  posterior  part  of  the  cords,  and  by  this 
means  they  could  be  voluntarily  shortened. 

(3.)  Intensittj  or  loudness  of  the  voice  depends  solely  on  the 
strength  of  the  current  of  air.  The  more  powerful  the  air-blast 
the  greater  amplitude  of  the  vibrations,  and  hence  the  greater 
the  sound  produced.  The  narrower  the  chink  of  the  glottis,  and 
the  tighter  the  parallel  cords  are  stretched,  the  less  is  the  amount 
of  air  and  the  weaker  is  the  blast  required  to  set  them  vibrating; 
and  vice  versa,  the  looser  the  cords  and  the  wider  apart  they  are, 
the  greater  the  volume  and  the  force  of  the  air-current  necessary 
for  their  complete  vibration.  Hence  it  is  that  an  intense  vibra- 
tion or  loud  note  can  be  produced  much  more  easily  with  notes 
of  a  high  pitch  than  with  very  low  notes,  and  we  find  singers 

41 


490  MANUAL   OF   PHYSIOLOGY. 

choosing  for  their  telling  crescendo  some  note  high  up  in  the  range 
of  their  voice. 

The  human  voice,  including  all  kinds  of  voice,  extends  over 
about  three  and  a  half  octaves.  Of  this  wide  range  a  single  in- 
dividual can  seldom  sing  more  than  two  octaves.  The  soprano, 
alto,  tenor,  and  bass  form  a  descending  series,  the  range  of  each 
one  of  which  considerably  overlaps  the  next  in  the  scale. 

During  the  ordinary  vocal  sounds,  the  air,  both  in  the  resonat- 
ing tubes  above  the  larynx  and  in  the  windpipes  coming  from 
below,  is  set  vibrating,  so  that  the  trachea  and  bronchi  act  as 
resonators  as  well  as  the  pharynx,  mouth,  etc.  This  may  be  rec- 
ognized by  placing  the  hand  on  the  thorax,  when  a  distinct 
vibration  is  communicated  from  the  chest-wall.  Such  tones  are, 
therefore,  spoken  of  as  chest  notes.  Besides  the  chest  tones  of 
the  ordinary  voice,  Ave  can  produce  notes  of  a  higher  pitch  and 
a  different  quality,  which  are  called  head  notes,  since  their  pro- 
duction is  not  accompanied  by  any  vibration  of  the  chest-wall. 
The  physical  contrivance  by  means  of  which  this  falsetto  voice  is 
brought  about  is  not  very  clearly  made  out.  The  following  are 
the  more  probable  views :  (1)  It  has  been  suggested  that  in 
falsetto  only  the  thin  edges  of  the  cords  vibrate,  the  internal 
thyro-arytenoid  muscles  keeping  the  base  of  the  cord  fixed  ;  while 
with  chest  tones  a  greater  surface  of  the  cord  is  brought  into 
play.  (2)  The  cords  are  said  to  be  wider  apart  in  falsetto  than 
in  chest  notes,  and  hence  the  trachea,  etc.,  ceases  to  act  as  a  reso- 
nator. (3)  Or  the  cords  may  be  arranged  so  that  only  one  part 
of  them,  the  anterior,  can  vibrate,  and  thus  they  act  as  shortened 
cords,  a  "  stop  "  being  placed  on  the  point  where  the  vibrations 
cease,  by  the  internal  thyro-arytenoid  muscle. 

The  production  of  a  falsetto  voice  is  distinctly  voluntary,  and 
is  probably  dependent  upon  some  muscular  action  in  immediate 
relation  to  the  cords,  for  it  is  always  associated  with  a  sensation 
of  muscular  exertion  in  the  larynx  as  well  as  with  changes  that 
take  place  in  the  conformation  of  the  mouth  and  other  resonat- 
ing tubes. 


SPEECH.  491 

Nervous  Mechanism  of  Voice. 

The  Dervous  mechauism  by  means  of  which  vocal  souuds  are 
produced  is  amoog  the  most  completely  coordinated  actions  that 
regulate  muscular  movements. 

Like  respiration,  vocalization  at  first  seems  a  simple  voluntary 
act,  sounds  of  various  kinds  being  produced  at  the  command  of 
the  will  of  the  individual.  No  doubt  the  respiratory  muscles, 
which  work  the  bellows  of  the  voice  organ,  are  under  the  control 
of  the  will  so  long  as  the  respiration  is  not  interfered  with.  The 
mouth  and  throat  muscles  which  shape  the  resonating  tube  are 
also  voluntary.  But  the  intrinsic  muscles  of  the  larynx  are  only 
voluntary  in  a  certain  sense,  while  in  another  they  are  distinctly 
involuntary,  as  may  be  seen  in  spasm  of  the  larynx ;  for  they 
are,  in  part  at  least,  controlled  by  impulses  which  arise  at  the 
organ  of  hearing  and  pass  to  some  coordinating  centre,  which 
arranges  the  finer  muscular  movements  necessarv  to  produce  a 
certain  note.  When  we  sing  any  note  struck  on  a  musical  instru- 
ment, we  set  the  expiratory,  mouth,  and  general  vocalizing  mus- 
cles in  readiness  for  the  proper  application  of  the  air  blast  by  a 
voluntary  act ;  but  the  exact  tuning  of  the  vocal  cords  is  accom- 
plished in  some  measure  reflexly  by  impulses  coming  from  the 
ear  to  a  special  coordinating  centre,  the  education  of  which  is 
commonly  in  advance  of  the  volition  centres,  and  therefore  can 
only  be  controlled  by  the  latter  in  persons  specially  educated  to 
music.  Some  persons  who  can  sing  a  given  note  with  promptness 
and  exactitude  without  any  effort,  would  find  much  difficulty  in 
overcoming,  by  volition,  the  accuracy  of  this  perfect  reflex  mech- 
anism. In  fact,  a  person  with  a  good  ear  finds  it  difficult  to  sing 
out  of  tune,  even  if  he  try. 

Though  we  feel  that  we  have  command  over  the  pitch  of  our 
sound-producing  organ,  we  owe  much  to  the  aid  of  our  sound- 
appreciating  organs  and  the  nerve  centres  which  they  have  in 
connection  with  them. 

Speech. 

The  variations  in  vocal  sounds  which  give  rise  to  speech  are  not 
produced  in  the  larynx,  but  in  the  throat,  mouth,  and  nose.   When 


492  MANUAL   OF   PHYSIOLOGY. 

unaccompauied  by  any  vocal  sound,  speech  only  gives  rise  to  a 
whisper;  but  when  a  vocal  tone  is  at  the  same  time  produced, 
we  have  the  ordinary  loud  speaking.  Since  vocal  tones  can  only 
be  produced  by  expiration,  so  we  can  only  speak  aloud  by  means 
of  an  expiratory  current  of  air;  but  an  inspiratory  current  may 
be  made  to  give  rise  to  a  kind  of  whisper. 

Speech  is  composed  of  two  kinds  of  sounds,  in  one  of  which  the 
sounds  must  be  accompanied  by  a  vocal  tone,  and  are  hence  called 
"  vowels ;"  in  the  other  no  vocal  tone  is  necessary,  but  changes  in 
shape  take  place  in  the  resonating  chambers,  so  as  to  give  rise  to 
noises  called  consonants.  As  the  pronunciation  of  the  consonants 
is  usually  accompanied  by  some  vowel  sound,  and  further  from 
the  fact  that  the  difference  between  the  vowels  is  brought  about 
by  changes  in  the  shape  of  the  mouth,  the  distinction  between 
the  two  sets  of  sound  is  rather  artificial  than  real. 

The  production  of  the  different  vowel  sounds  depends  upon 
such  a  change  being  brought  about  in  the  shape  of  the  mouth 
cavity  and  aperture,  that  a  resonator  with  a  difierent  individual 
note  is  formed  for  each  particular  vowel. 

The  sounds  called  consonants  are  caused  by  some  check  or  im- 
pediment being  placed  in  the  course  of  the  blast  of  air  issuing 
from  the  air-passages.  They  may  be  classified,  according  to  the 
part  at  which  the  obstruction  occurs,  as  follows: 

1.  Labials,  when  the  narrowing  takes  place  at  the  lips,  as  in 
pronouncing  b,  p,f,  v. 

2.  Dentals,  when  the  tongue  causes  the  obstruction  by  being 
pushed  against  the  hard  palate  or  the  teeth,  as  in  t,  d,  s,  I. 

3.  Gutturals,  when  the  posterior  part  of  the  tongue  moves  to- 
wards the  soft  palate  or  pharynx,  as  in  saying  k,  g,  gh,  ch,  r. 

Consonants  may  also  be  divided  into  different  groups,  accord- 
ing to  the  kind  of  movements  which  give  rise  to  them. 

1.  Explosives  are  produced  by  the  sudden  removal  of  the  ob- 
struction, as  with  p,  d,  L 

2.  Aspirates  are  continuous  sounds  caused  by  the  passage  of  a 
current  of  air  through  a  narrow  opening,  which  may  be  at  the 
lips,  as  in/,  at  the  teeth  as  with  s,  or  at  the  throat  as  in  ch. 


SPEECH.  493 

3.  Resonants  are  the  sounds  which  require  some  resonance  of 
the  vocal  cords,  and  the  air  current  is  suddenly  checked  by  clos- 
ure of  the  lips,  as  in  m,  or  the  dental  aperture  as  in  n  or  ng. 

4.  Vibratory,  of  which  r  is  the  example,  requires  a  peculiar 
vibration  of  the  vocal  cords,  while  either  the  dental  or  the  gut- 
tural aperture  is  partially  closed. 


CHAPTER  XXVIII. 

GENERAL  PHYSIOLOGY  OF  THE  NERVOUS  SYSTEM. 

Anatomical  Sketch. 

The  nervous  system  is  the  apparatus  by  which  the  distant  parts 
of  the  body  are  kept  in  constant  relationship  with  one  another, 
so  that  a  change  of  state  of  any  one  organ  is  communicated  to 
and  may  set  up  corresponding  changes  in  remote  parts  of  the  sys- 
tem. It  is  made  up  of  two  varieties  of  tissue,  both  of  which  pos- 
sess special  vital  properties.  The  one,  which  is  composed  of  thread- 
like, strands  of  protoplasm — 7ierve  fibres — connect  together  the 
elements  of  the  other  group,  the  nerve  corpuscles,  which  form 
either  peripheral  or  central  terminals.  Nerve  fibres  are  then 
simply  special  conducting  agents,  having  at  one  extremity  a  spe- 
cial terminal  or  nerve  cell  for  sending  impulses,  and  at  the  other 
extremity  other  cells  for  receiving  the  same.  These  terminal 
organs,  between  which  the  nerve  fibres  pass,  are  the  agents  which 
determine  the  direction  the  impulse  is  to  travel  along  the  nerve. 
The  sending  organ  is  sometimes  at  the  peripheral  end  of  the  nerve, 
and  the  receiver  in  the  nerve  centres,  as  in  the  case  of  an  ordinary 
cutaneous  nerve,  which  carries  impulses  from  the  skin  to  the 
brain  ;  or  these  duties  of  the  terminal  organs  may  be  reversed, 
as  in  the  case  of  the  nerves  conveying  impulses  from  the  brain 
to  the  muscles. 

The  former  kind  of  nerves  are  called  afferent  or  centripetal,  and 
the  latter  efferent  or  centrifugal.  Nerves  are  capable  of  carrying 
impulses  in  either  direction,  as  has  been  proved  by  cutting  the 
afferent  lingual  and  the  efferent  hypoglossal  nerves,  and  causing 
the  proximal  end  of  the  former  to  unite  with  the  distal  end  of  the 
latter,  which  is  distributed  to  the  muscles  of  the  tongue.  When 
the  union  has  taken  place  a  stimulus  applied  on  the  efferent  por- 
tion causes  the  muscles  to  move. 


ANATOMICAL   SKETCH. 


495 


Any  piece  of  protoplasm  can  conduct  impulses,  as  is  seen  in  the 
rapid  transmission  of  an  impulse  in  animals  and  textures  which 
have  no  special  conducting  elements  or  nerve  fibres.  Thus  in  the 
hydra  all  the  cells  act  as  nerves,  and  in  the  higher  animals  an  im- 
pulse, producing  a  wave  of  contraction,  can  pass  on  from  one  muscle 
cell  to  the  other  directly,  as  is  seen  in  the  ureter,  or  in  the  frog's 
heart.     The  only  essential  part  of  a  nervous  conductor  is  a  deli- 


FiG.  200. 


Fig.  201. 


Fig.  200. — Highly  magnified  view  of  three  medullated  and  two  non-medul- 
lated  nerve  fibres  of  frog,  stained  with  osmic  acid,  which  makes  the  medul- 
lary sheath  black.  N.  Nodes  of  Ranvier — where  the  axis  cylinder  can  be 
seen  to  pass  the  gap  in  the  medullary  sheath. 

Fig.  201 . — Transverse  section  of  nerve  fibres,  showing  the  axis  cylinders 
cut  across,  and  looking  like  dots  surrounded  by  a  clear  zone,  which  is  the 
medullary  sheath.     Fine  connective  tissue  separates  the  fibres  into  bundles. 


cate  protoplasmic  fibril.  Single,  thin,  thread-like  fibrils  are  com- 
monly found  carrying  impulses  in  the  nerve  centres.  But  in  the 
nerves  distributed  about  the  body  one  does  not  meet  these  single 
protoplasmic  threads  (except  where  the  fibrils  are  interwoven  to 
form  terminal  networks,  as  seen  in  the  cornea),  but  the  fibrils  are 
clustered  together  in  large  bundles,  so  as  to  make  one  nerve  fibre. 
This  bundle  of  protoplasmic  fibrils  is,  in  the  peripheral  nerves, 
always  covered,  and  is  called  the  axis  cylinder  of  the  nerve  fibre. 


496  MANUAL   OF   PHYSIOLOGY. 

In  some  nerve  fibres  there  is  but  one  very  thin,  transparent  cov- 
ering, termed  the  primitive  sheath,  while  in  others  there  is  a  thick 
layer  of  doubly-refracting  fluid  inside  the  primitive  sheath,  in 
immediate  contact  with  the  fibrils  of  the  axis  cylinder.  This  is 
called  the  medullary  sheath,  or  white  substance  of  Schwann,  be- 
cause its  peculiar  refractive  properties  make  it  look  white  when 
viewed  in  a  direct  light.  According  as  the  nerves  have  or  have 
not  this  medullary  sheath,  they  have  been  termed  "white"  or 
"gray."  The  former  are  by  far  the  most  plentiful,  since  they 
make  up  the  greater  part  of  the  ordinary  nerves,  while  the  gray 
fibres  only  predominate  in  the  sympathetic  nerve  and  its  ramifi- 
cations and  parts  of  the  special  sense  organs. 

An  ordinary  nerve,  then,  is  made  up  of  a  large  number  of  fibres 
held  together  by  connective  tissue,  and  each  of  the  fibres  contains 
a  vast  number  of  fibrils  within  its  sheath. 

Functional  Classification. 

Nerve  fibres  may  be  classified  according  to  their  function,  in 
the  following  way : 

I.  Afferent  nerves,  those  which  bear  impulses  from  the  surface 
to  the  nervous  centres.     These  may  be  further  divided  into : 

(a.)  Sensory  nerves,  when  the  impulse  they  convey  gives  rise  to 
a  "  perception."  The  perceptions  may  be  the  special  sensations 
which  are  transmitted  from  the  organs  of  special  sense,  or  those 
of  general  sensation,  giving  rise  to  pleasure  or  pain. 

(h.)  Reflex  nerves,  which  communicate  the  impulse  to  some 
other  nerve  elements,  and  thus  give  rise  to  some  new  forces,  with- 
out any  perception  of  the  stimulus.  According  to  the  result  of 
the  excitation  resulting  from  their  reflected  impulse  they  are 
termed — excito-motor,  excito-secretory,  and  excito-inhibitory,  etc. 

(c.)  Those  nerves  which  act  both  as  sensory  and  reflex  nerves ; 
these  are  the  most  numerous,  the  sensory  or  reflex  action  depend- 
ing upon  the  condition  of  the  nerve  centres. 

II.  Efferent  nerves,  which  carry  impulses  from  the  centres  to 
the  various  organs  throughout  the  body.  According  to  the  effect 
their  excitation  produces  they  are  termed ;  (a)  motor,  going  to 
muscles  and  causing  them  to  contract ;  (/9)  secretory,  the  stimula- 


MODE   OF   INVESTIGATION.  497 

tion  of  which  calls  forth  the  activity  of  a  gland  :  (y)  inhibitory, 
when  they  check  or  prevent  some  activity  by  the  impulses  which 
they  carry  ;  (5)  vasomotor  nerves,  which  regulate  the  contraction 
of  the  muscular  coat  of  the  bloodvessels  ;  (e)  trophic,  thermic,  elec- 
tric nerves  are  also  to  be  named,  the  two  former  being  of  doubtful 
existence,  and  the  latter  being  only  found  in  those  animals  which 
are  capable  of  emitting  electric  discharges,  such  as  the  electric 
fishes. 

III.  Intercentral  nerves  are  those  which  act  as  bonds  of  union 
between  the  several  ganglion  cells  of  the  nervous  centres,  which 
are  connected  in  the  most  elaborate  manner,  one  with  the  other. 
As  the  terminals  of  these  fibres  are  both  probably  receiving  and 
directing  agents,  the  delicate  strands  of  protoplasm  communicating 
between  them  probably  convey  impulses  in  different  directions, 
but  of  this  we  can  have  no  definite  idea,  although  such  a  supposi- 
tion would  aid  us  in  forming  a  mental  picture  of  the  manner  in 
which  the  wonderfully  complete  intercentral  communications  are 
accomplished. 

Mode  of  Investigation. 

In  order  to  investigate  the  functions  of  the  different  nerves  a 
knowledge  of  their  central  connections  and  their  peripheral  distri- 
bution is  necessary.  But  anatomical  knowledge,  however  perfect 
does  not  convey  an  adequate  notion  of  their  function,  as  may  be 
amply  seen  from  deductions  made  by  anatomists,  many  of  which 
have  not  borne  experimental  tests. 

The  procedure  adopted  in  testing  the  function  of  a  nerve  is  the 
following.  The  nerve  is  exposed  and  cut,  and  it  is  observed 
whether  there  be  any  loss  of  sensation  or  muscular  paralysis  in 
the  parts  to  which  it  passes.  The  cut  ends  are  then  stimulated, 
and  the  results  are  observed.  The  end  of  the  part  connected  with 
the  centres  is  spoken  of  as  the  central  or  proximal  end,  and  that 
belonging  to  the  part  leading  to  the  distribution  of  the  nerve,  is 
called  the  peripheral  or  distal  end.  If  the  nerve  be  purely  motor, 
stimulation  of  the  proximal  end  will  yield  no  result,  but  when  the 
distal  end  be  irritated,  active  movements  follow.  If,  on  the  other 
band,  it  be  a  sensory  nerve,  stimulation  of  the  distal  end  gives  no 

42 


498  MANUAL   OF   PHYSIOLOGY. 

result,  and  that  of  the  proximal  end  produces  signs  of  pain.  In 
a  compound  nerve  some  response  is  obtained  when  either  the  dis- 
tal or  proximal  end  is  irritated. 

Chemistry  of  Nerve-Fibres. 

The  axis  cylinder  of  nerves  is  probably  composed,  as  already 
mentioned,  of  protoplasm  ;  further  than  that  nothing  is  known  of 
its  chemical  properties.  The  medullary  sheath  yields  certain 
substances  which  are  related  to  the  fats,  and  can  be  extracted 
with  ether  and  chloroform.  Among  these  is  the  peculiar  com- 
pound nitrogenous  fat,  lecithin  containing  phosphorus,  also  cho- 
lesterin,  cerebrin,  and  kreatin. 

Electric  Properties  of  Nerves. 

Like  muscle,  nerves  may  be  regarded  as  having  a  state  of  rest 
and  a  state  of  activity,  but  the  two  states  are  not  obvious  in  the 
same  striking  way  as  they  are  in  muscle,  nor  do  we  know  much 
of  the  physical  properties  of  nerve.  While  at  rest,  however,  it 
shows  electric  phenomena  similar  to  those  which  have  already 
been  described  as  belonging  to  muscle  tissue.  These  electrical 
currents  are  contemporaneous  with  the  life  of  the  nerve,  and  they 
undergo  the  same  variation  as  occurs  in  muscle  when  the  nerve 
passes  into  the  active  state  ;  that  is,  when  it  transmits  an  impulse. 

The  so-called  natural  current  of  nerve  is  practically  the  same 
as  that  of  muscle,  passing  in  the  nerve  to  the  central  part  from 
the  cut  extremities  of  the  fibre;  that  is  to  say,  the  current  passes 
through  the  galvanometer  from  the  electrode  applied  to  the  mid- 
dle of  the  nerve  to  that  applied  to  the  extremity.  The  electro- 
motive force  of  a  small  nerve  is  much  less  than  that  of  a  muscle. 
In  a  frog's  sciatic  it  has  been  estimated  to  be  0.02  of  a  Daniell 
cell.  The  natural  current  of  the  frog's  nerve  is  said  to  increase 
in  intensity  in  proportion  to  the  increase  in  temperature  up  to 
about  20°  C,  after  which  it  decreases. 

Experiments  on  nerve  currents  must  be  carried  on  with  all  the 
precautions  mentioned  in  speaking  of  muscle  currents,  and  with 
the  non-polarizable  electrodes  there  figured  (page  445). 


neeve  stimuli.  499 

The  Active  State  of  Nerve-Fibres. 

Nerves  pass  into  a  state  of  activity  in  response  to  a  variety  of 
stimuli ;  their  changed  condition,  however,  cannot  be  readily 
recognized,  because  the  only  change  we  can  detect  in  the  nerve 
is  that  in  its  electric  state.  We  soon  know,  however,  when  a 
nerve  is  conducting  an  impulse,  if  it  be  connected  with  its  termi- 
nals. In  the  case  of  a  nerve  bearing  impressions  from  the  skin 
to  the  nerve  centres,  we  get  evidence  of  a  sensation  being  felt,  and 
when  the  nerve  is  motor — that  is,  bearing  impulses  from  the  cen- 
tres to  the  muscles,  we  judge  of  the  state  of  activity  of  the  nerve  by 
means  of  the  muscle  contraction  which  it  brings  about.  For  all 
ordinary  experimental  purposes  we  use  the  nerve  of  a  frog  with  the 
muscle  it  supplies  intact.  This  nerve-muscle  preparation  is  com- 
monly made  from  the  leg  of  a  frog,  the  sciatic  nerve  being  care- 
fully prepared  from  the  thigh,  while  the  gastrocnemius  is  cut  away 
from  all  its  attachments  except  that  to  the  femur,  which  is  retained 
as  a  point  of  fixation.  In  fact,  the  same  method  as  is  used  for 
the  indirect  stimulation  of  muscle  is  employed  for  the  study  of  the 
excitability  of  nerve-fibres. 

Nerve  Stimuli. 
Besides  the  normal  physiological  impulse  which  comes  from 
the  cells  in  connection  with  the  nerve-fibres,  they  may  be  made  to 
pass  into  the  active  state  by  a  variety  of  stimuli,  differing  little 
from  those  which  are  found  to  afiect  muscle,  when  applied  directly 
to  that  tissue.     They  may  be  enumerated  as  follows  : 

1.  Mechanical  stimulation.  Almost  any  mechanical  impulse, 
applied  to  any  part  of  a  nerve,  causes  its  excitation.  The  stim- 
ulus must  have  a  certain  degree  of  intensity,  and  have  a  definite, 
though  it  may  be  a  very  short,  duration.  If  mechanical  stimuli 
be  frequently  applied  to  a  nerve  in  the  same  place,  the  irritability 
of  the  part  is  soon  destroyed  ;  but  if  fresh  parts  of  the  nerve  be 
stimulated,  at  each  blow  the  nerve  passes  into  a  state  of  tetanus* 
as  shown  by  the  contraction  of  the  muscle  to  which  it  is  supplied. 

2.  Chemical  stimulation.  First,  must  be  named  drying  of  the 
fibre,  whether  this  be  caused  by  ordinary  evaporation,  or  facil- 
itated with  blotting  paper,  exposure  over  sulphuric  acid,  or  the 


500  MANUAL   OF   PHYSIOLOGY. 

addition  of  solutions  of  high  density,  such  as  syrup,  glycerin,  or 
strong  salt  solution.  Secondly,  strong  metallic  salts  or  acids 
irritate  nerves.  Thirdly,  alcohol  and  ether,  and  a  solution  of 
bile;  and  lastly,  even  weak  alkalies,  except  ammonia,  which  has 
no  effect  on  nerve,  although  it  acts  on  muscle  when  applied 
directly  to  that  tissue. 

3.  Thermic  stimulation  occurs  when  sudden  changes  are  brought 
about  approaching  either  of  the  extreme  temperatures  at  which  the 
nerve  can  act ;  i.e.,  near  5^  or  50'^  C. 

4.  Electric  stimulation  is  by  far  the  most  important  for  physiolo- 
gists, being  the  most  delicate,  the  most  easily  applied  and  regu- 
lated, and  the  least  injurious  to  the  nerve  tissue.  As  was  men- 
tioned with  respect  to  muscle,  any  sufficiently  rapid  change  of 
intensity  in  an  electric  current  passing  through  a  nerve  causes  in 
it  the  molecular  changes  we  call  excitation,  as  evidenced  by  the 
muscle  contracting,  and  the  natural  electric  currents  of  the  nerve 
undergoing  variation.  The  less  the  absolute  intensity  of  the  cur- 
rent, the  greater  the  effect  in  any  given  change  in  intensity 
causes.  The  muscle  of  a  nerve-muscle  preparation  contracts, 
when  a  weak  constant  current,  say  from  a  single  small  Daniell 
cell,  is  suddenly  allowed  to  pass  through  the  nerve.  This  is 
done  by  placing  a  part  of  the  nerve  in  the  circuit,  which  is  made 
complete  by  closing  a  key,  when  the  stimulation  is  to  be  ap- 
plied. This  form  of  stimulation  is  called  a  making  shock.  While 
the  current  is  allowed  to  pass  through  the  nerve  no  effect  is  pro- 
duced if  the  battery  be  quite  constant.  On  breaking  the  circuit 
by  opening  the  key  the  current  suddenly  ceases,  and  another  con- 
traction occurs,  this  is  called  the  breaking  shock.  At  each  making 
and  breaking  of  the  constant  current  a  stimulus  is  applied  to 
the  nerve,  and  transmitted  to  the  muscle,  and  it  has  been  found 
that  a  weaker  current  suffices  to  bring  about  a  contraction  when 
applied  to  the  nerve,  than  when  it  is  applied  to  the  muscle 
directly. 

If  a  strong  constant  current  be  allowed  to  pass  through  a  con- 
siderable length  of  a  nerve  for  some  little  time,  and  the  circuit  be 
then  suddenly  broken,  instead  of  a  single  contraction  tetanus  of 
the  muscle  results.     This  breaking  tetanus  (Ritter's  tetanus)  is 


STIMULATION    OF    NERVES.  501 

easily  produced  when  the  positive  pole  or  anode  is  next  the  muscle. 
Sometimes  in  particular  conditions  of  the  nerve,  and  with  certain 
strength  of  stimulation,  a  making  tetanus  also  occurs,  but  more 
rarely  and  only  when  the  negative  pole  is  next  the  muscle. 

When  a  constant  current,  such  as  we  get  directly  from  a  Daniell 
cell,  is  used,  that  part  of  the  nerve  between  the  stimulating  points 
through  which  the  current  passes  is  found  not  to  be  equally  af- 
fected throughout  its  entire  length,  but  one  single  point  is  stimu- 
lated whence  the  impulse  spreads.  This  may  be  the  point  where 
either  of  the  poles  is  in  contact  with  the  nerve  ;  and  further,  a  dif- 
ferent pole  starts  from  the  stimulus,  according  as  the  circuit  is  made 
or  broken.  With  a  making  shock  the  stimulation  takes  place  at 
the  negative  pole  or  cathode,  and  with  a  breaking  shock  at  the 
positive  pole  or  anode.  That  is  to  say,  the  point  where  the  current 
leaves  the  nerve  is  affected  at  the  make,  and  the  point  where  the 
current  enters  the  nerve  is  affected  at  the  break  of  the  current. 

It  has  also  been  found  that,  other  things  being  equal,  the  making 
shock  is  a  more  powerful  stimulus  than  the  breaking  shock ;  i.e., 
a  weak  current  will  sooner  cause  a  contraction  when  the  circuit 
is  made  than  when  it  is  broken. 

This  remarkable  fact  that  the  impulse  starts  from  the  anode  in 
a  breaking  shock  is  proved  by  means  of  the  breaking  tetanus  just 
alluded  to.  It  has  been  found  that  when  the  positive  pole  or 
anode  is  next  to  the  muscle  the  breaking  tetanus  lasts  longer  and 
is  stronger  than  when  the  anode  is  placed  at  a  greater  distance 
from  the  muscle  than  the  cathode ;  and  further,  when  the  anode 
is  beyond  the  cathode  section  of  the  nerve  in  the  intrapolar  region, 
during  stimulation  the  tetanus  stops  it  at  once,  because  the  point 
from  which  the  stimulus  comes  is  thereby  cut  off  from  the  muscle. 
Section  has  no  effect  if  the  anode  be  next  the  muscle,  the  tetanus 
proceeding  in  a  normal  way,  only  the  inactive  pole  being  cut  away 
from  the  muscle.  That  the  stimulus  occurs  at  the  cathode  in  a 
making  current  may  also  be  demonstrated  by  the  fact  that  it  takes 
a  certain  measurable  time  for  the  impulse  to  travel  along  the  nerve. 
If  then  the  cathode  be  placed  far  from  the  muscle  and  the  anode 
near  it,  the  contraction  after  a  breaking  shock,  when  the  stimulus 
starts  from  the  anode,  will  occur  sooner  than  that  which  follows 


502  MANUAL   OF   PHYSIOLOGY. 

the  viakinrj.  when  the  stimulus  starts  from  the  cathode,  because 
the  impulse  has  a  less  distance  of  nerve  to  traverse  in  the  former 
case. 

In  ordinary  experiments  on  nerve,  a  constant  current,  i.e.,  one 
coming  directly  from  a  battery,  is  seldom  used,  because  there  is 
no  means  of  regulating  or  varying  the  strength  of  the  stimulation, 
and  it  is  not  convenient  to  make  and  break  the  current  in  order  to 
excite  the  tissue  continuously.  And  further,  the  more  rapid 
current  induced  in  one  coil  of- wire — the  secondary  coil — by  the 
making  or  breaking  of  a  current  passing  through  another  coil — 
the  primary  coil — is  more  effective  and  suitable  for  physiological 
purposes.  The  strength  of  the  induced  current  being  approxi- 
mately in  inverse  proportion  to  the  square  of  the  distance  between 
the  two  coils — moving  the  secondary  away  from  the  primary  coil 
gives  a  ready  means  of  varying  and  regulating  the  strength  of 
the  stimulus  without  any  special  care  being  devoted  to  the  exact 
strength  of  the  element  used. 

Du  Bois  Reymoud's  Inductorium  is  the  instrument  commonly 
used  in  physiological  laboratories.  In  it  the  secondary  coil  can 
be  moved  away  from  the  primary  on  a  slide  which  is  graduated, 
and  the  primary  current  may  be  made  to  pass  through  a  magnetic 
interrupter  so  as  to  cause  a  rapid  succession  of  breaks  and  makes, 
and  thus  give  a  series  of  stimulations  one  after  another,  which  is 
necessary  in  order  to  produce  tetanus.  A  drawing  and  further 
description  of  the  instrument  will  be  found  at  p.  451. 

Velocity  of  Nerve  Force. 

It  has  already  been  stated  that  nerve  fibres  are  capable  of  con- 
ducting impulses  in  either  direction — from  or  to  the  nervous 
centres — the  position  and  the  character  of  the  terminal  organs 
determining  the  direction  in  which  the  nerve  force  usually  travels. 
In  the  ordinary  peripheral  nerves  there  are  generally  both  kinds 
— efferent  and  afferent  fibres  carrying  impulses  in  different  direc- 
tions without  interfering  with  one  another. 

When  we  reflect  that  the  passage  of  an  impulse  along  a  nerve 
is  brought  about  by  a  molecular  change  in  the  axis  cylinder,  we 
are  at  once  struck  with  the  rapidity  with  which  impressions  are 


VELOCITY  OF   NERVE  TRANSMISSION.  503' 

transmitted  from  one  part  of  the  body  to  another.  This  surprising 
velocity  is,  however,  only  relatively  great.  When  we  compare  it 
with  the  velocity  of  the  electric  current  or  of  light,  we  at  once  see 
how  incomparably  slower  the  rate  of  nerve  impulse  is,  and  that 
it  may,  with  more  advantage,  be  compared  with  rates  of  motion 
commonly  under  our  observation.  To  take  every -day  examples  : 
viz.,  nine  metres  per  second  is  about  as  fast  as  the  quickest  runner 
can  accomplish  his  100  yards ;  race-horses  can  gallop  about  15 
metres  a  second  for  a  mile  or  so  :  a  mail  train  at  full  speed  travels 
at  about  30  metres  a  second,  and  the  velocity  of  nerve  force  has 
been  estimated  to  be  in  cold-blooded  animals  27  metres  per  second  ; 
and  in  man  about  33  metres  per  second.  So  that  the  intercom- 
munications between  man's  brain  and  the  various  parts  of  his 
body  only  travel  about  the  same  rate  as  an  express  train,  and 
about  twice  as  fast  as  the  quickest  horse  can  gallop. 

In  order  to  measure  the  rate  of  transmission  of  nerve  force,  dif- 
ferent methods  may  be  employed ;  the  simplest  of  which  is  to  make 
a  muscle  draw  two  curves  one  over  the  other,  with  a  good  myo- 
graph such  as  described  in  Chapter  XVI.,  in  one  of  which  the 
stimulation  is  applied  to  the  nerve  close  to  the  muscle,  and  in 
the  other,  as  far  as  possible  away  from  the  muscle.  The  difference 
in  length  of  the  latent  period,  as  estimated  by  the  tuning-fork 
tracing,  corresponds  to  the  time  the  impulse  has  taken  to  travel 
along  the  part  of  the  nerve  between  the  two  points  of  stimulation. 

Utilizing  the  fact  that  the  extent  of  deflection  of  the  needle  of  a 
galvonometer  is  in  proportion  to  the  duration  of  a  current  of  known 
strength  passing  through  it  for  a  short  time,  an  accurate  measure- 
ment of  the  difference  in  time,  of  remote,  and  near  or  direct  stimu- 
lation of  a  nerve,  may  be  made.  By  a  special  mechanism  the 
time-measuring  current  is  sent  through  the  galvanometer  at 
the  same  moment  that  the  stimulating  current  goes  through  the 
nerve,  and  the  instant  the  muscle  begins  to  contract,  it  breaks  the 
current  passing  through  the  galvanometer,  so  that  this  time- 
measuring  current  lasts  only  from  the  moment  when  the  nerve  is 
stimulated  until  the  muscle  begins  to  contract. 


504  MANUAL   OF   PHYSIOLOGY. 

The  Electric  Ciian(je  in  Nerve. 

Negative  Variation. — The  natural  current  of  a  nerve,  like  that 
of  muscle,  undergoes  a  diminution  at  the  moment  the  nerve  is 
stimulated  ;  this  is  termed  the  negative  variation.  It  occurs  with 
any  other  form  of  stimulation  as  well  as  when  an  electric  shock 
is  used,  so  it  is  not  dependent  on  an  escape  of  the  stimulating 
current.  In  the  case  of  a  single  stimulation,  the  negative  varia- 
tion is  so  rapidly  over — lasting  only  .0005  sec,  that  the  inertia 
of  the  needle  of  the  galvanometer  prevents  the  change  in  the  cur- 
rent being  indicated.  In  tetanus,  however,  it  makes  a  decided 
impression  on  the  galvanometric  needle.  The  strength  of  the 
negative  variation  depends  on  the  condition  of  the  nerve  and  the 
strength  of  the  stimulus;  being  stronger  when  the  nerve  is  fresh 
and  irritable  and  has  a  good  natural  current,  and  when  a  strong 
stimulus  is  applied. 

The  negative  variation  of  the  natural  currents  passes  along  the 
nerve  from  the  point  of  stimulation  in  both  directions,  just  as  does 
the  nerve  impulse;  and  with  a  galvanometer  the  electric  change 
may  be  traced  from  the  nerve  to  the  muscle.  Moreover,  it  has 
been  made  out  that  the  negative  variation  travels  along  the  nerve 
at  just  the  same  velocity  as  the  impulse  does  from  the  point  of 
stimulation  ;  namely,  about  27  metres  per  second  ;  and  further, 
this  rate  is  said  to  be  influenced  in  the  same  way  by  the  passage 
of  a  constant  current  through  the  nerve  (to  be  presently  described) 
as  is  the  impulse  derived  from  stimulus.  These  points  seem  to 
lead  to  the  belief  that  the  nerve  impulse  and  the  negative  varia- 
tion are  identical.  This  peculiar  electric  change  and  its  accom- 
panying impulse  pass  then  along  the  nerves  as  a  kind  of  wave  of 
activity,  the  speed  and  the  duration  of  which  we  know  to  be  27 
metres  per  sec,  and  .0005  of  a  sec  respectively;  the  length  of  the 
wave  we  therefore  calculate  to  be  about  18  millimetres. 

Electrotonus. 

If  the  two  poles  leading  to  a  galvanometer  be  applied  to  the 
centre  and  to  the  end  of  a  nerve  respectively,  so  as  to  indicate 
the  natural  current,  and  at  the  same  time  another  part  of  the 


ELECTROTONU8. 


505 


nerve  be  placed  in  the  circuit  of  a  constant  current  from  a  battery, 
when  the  circuit  of  the  constant  (now  called  polarizing)  current 
is  completed,  a  change  is  found  to  take  place  in  the  natural  cur- 
rent ;  this  is  called  eleetrotonus.  Instead  of  the  natural  currents 
from  the  equator  to  the  pole  of  the  nerve,  a  current  is  found  to 
pass  through  the  entire  length  of  the  nerve  in  the  same  direction 
as  the  polarizing  current  from  the  battery.  This  electrotonic  cur- 
rent is  not  proportional  to  the  strength  of  the  natural  currents, 
and  is  to  be  recognized  when  the  latter  are  no  longer  to  be  found. 

Fig.  202. 


Diagram  to  illustrate  Eleetrotonus. — n  n**.  Portion  of  nerve.  G  G''. 
Galvanometers,  d.  Battery  from  which  polarizing  current  can  be  sent  into 
nerve  by  closing  key  K.  The  direction  of  the  polarizing  and  electrotonic 
currents  is  indicated  by  the  arrows,  and  is  seen  to  be  the  same. 


The  electrotonic  current  is  stronger  with  a  strong  polarizing  cur- 
rent, and  is  most  marked  in  the  immediate  neighborhood  of  the 
poles,  fading  gradually  away  as  one  passes  to  the  remoter  parts  of 
the  nerve.  The  electrotonic  state  is  not  to  be  attributed  to  an 
escape  of  the  constant  polarizing  current,  because  it  decreases 
gradually  with  the  waning  of  the  physiological  activity  of  the 
nerve,  and  ceases  at  the  death  of  the  nerve  long  before  the  tissue 
has  lost  its  power  of  conducting  electric  currents.  Moreover,  it 
has  been  shown  that  a  ligature  applied  to  the  nerve  so  as  to  destroy 


506  MANUAL   OF   PPIYSIOLOGY. 

its  physiological  continuity,  but  not  its  power  of  carrying  electric 
currents,  prevents  the  electrotonic  current  passing  to  the  part  of 
the  nerve  which  is  thus  separated. 

The  condition  of  the  portion  of  the  nerve  near  the  anode — 
positive  pole — is  found  to  differ  somewhat  from  that  near  the 
cathode — negative  pole — and  hence  it  is  found  convenient  to 
speak  of  the  region  of  the  anode  being  in  the  aneledrotonic,  and 
that  near  the  cathode  being  in  the  cat  electrotonic  condition.  A 
certain  time  is  required  for  the  production  of  anelectrotonus  and 
catelectroiomis,  a  current  of  less  duration  than  .0015  of  a  second 
being  unable  to  bring  about  the  change;  the  negative  variation 
is,  therefore,  over  before  the  electrotonus  has  commenced. 

Irritability  of  Nerve  Fibres. 

The  irritability  of  nerves  varies  according  to  certain  conditions 
and  circumstances.  While  uninjured  in  the  body,  the  irritability 
of  a  nerve  depends  upon — 

1.  A  perfect  supply  of  blood  so  as  to  bear  to  it  the  necessary 
quantity  of  new  material  as  nutriment,  and  to  carry  off  any 
injurious  effete  matters  that  may  be  produced  by  its  molecular 
changes. 

2..  A  suitable  amount  of  rest.  Prolonged  activity  causes  fatigue 
and  loss  of  irritability,  no  doubt  from  the  same  causes  as  were 
mentioned  as  bringing  about  fatigue  in  muscles.  The  chemical 
changes  taking  place  in  nerves,  however,  have  not  yet  been  made 
out  with  any  degree  of  accuracy. 

3.  Uninjured  connection  with  the  nerve  centres.  When  a  spinal 
nerve  is  cut,  the  part  connected  with  the  periphery  rapidly  under- 
goes degenerative  changes  which  seem  to  depend  upon  faulty 
nutrition,  since  they  are  accompanied  by  structural  changes — 
fatty  degeneration.  This  appears  to  commence  in  a  very  short 
time  after  the  section — commonly  in  about  five  days.  The  part 
of  the  nerve  remaining  in  direct  connection  with  the  cord,  retains 
its  irritability  for  a  very  much  longer  time. 

In  the  artificial  stimulation,  by  means  of  electric  shocks  applied 
to  the  nerve  of  a  cold-blooded  animal,  there  are  many  minor  con- 
ditions which  have  considerable  influence  on  the  irritability,  as 


ELECTROTONIC  STATES   OF   NERVES.  507 

evidenced  by  the  response  given  by  the  attached  muscle  to  weak 
stimuli.     The  more  im})ortaut  of  these  are  : 

1.  Temperature  changes.  In  the  case  of  a  frog's  nerve,  a  rise 
of  temperature  to  30-40°  C.  causes  an  increase  in  its  excitability. 
Also  a  fall  of  temperature  below  zero  tends  to  make  the  nerve 
more  easily  excited.  Both  these  conditions  have,  however,  a  very 
fleeting  effect,  for  the  nerve  soon  dies  at  the  temperature  named, 
and  most  probably  the  increased  irritability  is  only  to  be  taken 
as  a  sign  of  approaching  death.  It  thus  appears  that  a  medium 
temperature  is  the  optimum  for  nerve  work. 

2.  The  part  of  the  nerve  stimulated,  is  also  said  to  have  some 
effect  on  the  result  of  a  given  strength  of  stimulus.  The  further 
from  the  muscle,  the  more  powerful  the  contraction  produced, 
other  things  being  equal.  So  that  the  impulse  is  supposed  to 
gather  force  as  it  goes,  as  in  the  case  of  a  falling  body,  and  hence 
has  been  spoken  of  as  the  avalanche  action  of  nerve  impulse. 

3.  A  new  section  of  the  nerve  is  said  to  increase  its  irritability, 
as  does,  indeed,  any  slightly  stimulating  influence,  such  as  dry- 
ing, and  chemical  or  mechanical  meddling  of  any  kind.  This 
increase  in  irritability  probably  depends  upon  injurious  changes 
going  on  in  the  nerve,  as  the  influences  just  alluded  to  lead  to 
complete  loss  of  excitability  if  carried  too  far. 

4.  The  most  remarkable  change  in  the  excitability  of  a  nerve, 
is  that  brought  about  by  the  action  of  a  constant  current  passing 
through  the  nerve,  so  as  to  set  up  the  conditions  just  described 
as  anelectrotonus  and  catelectrotonus.  (a)  The  irritability  of  the 
nerve  is  considerably  increased  in  the  region  near  the  cathode, 
and  it  is  notably  diminished  in  the  neighborhood  of  the  anode. 

(/S)  The  increase  of  irritability  is  in  proportion  to  the  intensity 
of  the  catelectrotonic  state,  and  the  decrease  in  proportion  to  the 
intensity  of  the  anelectrotonus.  Thus  the  increase  is  most  marked 
in  the  immediate  neighborhood  of  the  cathode,  and  fades  with 
the  distance  from  the  negative  pole;  and  similarly,  the  decrease 
is  strongest  at  the  anode,  and  become  less  and  less  as  it  passes 
away  from  the  positive  pole.  In  the  same  way,  in  the  part  of  the 
nerve  between  the  two  poles — the  intrapolar  region — the  de- 
crease and  increase  of  irritability  become  less  marked  towards  the 


508 


MANUAL   OF    PHYSIOLOGY. 


middle  point,  between  the  cathode  and  the  anode,  so  that  here 
we  find  an  unaffected  part,  which  has  been  called  the  indifferent 
point. 

It  is  a  remarkable  fact  that  this  indifferent  point  is  not  always 
midway  between  the  two  poles,  but  decreases  its  distance  from  the 
cathode  in  proportion  as  the  polarizing  current  is  made  stronger. 
That  is  to  say  (>),  with  strong  polarizing  currents  the  indifferent 
point  is  near  the  cathode  (B)  ;  with  weak  currents  it  lies  near  the 
anode  (A)  (Fig.  203). 

Besides  becoming  less  irritable  in  proportion  as  the  polarizing 
current  becomes  more  powerful  (<5),  the  anelectrotonic  region  of 

Fig.  203. 


Diagram  illustrating  the  variations  of  irritability  of  different  parts  of  a 
nerve  during  the  passage  of  polarizing  currents  of  varying  strength  through 
a  portion  of  it. — A=Anode;  B=Cathode;  ab  =  Intrapolar  district;  yj  = 
Effect  of  weak  current:  jr^  =  Effect  of  medium  current;  2/3  =  Effect  of 
strong  current.  The  degree  of  effect  is  shown  by  the  distance  of  the  curves 
from  the  straight  line.  The  part  of  curve  below  the  line  corresponds  to 
decrease,  that  above  to  increase  of  irritability.  Where  the  curves  cross  the 
line  is  called  the  indifferent  point.  With  strong  currents  this  approaches 
the  cathode.     (From  Foster  after  Pfliiger.) 

the  nerve  loses  its  ability  to  conduct  impulses,  and  may,  finally, 
with  a  very  strong  current,  even  when  applied  for  a  short  time, 
become  quite  incapable  of  conducting  an  impulse. 

If  the  polarizing  current  be  now  opened,  so  as  to  stop  its  pass- 
age through  the  nerve,  and  remove  the  anelectrotonic  and  the 
catelectrotonic  states  (e),  a  kind  of  rebound  occurs  in  the  condi- 
tion of  both  the  altered  regions,  and  the  part  which  has  just 
ceased  to  be  catelectrotonic,  and  was,  therefore,  over-irritable 


L.AW   OF   CONTRACTION. 


509 


becomes,  by  a  kind  of  negative  modification,  very  much  lowered 
in  its  irritability ;  while  on  the  other  hand,  the  anelectrotonic 
part,  by  a  positive  rebound,  becomes  more  excitable  than  in  its 
normal  state.  The  rebound  over  the  line  of  normal  irritability 
lasts  but  a  very  short  time ;  but  still,  as  we  shall  see  presently,  it 
is  of  greater  duration  than  the  passage  of  the  negative  variation 
along  the  nerve. 

Fig.  204. 


Diagram  showing  the  meaning  of  the  terms  ascending  and  descending  cur- 
rents, used  in  speaking  of  the  law  of  contraction.  The  end  of  the  vertebral 
column,  sciatic  nerves,  and  calf  muscles  of  the  frog  are  shown,  while  the 
arrows  indicate  the  direction  of  the  ascending  current  A,  on  the  left,  and 
the  descending  current  d,  on  the  right,  according  as  the  positive  pole  c  of 
the  battery  is  below  or  above. 


Law  of  Contraction. 

Upon  the  foregoing  facts  (a-e),  and  others  already  mentioned, 
— viz.,  that  the  impulse  starts  in  the  nerve  from  different  poles 
and  with  different  force,  with  a  making  and  a  breaking  shock — 
depends  the  law  of  contraction,  which  would  be  difficult  to  under- 
stand without  bearing  in  mind  all  these  interesting  points. 


610 


MANUAL   OF    PHYSIOLOGY. 


It  was  found  that,  with  the  same  strength  of  stimulation,  not 
only  were  different  degrees  of  contraction  produced  with  making 
and  breaking  shocks,  but  also  that,  other  things  being  similar,  a 
different  result  followed  when  the  current  was  sent  through  the 
nerve  in  an  upward  direction  {i.e.,  from  the  muscle),  and  when  it 
was  sent  in  a  downward  direction  (i.e.,  towards  the  muscle).  The 
stimulating  current  is  spoken  of,  in  the  former  case,  as  an  ascending 
current,  and  in  the  latter  as  a  descending  current. 

The  following  is  a  tabular  view  of  the  law  of  contraction  : 


Weak  Stimulation. 

Medium  Stimulation. 
Strong  Stimulation. 

Ascending  Currents. 

Descending  Currents. 

Make  =  Contraction. 
Break  =  No  response. 

Make  =  Contraction. 
Break  =  Xo  response. 

Make  =  Contraction. 
Break  =  Contraction. 

Make  ==  Contraction. 
Break  =  Contraction. 

Make  =  No  response. 
Break  =  Contraction. 

Make  =  Contraction. 
Break  =  No  response. 

To  explain  this  law  the  following  points  must  be  kept  in  view: 

1.  In  a  breaking  shock  it  is  the  disappearance  of  anelectrotonus 

which  causes  the  stimulation. 

2.  In  a  making  shock  it  is  the   appearance  of  catelectrotonus 

which  causes  the  stimulation. 

3.  With  the  same  current  the  make  is  more  powerful  than  the 

break. 

4.  Anelectrotonus  causes  reduction  of  irritability  and  conduc- 

tivity. 

5.  Catelectrotonus  causes  increase  of  irritability. 

6.  "With  ascending  currents  the  part  of  the  nerve  next  to  the 

muscle  is  in  a  state  of  reduced  functional  activity  (anelec- 
trotonus). 

7.  With  descending  cuj-rents  the  part  of  the  nerve  next  the 

muscle  is  in  a  state  of  exalted  activity  (catelectrotonus). 

8.  The  reduction  or  exaltation  of  activity  is  much  greater  with 

strong  currents. 


NERVE   TERMINALS.  511 

That  only  making  shocks  cause  contraction  with  very  weak 
currents,  simply  depends  on  the  greater  efficacy  of  the  entrance 
of  catelectrotonus  into  the  nerve,  which  causes  the  making  stim- 
ulation. 

That  contraction  follows  in  all  four  cases,  with  medium  stimu- 
lation, is  explained  by  assuming  that  the  depression  of  the  func- 
tional activity  of  the  nerve  is  not  sufficient  to  affect  its  conduct- 
ivity. 

The  want  of  response  to  a  making  shock,  in  the  case  of  the 
strong  descending  current,  depends  upon  the  fact  that  the  part  of 
the  nerve  near  the  muscle,  around  the  anode,  is  in  a  state  of 
lowered  activity,  and  is,  therefore,  unable  to  conduct  the  impulse 
which  has  to  pass  through  this  region  from  the  cathode,  where 
the  stimulation  takes  place,  in  order  to  reach  the  muscle. 

The  absence  of  contraction  at  the  breaking  of  a  strong  descend- 
ing current,  is  caused  by  the  same  lowering  of  the  conductivity  of 
the  nerve  between  the  point  of  stimulation  and  the  muscle,  because 
at  the  cessation  of  strong  catelectrotonus,  the  region  near  the 
cathode  rebounds  from  exalted  to  depressed  activity,  and  at  the 
moment  of  stimulation  the  greater  part  of  the  intrapolar  region  is 
an  electrotonic. 

The  specific  use  of  nerve  fibres  in  the  body  of  the  higher  animals 
may  be  thus  briefly  stated.  They  form  a  means  of  extremely 
rapid  intercommunication  between  distant  parts.  The  protoplasm 
of  the  axis  cylinder  has  undergone  a  special  modification,  by  which 
it  is  enabled  to  conduct  impulses  much  more  quickly  than  ordi- 
nary protoplasm  does.  Even  the  most  nearly-related  substance, 
muscle  tissue,  transmits  impulses  about  thirty  times  more  slowly 
than  a  nerve  fibre.  A  highly  organized  animal  body,  without 
nerve  fibres,  would  be  in  a  worse  condition  than  a  highly  organ- 
ized state  without  a  telegraph  or  even  a  postal  system. 

Nerve  Corpuscles  or  Terminals. 

These  are  the  real  actors  in  the  nervous  operations,  while  the 
fibres  are  merely  their  means  of  communicating  with  one  another. 
One  great  set  of  terminals  is  placed  on  the  surface  of  the  body, 


512  MANUAL   OF    PHYSIOLOGY. 

and  is  adapted  to  the  reception  of  the  various  external  influences 
which  are  brought  to  bear  on  it  from  without  by  its  surroundings. 
These  receivers  of  extrinsic  stimuli  are  necessarily  much  varied 
so  as  to  be  capable  of  appreciating  all  the  different  kinds  of  stim- 
ulation presented  to  them.  They  are  either  distributed  over  the 
entire  surface  so  as  to  meet  with  general  mechanical  and  thermic 
changes,  or  they  are  further  specialized  for  the  reception  of  lumi- 
nous, sonorous,  odorous  or  gustatory  impulses.  In  the  latter  cases 
the  special  terminals  are  collected  into  one  part,  and  usually  form 
complex  organs,  which  will  be  described  presently  in  the  chapters 
on  the  special  senses.     Another  great  set  of  terminals  are  placed 

Fig.  205. 


Tactile  nerve  endings,  composed  of  small  capsules,  in  which  the  black  axis- 
cylinder  of  the  nerve  (a)  and  {n)  meets  with  many  protoplasmic  units. 

in  the  deeper  textures,  when  they  act  as  local  distributing  agents ; 
such  as  the  nerve  plates  on  skeletal  muscles,  and  the  ganglionic 
networks  in  the  wall  of  the  intestine.  In  many  instances,  how- 
ever, the  exact  mode  of  connection  between  the  nerve  and  the 
protoplasm  of  the  tissue  elements,  to  which  it  bears  impulses,  has 
not  been  satisfactorily  made  out.  In  the  remaining  class  of  nerve 
terminals  the  cells  are  grouped  together  so  as  to  form  larger  and 
smaller  colonies,  and  more  definitely  deserve  the  name  of  nerve 
or  ganglion  cells.  These  are  the  central  terminals,  and  are  placed 
either  in  the  cerebro-spinal  axis,  or  in  swellings  of  the  nerves 
called  sporadic  ganglia. 

Of  these  nerve  cells  there  are  many  varieties,  all  of  which  have 
the  following  characteristics :  The  cells  are  of  considerable  size. 


FUNCTIONS   OF    NERVE   CELLS. 


513 


and  have  processes  branching  off  from  them,  by  means  of  which 
they  communicate  with  the  nerve  fibres.  These  processes  may 
be  single  or  many,  hence  they  are  spoken  of  as  uni-,  bi-,  or  mul- 
tipolar cells,  etc.  The  nucleus  is  commonly  very  distinct,  and 
contains  a  well-marked  nucleolus.  The  abundant  protoplasm, 
which  is  usually  contained  in  a  delicate  cell  wall,  is  in  direct  con- 

FiG.  205*. 


Multipolar  cells  from  the  anterior  gray  column  of  the  spinal  cord  of  the 
dog-fish  (a)  lying  in  a  texture  of  fibrils;  (6)  prolongation  from  cells;  (c) 
nerve-fibres  cut  across.     (Cadiat.) 

nection  with  the  axis  cylinder  of  the  nerve  fibres,  with  which  it 
communicates  by  means  of  thin  strands  of  protoplasm  that  pass 
out  from  the  cell  by  the  processes.  A  delicate  striation  of  the 
protoplasm  may  sometimes  be  recognized,  indicating  the  course 
of  the  nerve  fibrils  as  they  run  into  the  cells  from  the  processes. 

The  Functions  of  Nerve  Cells. 

Any  mass  of  living  protoplasm,  such  as  an  amoeba,  can  receive 
extrinsic  impulses,  which  affect  directly  its  conditions,  and  though 
the  impression  may  be  very  localized  in  its  application,  yet  all 
the  parts  of  the  cell  participate  in  the  sensation,  and  probably 
take  part  in  the  resulting  movement. 

43 


514  MANUAL   OF    PHYSIOLOGY. 

Besides  those  acts  of  which  we  can  recognize  the  cause,  many 
others  occur  in  an  amoeba  which  we  are  not  able  to  trace  to  any 
definite  cause  other  than  the  energies  derived  from  its  special 
powers  of  assimilation.  We  say,  then,  that  not  only  can  an 
amoeba  feel  local  stimulation,  transmit  the  impulse  to  remoter 
parts  of  its  body,  and  respond  by  movement  to  the  stimulus,  but, 
moreover,  as  the  result  of  intrinsic  processes  of  a  chemical  nature, 
it  can  initiate  impulses  which  appear  as  motions,  etc.  We  may 
conclude  from  this  fact  alone  that  automatic  action  is  one  of  the 
vital  properties  of  protoplasm. 

Now  in  the  nerve  centres  we  find,  certainly  in  all  the  more 
complex  animals,  that  each  of  these  kinds  of  action  is  commonly 
distributed,  so  that  different  individual  cells  have  each  a  different 
act  to  perform,  and  thus  an  important  division  of  labor  takes 
place.  The  first  act  is  performed  by  a  wonderfully  elaborate  set 
of  special  organs  adapted  to  the  reception  of  the  various  extrinsic 
impulses  or  sensations  from  without.  The  excitation  is  then  sent 
by  nerve  fibres  to  another  great  group  of  central  nerve  cells, 
which  are  apparently  employed  solely  in  receiving  the  stimuli 
from  the  peripheral  organs,  and  then  distributing  the  impulses  to 
their  neighbors,  which  can  direct,  modify,  analyze,  classify,  re- 
distribute, or  check  the  impulses,  so  that  other  nerve  cells  may 
have  the  least  possible  amount  of  trouble,  and  at  the  same  time 
lose  none  of  the  advantage  that  is  to  be  gained  from  the  income 
derived  from  stimulus  coming  from  without.  Connected  with  the 
last  group  is  another,  the  nerve  cells,  which  lie  out  of  the  reach 
of  the  ordinary  peripheral  impulses,  but  are  capable  of  develop- 
ing within  themselves  energies,  and  can  initiate  impulses  with  no 
other  aid  than  that  of  their  nutrition  and  the  chemical  changes 
resulting  from  their  assimilation. 

These  impulses  are  distributed  to  the  peripheral  active  tissues, 
muscles,  glands,  etc.,  probably  through  the  medium  of  other  sets 
of  cells  analogous  to  the  last  group  situated  in  the  nerve  centres 
as  well  as  to  the  local  distributors  which  act  as  unions  between  the 
other  textures  and  the  nerve  fibres. 

The  functions  of  these  nerve  cells  which  form  centres  of  action 
may  be  classified  thus  : 


FUNCTIONS   OF   GANGLION    CELLS.  515 

1.  Reflection.  Many  nervous  cells  are  capable  of  reflecting  an 
impulse  received  by  an  afferent  nerve ;  that  is  to  say  they  send  it 
by  an  efferent  nerve  to  some  active  tissue,  such  as  a  muscle  or 
gland.  This  kind  of  direction  is  spoken  of  as  a  simple  reflex  action. 
For  instance,  if  a  grain  of  red  pepper  be  placed  on  the  tongue, 
the  stimulus  soon  travels  from  the  peripheral  receiving  terminal, 
along  an  afferent  nerve,  to  its  central  terminal,  which  reflects  the 
impulse  to  the  efferent  nerve,  going  to  the  salivary  gland,  and  the 
result  is  an  increased  secretion  of  saliva. 

2.  Coordination.  There  are  but  few  reflex  acts  that  do  not 
require  the  cooperation  of  several  cells,  and  these  work  together 
in  an  orderly  manner,  the  resulting  activity  being  well  arranged 
and  usually  adapted  to  some  purpose.  The  first  act  of  the  re- 
ceiving cells  of  this  reflex  centre  must  then  be  to  distribute  and 
direct  the  impulse  into  those  channels  which  lead  to  groups  of  cells 
capable  of  sending  impulses  in  an  orderly  and  definite  direction. 
This  directing  and  arranging  power  is  spoken  of  as  coordination, 
and  probably  is  an  attribute  common  to  all  nerve  cells. 

3.  Augmentation.  Usually  the  force  of  the  reflected  efferent 
impulse  bears  a  direct  relation  to  the  afferent  impulse  as  determined 
by  the  strength  of  the  stimulus.  Thus  if  the  amount  of  pepper  on 
the  tongue  be  much  increased,  not  only  is  the  flow  of  saliva  greater, 
but  the  stimulus  spreads  from  one  central  cell  to  another  until  the 
neighboring  centres  are  affected.  Thus  we  often  find  the  lachrymal 
glands  are  also  influenced  by  very  strong  stimulation  of  the  tongue, 
and  pour  out  their  secretion,  as  is  said,  "in  sympathy"  with  the 
mouth  glands.  But  the  amount  of  the  afferent  impulse  is  not  the 
only  factor  in  determining  the  amount  of  response  to  be  reflected 
along  the  efferent  channels.  Some  nerve  cells  have  a  distinct 
power  of  increasing  the  amount  of  response  to  be  given  to  a  given 
stimulus.  When  an  irritant  falls  near  the  laryngeal  opening,  a 
very  different  effect  is  produced,  and  the  vastly  greater  response 
to  an  equal  stimulus  depends  rather  on  the  augmenting  power  of 
some  central  cells  than  upon  any  greater  sensibility  of  the  local 
mechanisms. 

4.  Inhibition.  Under  certain  circumstances,  such  as  pre-occu- 
patioD,  etc.,  which  will  be  more  fully  explained  presently,  nerve 


516  MANUAL   OF    PHYSIOLOGY. 

cells  withhold  the  transmission  of  a  stimulus,  or  lessen  the  im- 
pulse reflected  so  as  to  produce  little  or  no  effect;  this  is  called 
inhibition. 

5.  Automatism.  Nerve  cells  are  supposed  to  have  the  power 
of  originating  impulses;  e.g.,  those  carrying  on  operations  which 
require  to  be  of  a  more  or  less  permanent  kind,  such  as  the  closure 
of  the  sphincter  muscles  and  the  partial  contraction  of  the  muscle 
cells  of  the  arteries.  Automatic  actions  are  sometimes  spoken  of  as 
those  that  are  continuous  and  those  that  undergo  rhythmical 
changes.  If  carefully  examined,  however,  most  of  the  so-called 
constant  automatic  nervous  actions  will  be  found  to  show  some 
traces  of  rhythmic  relaxation.  Nerve  cells,  with  automatic 
properties,  may  be  exercised  in  preventing  reflex  actions  having 
their  full  effect,  and  thus  they  act  as  aids  to  the  controlling 
part  played  by  the  reflex  cells.  And  in  the  same  way  automatic 
cells  may  be  influenced  and  even  regulated  by  impulses  coming 
from  the  periphery  to  reflex  centres  in  the  vicinity,  which  join 
forces  with  the  automatic  centre.  Thus  the  act  of  respiration 
may  be  performed  by  pure  automatism,  and  its  centre  supplies 
a  good  example  of  an  automatic  group  of  cells.  As  a  matter  of 
fact,  however,  the  respirations  are  regulated  by  a  reflex  mechanism, 
the  channels  of  which  reside  in  the  vagus  nerve. 

Moreover  it  is  in  the  nerve  cells  that  we  must  seek  mental 
activity,  under  which  term  may  be  considered  perception,  volition, 
thought,  and  memory.  It  is  very  difficult  to  allocate  the  due  pro- 
portions of  reflection,  coordination,  augmentation,  inhibition,  au- 
tomatism, etc.,  requisite  for  the  development  of  what  we  must  call 
mental  faculties,  but  there  can  be  no  doubt  that  the  mind  must 
be  the  resultant  of  a  long  series  of  external  and  internal  excita- 
tions, modified  by  intrinsic  influence,  and  acting  upon  innumerable 
groups  and  masses  of  nerve  cells,  the  general  outline  of  which  has 
been  rough-hewn  by  hereditary  tendency. 


CHAPTER  XXIX. 

SPECIAL  PHYSIOLOGY  OF  NEEVES. 

Spinal  Nerves. 

The  thirty-one  pairs  of  nerves  which  leave  the  vertebral  canal 
by  the  openings  between  the  vertebrae  are  called  spinal  nerves,  in 
contradistinction  to  the  cranial  nerves,  which  come  out  through 
the  base  of  the  skull.  They  are  attached  to  the  spinal  marrow 
by  two  bands,  the  anterior  and  posterior  "  roots,''  which  unite 
together  in  the  inter- vertebral  canal  to  form  the  trunk  of  the 
nerve.  Just  before  the  junction  of  the  two  roots  the  posterior 
one  is  enlarged  by  a  ganglionic  swelling. 

The  spinal  nerves  are  all  "  mixed  nerves,"  that  is  to  say,  they 
contain  both  efferent  and  afferent  fibres ;  but  these  two  sets  of 
fibres  run  separately  in  the  anterior  and  posterior  roots  of  each 
nerve.  The  spinal  nerves  are  thus  joined  to  the  spinal  marrow 
by  two  nervous  cords,  each  one  of  which  contains  only  efferent 
or  afferent  channels.  About  seventy  years  ago  Charles  Bell  dis- 
covered that  the  anterior  roots  carry  the  efferent  fibres  and  the 
posterior  the  afferent.  Hence,  the  anterior  are  commonly  spoken 
of  as  the  motor  roots,  and  the  posterior  as  the  sensory  roots  of  the 
spinal  nerves.  The  experiment  to  show  this  difference  is  simple, 
but  requires  very  delicate  manipulation.  If  the  anterior  roots  of 
the  nerves  supplying  the  hind  leg  of  a  recently-killed  frog  be 
divided,  the  muscles  of  the  limb  are  cut  off  from  the  centres  in 
the  spinal  cord,  and  therefore  the  leg  hangs  limply,  and  does  not 
move  if  it  is  pinched  when  the  frog  is  suspended ;  whereas  the 
limb  on  the  sound  side,  upon  which  the  anterior  roots  are  intact, 
will  move  energetically  when  the  motionless  one  is  irritated.  If 
the  distal  ends  of  the  divided  anterior  roots  be  stimulated,  the 
muscles  of  the  paralyzed  limb  are  thrown  into  action  ;  but  stimu- 
lation of  the  proximal  end  gives  no  result.     Further,  if  the  two 


518  MANUAL  OF   PHYSIOLOGY. 

webs  of  this  frog  be  compared,  the  bloodvessels  running  across 
the  transparent  part  of  the  web  on  the  injured  side  will  be  found 
to  be  fuller  than  those  in  the  web  of  the  other  limb,  but  if  the 
distal  ends  of  the  motor  roots  are  stimulated,  the  dilated  blood- 
vessels return  to  their  normal  calibre.  By  these  experiments  we 
are  shown  that,  together  with  fibres  to  the  skeletal  muscles,  effer- 
ent fibres  carrying  impulses  to  the  muscular  walls  of  the  vessels 
are  contained  in  the  anterior  roots  of  the  spinal  nerves. 

The  fact  that  when  the  leg  on  the  side  where  the  anterior  roots 
have  been  severed  is  stimulated,  the  other  moves,  is  sufiicient  to 
show  that  the  sensory  connections  between  its  surface  and  the 
cord  are  not  destroyed  by  cutting  those  anterior  roots  ;  and  we 
may  conclude — taking  the  other  facts  just  mentioned  into  account 
— that  the  afferent  fibres  are  situated  in  the  posterior  roots. 

We  can  confirm  this  result  by  cutting  the  posterior  roots  on 
one  side  of  a  recently-killed  frog,  and  repeating  the  stimulation 
of  the  feet. 

Pinching  the  limb  whose  posterior  roots  are  cut,  gives  rise  to 
no  response,  because  the  impulses  cannot  reach  the  spinal  cord  ; 
but  stimulation  of  the  sound  foot  causes  obvious  movements  of 
both  legs.  This  shows  that  the  section  of  the  posterior  roots  of 
one  limb  cuts  off  the  afferent  (sensory)  communication  on  the 
side  operated  on,  but  that  the  efferent  (motor)  impulses  can  pass 
freely  to  the  muscles,  even  when  the  posterior  roots  are  divided, 
for  the  limb  moves  on  pinching  the  other  foot.  Further,  if  the 
proximal  ends  of  the  cut  posterior  roots  be  stimulated,  motions 
are  produced  showing  that  the  centres  in  the  spinal  cord  are  in- 
fluenced by  the  afferent  impulses  carried  by  those  posterior  roots. 
On  the  other  hand,  if  the  distal  ends  of  the  cut  roots  be  stimu- 
lated no  movement  results. 

It  has  been  sometimes  found  that  stimulation  of  the  anterior 
roots  seemed  to  cause  pain,  as  shown  by  the  motion  of  other  parts 
besides  those  to  which  this  root  was  itself  distributed  ;  and  it  was 
believed  that  some  sensory  fibres  must  run  in  the  anterior  roots. 
But  it  has  been  since  found  that  if  the  posterior  roots  be  first  cut 
these  signs  of  pain  are  not  shown  when  the  anterior  roots  are 
stimulated.     From  this  it  has  been  concluded  that  the  apparent 


SPINAL.  GANGLIA. 


519 


sensory  channels  of  the  motor  roots  are  nothing  more  than  some 
sensory  fibres  which  pass  from  the  nerve  trunk  a  little  way  up 
the  motor  root,  and  then  turn  back  and  descend  again  to  the 
junction  of  the  roots,  whence  they  pass  along  the  posterior  root 
to  the  cord.  These  fibres  are  named  the  "recurrent  sensory 
fibres,"  and  the  recurrent  sensibility  of  the  anterior  roots  is  not 
regarded  as  any  serious  departure  from  Bell's  law. 

The  course  of  the  secretory,  etc.,  nerves  probably  follows  that 
of  the  motor  channels  at  their  exit  from  the  cord.     Their  per- 

FiG.  206. 


Section  through  spinal  ganglion  of  a  cat,  showing  ganglion  cells  interspersed 
between  the  fibres.     (Low  power.) 


ipheral  distribution,  and  that  of  the  vasomotor  nerves,  are  inti- 
mately connected  with  the  sympathetic  system,  and  will  be  con- 
sidered further  on. 

Of  the  function  of  the  ganglia  on  the  posterior  roots  of  the 
spinal  nerves  but  little  is  positively  known.  There  is  no  evidence 
of  their  being  centres  of  reflex  action,  nor  can  they  be  shown  to 
possess  any  marked  automatic  activity.  From  the  fact  that  when 
a  mixed  nerve  is  divided  the  end  cut  ofi"  from  the  ganglion  de- 
generates after  a  few  days,  these  ganglia  are  supposed  to  preside 
over  the  nutrition  of  the  tissue  of  the  nerve  itself.  And  if  the 
roots  be  cut,  that  part  of  the  posterior  root  attached  to  the  cord 
degenerates,  while  the  piece  connected  with  the  ganglion  is  well 


520 


MANUAL  OF   PHYSIOLOGY. 


nourished.  This  is  not  the  case  if  the  anterior  root  be  divided, 
but,  on  the  contrary,  that  portion  next  the  cord  is  well  nourished, 
while  that  going  to  join  the  posterior  root  is  degenerated. 

It  would  thus  appear  that  the  trophic  function  of  the  ganglia 
is  restricted  to  the  sensory  nerves,  while  the  nutrition  of  the  motor 
nerves  is  provided  for  by  nervous  centres  situated  higher  up. 


The  Cranial  Nerves. 

The  nerves  which  pass  out  through  the  foramina  in  the  base  of 
the  skull  must  be  considered  separately, 
as  the  function  of  each  of  them  shows 
some  peculiarity.  Some  are  exclusively 
nerves  of  special  sense,  and  may  be  most 
conveniently  described  when  the  special 
sense  organs  are  under  consideration. 
Some  are  simple,  being  purely  motor  in 
function,  while  others  are  exceedingly 
complex,  containing  many  kinds  of  fibres. 
They  may  be  taken  in  the  order  of  their 
functional  relationships,  motor  and 
mixed.  Those  which  relate  to  the  spe- 
cial senses  will  be  considered  in  future 
chapters. 


Two  cells  from  the  for- 
mer seen  under  a  high 
power,  showing  the  fine 
protoplasm  here  and  there 
retracted  from  the  cell- 
wall. 


III. — The  Motor  Oculi  Nerve. 


The  nerves  of  the  third  pair  are  the 
chief  motor  nerves  of  the  eyes.  They 
arise  from  the  gray  matter  on  the  floor 
and  roof  of  the  aqueduct  of  Sylvius,  and  pass  out  of  the  brain- 
substance  near  the  pons  from  between  the  fibres  of  the  peduncle, 
and  then  run  between  the  posterior  cerebral  and  superior  cere- 
bellar arteries.  They  pass  into  the  orbits  in  two  branches,  and 
are  distributed  to  the  following  orbital  muscles:  (1)  elevator  of 
the  eyelid ;  (2)  the  superior,  (3)  inferior,  and  (4)  internal  recti ; 
and  (5)  the  inferior  oblique.  They  also  contain  fibres  which 
carry  efferent  impulses  to  the  (1)  circular  muscle  of  the  iris,  and 


CRANIAL,   NERVES.  521 

to  the  (2)  ciliary  muscle.  The  latter  branches  reach  the  eye  by 
a  short  twig  from  the  inferior  oblique  branch,  which  goes  to  the 
ciliary  ganglion,  and  thence  enter  the  ciliary  nerves. 

The  action  of  the  orbital  muscles  is,  in  the  main,  under  the 
control  of  the  will,  though  they  afford  good  examples  of  peculiar 
coordination  and  involuntary  association  of  movements.  The 
contraction  of  the  pupil  by  the  action  of  the  circular  muscle 
(sphincter  pupillse)  is  a  bilateral  reflex  act,  the  afferent  impulse 
of  which  originates  in  the  retina,  passes  along  the  optic  nerves, 
and  is  transmitted,  probably  in  the  corpora  quadrigemina,  to  both 
the  third  nerves.  The  central  extremities  of  the  third  nerves 
must  have  an  intimate  connection  with  each  other  and  with  the 
optic  nerves,  for  the  diminution  in  size  of  the  pupils  follows  accu- 
rately the  increase  in  intensity  of  the  light  to  which  even  one  of 
the  retinse  is  exposed.  In  retinal  blindness  and  after  section  of 
the  optic  nerve  the  pupil  is  dilated.  The  action  of  the  ciliary 
muscle  may  be  said  to  be  voluntary,  since  we  can  voluntarily 
focus  our  eyes  for  near  or  far  objects.  Contraction  of  the  sphincter 
pupillae  and  of  the  internal  rectus  is  associated  with  the  contrac- 
tion of  the  ciliary  muscle  in  accommodation. 

Injury  or  disease  of  the  third  nerve  within  the  cranium  gives 
rise  to  the  following  group  of  phenomena :  (1.)  Drooping  of  the 
upper  lid  (Ptosis).  (2.)  Fixation  of  the  eye  in  the  outer  angle 
(Luscitas).  (3.)  Dilatation  and  immobility  of  pupil  (Mydriasis). 
(4.)  Inability  to  focus  the  eye  for  short  distances. 

IV. — The  Trochlear  Nerve. 

This  thin  nervous  filament  arises  under  the  Sylvian  aqueduct, 
and  passes  into  the  superior  oblique  muscle,  to  which  it  carries 
voluntary  impulses,  which  are  involuntarily  associated  with  those 
of  the  other  muscles  moving  the  eyeball.  Paralysis  of  this  mus- 
cle causes  no  very  obvious  impairment  in  the  motions  of  the  eye- 
ball when  the  head  is  held  straight,  but  it  is  accompanied  by 
double  vision,  .so  there  must  be  some  displacement  of  the  eyeball. 
When  the  head  is  turned  on  one  side  the  eye  follows  the  position 
of  the  head  instead  of  being  held  in  its  primary  position.     In 

44 


622  MANUAL   OF   PHYSIOLOGY. 

paralysis  of  this  nerve  a  double  image  is  seen  only  when  looking 
downwards,  and  the  image  on  the  affected  side  is  oblique  and 
below  that  seen  by  the  sound  eye. 

VI. — The  Abductor  Nerve  of  the  Eye. 

This  arises  in  the  floor  of  the  fourth  ventricle,  and  appears  just 
below  the  pons  Varolii.  It  is  the  motor  nerve  of  the  external 
rectus  muscle  of  the  eye.  Paralysis  or  section  of  it  causes  inter- 
nal squint. 

VII. — (PoRTio  Dura)  Motor  Nerve  of  the  Face. 

This  nerve  arises  from  a  gray  nucleus  in  the  floor  of  the  fourth 
ventricle.  It  passes  with  the  other  part  of  the  seventh  (portio 
mollis)  or  auditory  nerve  into  the  internal  auditory  meatus  of  the 
temporal  bone.  It  first  passes  out  towards  the  hiatus,  and  then 
turns  at  a  right  angle  to  form  a  knee-like  swelling  (geniculate 
ganglion),  and  then  runs  backwards  along  the  top  of  the  inner 
wall  of  the  drum,  and  passing  downwards  through  a  special  canal 
in  the  bone,  comes  out  at  the  stylo-mastoid  foramen,  and  finally 
spreads  out  on  the  side  of  the  face.  It  is  essentially  an  efferent 
nerve,  being  partly  motor  and  partly  secretory,  though  its  con- 
nections have  caused  afferent  functions  to  be  ascribed  to  it.  Its 
distribution  may  be  thus  briefly  summarized  : 

i.  Motor  fibres. — (1.)  To  the  muscles  of  the  forehead,  eyelids, 
nose,  cheek,  mouth,  chin,  outer  ear,  and  the  platysma,  which  may 
be  grouped  together  as  the  muscles  of  expression.  (2.)  To  some 
muscles  of  mastication,  viz.,  buccinator,  posterior  belly  of  digastric, 
and  the  stylo-hyoid — all  the  foregoing  being  supplied  by  external 
branches — while  in  the  temporal  bone  it  gives  a  branch  to  (3) 
the  stapedius  muscle,  and  also  a  branch  from  the  geniculate  gan- 
glion named  the  great  superficial  petrosal  nerve,  which  after  a  cir- 
cuitous course  is  supplied  to  the  elevator  and  azygos  muscles  of 
the  palate  and  uvula. 

ii.  Secretory  fibres. — (1.)  To  the  parotid  gland  by  the  small  su- 
perficial petrosal  nerve,  which  sends  a  branch  to  the  otic  ganglion, 
whence  the  fibres  pass  to  the  auriculo-temporal  nerve,  and  then 


THE    FIFTH    CRANIAL   NERVE.  523 

on  to  the  gland.  (2.)  To  the  submaxillary  gland  by  the  chorda 
tympani,  which  after  traversing  the  tympanum  leaves  the  ear  by 
a  fissure  at  its  anterior  extremity,  then  joins  the  lingual  branch 
of  the  fifth  to  separate  from  it  and  pass  into  the  submaxillary 
ganglion  which  lies  in  close  relation  to  the  gland  (compare  Figs. 
64  and  65). 

iii.  Vaso-motor,  or  vaso-inhibitory  influences,  are  chiefly  con- 
nected with  the  secretory  function,  since  dilatation  of  the  vessels 
of  the  glands  accompanies  the  increased  secretion  that  follows 
stimulation  of  the  nerves  going  to  the  glands. 

iv.  The  following  afferent  impulses  are  said  to  travel  along  the 
track  of  the  portio  dura  and  its  branches:  (1.)  Special  taste-sen- 
sations, which  are  chiefly  located  in  the  chorda  tympani  branch, 
may  be  explained  by  the  branches  of  communication  which  pass 
from  the  trunk  and  petrous  ganglion  of  the  glosso-pharyngeal  to 
the  portio  dura  at  its  exit  from  the  foramen,  or  by  the  connection 
in  the  drum  of  the  ear  between  the  tympanic  branch  of  the  glos- 
so-pharyngeal and  the  geniculate  ganglion  of  the  portio  dura 
through  the  lesser  superficial  petrosal  nerve.  (2.)  Ordinary  sen- 
sations, which  are  also  located  in  the  chorda  tympani,  are  said  to 
traverse  this  nerve  in  an  afferent  direction  until  it  comes  near  the 
otic  ganglion,  when  the  sensory  fibres  leave  the  chorda  and  pass 
to  the  inferior  division  of  the  fifth  nerve  through  the  otic  ganglion. 

Injury  of  the  facial  nerve  in  any  of  the  deeper  parts  of  its 
course  gives  rise  to  the  striking  group  of  symptoms  known  as  facial 
paralysis,  the  details  of  which  are  too  long  to  be  given  here. 
When  it  is  remembered  that  muscles  aiding  in  expression,  masti- 
cation, deglutition,  hearing,  smelling,  and  speaking,  are  paralyzed, 
and  that  taste,  salivary  secretion,  and  possibly  ordinary  sensation 
are  impaired,  one  can  form  some  idea  of  the  complex  pathological 
picture  such  a  case  presents. 

V. — N.  Trigeminus,  or  Trifacial  Nerve. 

This  nerve  transmits  both  efferent  and  afferent  impulses,  which, 
however,  are  carried  by  two  different  strands  of  fibres.  The  motor 
part,  which  arises  from  a  gray  nucleus  in  the  floor  of  the  fourth 


524  MANUAL   OF    PHYSIOLOGY. 

ventricle,  is  much  the  smaller  of  the  two,  and  has  been  compared 
to  the  anterior  root  of  a  spinal  nerve.  The  large  sensory  division 
springs  from  a  very  extensive  tract,  which  can  be  traced  from  the 
pons  Varolii  through  the  medulla  to  the  lower  limit  of  the  olivary 
body,  and  on  to  the  posterior  cornua  of  the  spinal  marrow.  This 
set  of  fibres  has  been  likened  to  the  posterior  root  of  a  spinal 
nerve,  being  somewhat  analogous  to  it  in  origin,  function,  and  the 
fact  that  there  is  a  large  ganglion  on  it  within  the  cranium. 

The  distribution  and  peripheral  connections  of  this  nerve  are 
somewhat  complicated,  and  should  be  carefully  studied  when  the 
manifold  functions  of  its  branches  are  being  considered.  The 
various  impulses  conveyed  by  the  trifacial  nerves  may  be  thus 
enumerated  : 

i. — Efferent  Fibres. 

1.  Motor. — To  the  muscles  of  (1)  mastication,  viz.,  temporal 
masseters,  both  pterygoids,  mylohyoid,  and  the  anterior  part  of 
the  digastrica  ;  (2)  to  the  tensor  muscle  of  the  soft  palate  ;  and  (3) 
to  the  tensor  tympani.  (4)  In  some  animals  (rabbit)  nerve  fila- 
ments also  pass  to  the  dilator  muscle  of  the  iris,  reaching  the  eye- 
ball by  the  ciliary  ganglion. 

2.  Secretory. — The  efferent  impulses  which  stimulate  the  cells 
of  the  lachrymal  gland  to  increased  action,  pass  along  the  branches 
of  the  ophthalmic  division  of  this  nerve. 

3.  Vaso-motor. — The  nerves  governing  the  muscles  of  the  blood- 
vessels of  the  eye,  of  the  lower  jaw,  and  of  the  mucous  membrane 
of  the  cheeks  and  gums. 

4.  Trophic. — On  account  of  the  impairment  of  nutrition  of  the 
eye  and  the  mucous  membrane  of  the  mouth,  which  occurs  after 
injury  of  the  fifth  nerve,  it  is  said  to  carry  fibres  which  preside 
over  the  trophic  arrangements  of  these  parts. 

ii. — Afferent  Fibres. 

1.  Sensory, — All  three  divisions  of  the  trifacial  nerve  may  be 
said  to  terminate  in  cutaneous  nerves,  by  which  the  ordinary  sen- 
sory impulses  are  carried  from — (1)  the  entire  skin  of  the  face, 
and  the  anterior  surface  of  the  external  ear ;  (2)  from  the  exter- 


THE    FIFTH    CEANIAL,    NERVE.  525 

nal  auditory  meatus ;  (3)  from  the  teeth  aud  the  periosteum  of 
the  jaws,  etc. ;  (4)  from  the  mucous  membrane  lining  the  cheeks, 
the  floor  of  the  mouth,  and  the  anterior  part  of  the  tongue  ;  (5) 
from  the  lining  membrane  of  the  nasal  cavity  ;  (6)  from  the  con- 
junctiva, the  ball  of  the  eye,  and  the  orbit  generally ;  (7)  and 
from  the  dura  mater,  including  the  tentorium. 

2.  Excito-motor. — Some  of  the  fibres  which  have  just  been  enu- 
merated as  carrying  ordinary  sensory  impressions  have  special 
powers  of  exciting  coordinated  reflex  motions.  Thus  the  sensory 
fibres  from  the  conjunctiva  and  its  neighborhood  are  the  afferent 
channels  in  the  common  reflex  acts  of  winking  and  closing  the 
eyelids ;  and  the  fibres  from  the  nasal  mucous  membrane  com- 
monly excite  the  complexly  coordinated  involuntary  act  of 
sneezing. 

3.  Exeito-secretory. — In  the  same  way,  as  in  the  case  of  reflex 
motion,  secretion  is  reflexly  excited  by  the  fibres  which  carry 
afferent  impulses  to  the  medulla  from  the  anterior  part  of  the 
tongue  when  the  latter  is  strongly  stimulated,  and  thus  excite 
activity  of  the  salivary  secretion  ;  and  severe  stimulation  of  the 
mucous  membrane  of  the  nose  or  of  the  eye  causes  impulses  to 
pass  to  the  secretory  centre  of  the  lachrymal  glands,  which  are 
frequently  thus  reflexly  excited. 

Very  intense  stimulation  of  almost  any  of  the  afferent  nerves 
may  excite  these  reflex  phenomena.  Thus  the  most  stoic  person 
will  experience  active  secretions  of  saliva  and  lachrymal  fluid, 
as  well  as  spasmodic  closure  of  the  lids  during  the  extraction  of 
a  tooth.  Even  the  bold  use  of  a  blunt  razor  will  cause  the  tears 
to  flow  down  the  cheeks  by  sending  exeito-secretory  impulses 
along  the  branches  of  the  inferior  and  superior  maxillary  divi- 
sion of  this  nerve. 

4.  Tactile  impulses  are  appreciated  by  the  anterior  part  of  the 
tongue  with  remarkable  delicacy, and  are  conveyed  by  the  lingual 
branch  of  the  fifth  nerve ;  and  most  of  the  cutaneous  fibres  are 
also  capable  of  receiving  tactile  stimulation. 

5.  Taste. — The  tastes  that  are  appreciated  by  the  anterior  part 
and  the  edges  of  the  tongue  are  carried  by  fibres  which  lie  in  the 
peripheral  branches  of  this  nerve.     These,  however,  probably 


526  MANUAL  OF   PHYSIOLOGY. 

beloug  chiefly,  if  not  aitogetlier,  to  the  chorda  tyrapani,  and  leave 
this  lingual  branch  of  the  fifth  to  join  the  seventh  nerve  on  their 
way  to  the  trunk  of  the  glosso-pharyngeal. 

There  are  four  ganglia  in  close  relation  to  the  branches  of  the 
fifth  nerve  which  have  certain  points  of  similarity,  and  may, 
therefore,  be  considered  together,  although  their  diflferent  posi- 
tions show  that  they  are  engaged  in  the  performance  of  very  dif- 
ferent functions. 

We  have  not  yet  been  able  to  ascertain  the  value  of  these  little 
points  of  junction  of  motor,  sensory,  vaso-motor,  and  secretory 
fibres,  because,  so  far,  we  are  unable  to  attribute  to  the  cells  of  the 
ganglia  either  reflecting  or  controlling  action,  or  any  automatic 
power. 

They  have  all  efferent  (motor  and  secretory)  and  afferent  (sen- 
sory) connections  with  the  nervous  centres,  and  also  connections 
with  the  main  channels  of  the  sympathetic  nerves.  These  are 
spoken  of  as  the  roots  of  the  ganglia.  Their  little  branches  are 
generally  mixed  nerves. 

The  Ciliary  or  Ophthalmic  Ganglion. 

This  ganglion  lies  in  the  orbit.  It  has  three  roots,  which  come 
from — (1)  the  inferior  oblique  branch  of  the  third  nerve,  by  a 
short  slip,  which  forms  the  motor  root ;  (2)  from  the  nasal  branch 
of  the  ophthalmic  division  of  the  fifth,  and  from  the  carotid  plexus 
of  the  sympathetic.  The  branches  go  mostly  to  the  ball  of  the 
eye,  and  may  be  divided  into  those  which  are  afl^erent  and  efl^erent. 
The  aflTerent  are  only  sensory  branches,  connecting  the  cornea 
and  its  neighboring  conjunctiva  with  the  centres.  The  efferent, 
or  motor  fibres,  are  those  that  go  to  the  dilator  pupillje  (coming 
mostly  from  the  sympathetic),  and  the  vaso-motor  fibres  going  to 
the  choroid  coat,  iris,  and  the  retina. 

The  Sphenopalatine  or  Nasal  Ganglion. 

This  lies  on  the  second  division  of  the  fifth  nerve,  from  which 
it  gets  its  sensory  root.  Its  motor  root  comes  from  the  seventh 
by  the  great  superficial  petrosal  nerve,  and  its  sympathetic  root 


THE   GLOSSO-PHAEYNGEAL,   NERVE,  527 

from  the  carotid  plexus  by  the  branch  joining  this  nerve.  These 
enter  the  ganglion  together,  and  are  commonly  spoken  of  as  the 
vidian  nerve.  Afferent  (sensory)  impulses,  from  the  greater  part 
of  the  nasal  cavity,  pass  through  this  ganglion.  Its  efferent 
branches  are — (1)  motor  to  the  elevator  of  the  soft  palate  and  the 
azygos  uvulae  ;  (2)  vaso-motor,  which  come  from  the  sympathetic ; 
and  (3)  secretory,  which  supply  the  glands  of  the  cheek,  etc. 

Otic  or  Ear  Ganglion. 

The  otic  ganglion  lies  under  the  foramen  ovale,  where  the  in- 
ferior division  of  the  fifth  comes  out  of  the  cranium.  Its  roots 
are — (1)  motor ;  and  (2)  sensory,  from  the  inferior  division  of  the 
fifth  ;  and  (3)  sympathetic,  made  up  of  a  couple  of  fine  filaments 
from  the  plexus,  around  the  meningeal  artery.  By  its  branches 
it  communicates  with  the  seventh,  chorda  tympani,  and  sends 
filaments  to  the  parotid  gland. 

The  Submaxillary  Ganglion. 

This  is  on  the  hyoglossus  muscle  in  close  relation  to  the  lin- 
gual branch  of  the  fifth,  from  which  it  gets  a  sensory  root.  The 
chorda  tympani  passes  to  the  ganglion,  carrying  efferent  impulses 
through  it  to  the  gland.  Its  sympathetic  branches  come  from 
the  plexus  around  the  facial  artery. 

VIII. — The  Glossopharyngeal  Nerve. 

This  nerve,  forming  part  of  the  eighth  pair,  springs  from  the 
floor  of  the  fourth  ventricle  above  the  nucleus  of  the  vagus.  It 
is  a  mixed  nerve,  the  functions  of  which  may  thus  be  classified. 

Afferent  fibres,  which  are  of  various  kinds,  viz. : 

(1.)  Sensory  fibres,  carrying  impulses  from  the  anterior  surface 
of  the  epiglottis,  the  base  of  the  tongue,  the  soft  palate,  the  ton- 
sils, the  Eustachian  tube  and  tympanum. 

(2.)  Excito-motor.  This  nerve  is  a  very  important  exciter  of 
reflex  movements  in  swallowing  and  vomiting,  when  a  stimulus 
is  applied  to  the  glosso-palatine  arch. 


628  MANUAL   OF   PHYSIOLOGY. 

(3.)  Excito-secretory ;  the  stimulation  of  the  back  of  the  tongue 
gives  rise  to  a  copious  flow  of  saliva  by  means  of  reflex  action. 

(4.)  Taste  sensations  are,  for  the  most  part,  carried  by  this 
nerve  ;  they  are  conveyed  from  special  nerve-endings  in  the  back 
of  the  tongue  (see  Taste). 

The  efferent  fibres  are  not  so  varied,  being  simply  motor  to  the 
middle  constrictor  of  the  pharynx,  the  stylo-pharyngeus,  the 
elevator  of  the  soft  palate,  and  the  azygos  uvula;. 

The  Spinal  Accessory  Nerves. 

These  also  form  part  of  the  eighth  pair  of  nerves,  and  arise 
from  the  oblong  and  the  spinal  marrow,  as  low  down  as  the 
seventh  cervical  vertebra.  The  lower  fibres  leave  the  lateral 
columns  at  their  posterior  aspect,  and  then  run  up  between  the 
denticulate  ligament  and  the  posterior  roots  of  the  spinal  nerves 
to  enter  the  cranial  cavity.  On  their  way  out  of  the  cranium 
they  divide  into  two  parts,  one  of  which  becomes  amalgamated 
with  the  vagus,  and  the  other  passes  down  the  side  of  the  neck 
as  the  motor  nerve  of  the  sterno-mastoid  and  trapezius  muscles. 
Physiologically,  it  may  be  compared  with  the  anterior  root  of  a 
spinal  nerve,  and  the  part  accessory  to  the  vagus  most  probably 
supplies  that  nerve  with  most  of  its  motor  branches. 

The  Vagus  Nerve. 

The  vagus  arises  from  the  lower  part  of  the  floor  of  the  fourth 
ventricle,  and  has  connections  with  many  of  the  important  groups 
of  nerve  cells  in  this  neighborhood. 

The  functions  of  its  widely-distributed  fibres  may  be  thus 
briefly  stated  : 

A.  The  Efferent  Fibres  may  be  divided  into — 

1.  Motor-nerve  channels,  going  to  a  great  portion  of  the  alimen- 
tary tract  and  the  air-passage ;  the  following  muscles  getting 
their  motor  supply  from  the  branches  of  the  vagus — the  pharyn- 
geal constrictors,  some  of  the  muscles  of  the  palate,  the  CBSoph- 
agus,  the  stomach,  and  the  greater  part  of  the  small  intestine. 
Motor  impulses  also  pass  along  the  trunk  of  the  vagus — though 


THE   VAGUS    NERVE.  529 

leaving  the  cord  by  the  roots  of  the  accessory  nerve — to  the  in- 
trinsic muscles  of  the  larynx  ;  these  fibres  lie  in  the  inferior  or 
recurrent  laryngeal  nerve  except  that  to  the  crico-thyroid,  which 
lies  in  the  superior  laryngeal  branch.  The  tracheal  muscle  and 
the  smooth  muscle  of  the  bronchial  walls  are  also  under  the  con- 
trol of  the  pulmonary  branches  of  the  vagus. 

2.  V(xso-motor  fibres  are  said  to  be  supplied  to  the  stomach  and 
small  intestine.  These  fibres  are  probably  derived  from  some  of 
the  numerous  connections  with  the  sympathetic. 

3.  Inhibitory  impulses  of  great  importance  for  the  regulation  of 
the  forces  of  the  circulation  pass  along  the  vagus  to  the  ganglia 
of  the  heart.  As  already  explained  in  detail  (see  p.  276),  these 
fibres  are  always  acting,  as  shown  by  the  fact  that  section  of  the 
vagi  causes  a  considerable  quickening  of  the  heart-beat.  On  the 
other  hand,  if  the  distal  end  of  the  cut  vagus  be  stimulated,  the 
heart  beats  more  slowly,  and  in  some  animals  may  come  to  a 
standstill  in  a  condition  of  relaxation. 

B.  The  Afferent  Fibres,  still  more  widely  spread,  are  im- 
portant for  the  functions  of  the  various  viscera.     They  are : 

1.  Setisory  fibres  carry  impulses  from  the  pharynx,  oesophagus, 
stomach,  and  intestine,  and  from  the  larynx,  trachea,  bi'onchi, 
and  the  lungs  generally.  The  pneumonia  which  follows  section 
of  the  vagi  depends  on — (1)  the  removal  of  the  sensibility,  and 
the  ease  with  which  foreign  matters  can  enter  the  air-passages  ; 
or  (2)  the  violent  breathing  necessary  when  the  motor  nerves  of 
the  larynx  are  cut ;  or  (3)  the  injury  of  trophic  or  vaso-motor 
fibres. 

2.  Excito-motor  nerves.  There  is  no  nerve  that  can  be  com- 
pared with  the  vagus  in  the  variety  of  the  reflex  phenomena  in 
which  it  participates.  Afferent  fibres  in  this  nerve  cause  spasm 
of  the  muscles  of  the  thorax  and  govern  the  respiratory  rhythm, 
and  preside  over  the  inhalation  of  the  air  and  excite  the  expira- 
tory muscles.  Thus  irritation  of  the  mucous  membrane  at  the 
root  of  the  tongue,  the  folds  of  the  epiglottis,  larynx,  trachea,  or 
bronchi,  caus&s  spasmodic  fits  of  coughing.  Irritation  of  the 
pharyngeal  or  the  gastric  fibres  gives  rise,  by  reflex  stimulation, 
to  the  act  of  vomiting. 


530  MANUAL   OF  PHYSIOLOGY. 

Stimulation  of  the  proximal  cut  end  of  the  trunk  of  the  vagus 
causes  inspiratory  effort  and  cessation  of  the  breathing  move- 
ments in  the  position  of  inspiration.  Stimulation  of  the  central 
cut  end  of  the  superior  laryngeal  branch,  causes  reflex  spasm  of 
the  muscles  of  the  larynx  and  a  fixation  of  the  expiratory  mus- 
cles in  the  position  of  expiration.  The  fibres  which  regulate  the 
respiratory  rhythm  consist  of  two  sets,  probably  passing  from  the 
lungs  to  the  inspiratory  and  expiratory  centres,  and  causing  each 
to  act  before  its  ordinary  automatism  would  transmit  any  dis- 
charge of  impulse  to  the  thoracic  muscles. 

In  the  laryngeal  branches  are  fibres  which  bear  centrifugal 
impulses  to  the  vaso-motor  centres  in  the  medulla,  and  excite  the 
centres  to  action.  These,  which  may  be  grouped  with  the  excito- 
motor  channels,  are  spoken  of  as  "  pressor  ^^  fibres,  from  the  influ- 
ence they  exert  upon  the  pressure  of  the  blood  in  the  arteries. 

3.  Excito -inhibitory  fibres  pass  from  the  heart  to  the  vaso-motor 
centre.  Stimulation  of  these  fibres,  which  take  somewhat  different 
courses  in  different  animals,  checks  the  tonic  action  of  the  vaso- 
motor centre,  and  greatly  reduces  the  blood  pressure.  Hence 
these  fibres  form  the  depressor  nerve.  Its  terminals  in  the  heart 
are  stimulated  by  distension  of  that  organ ;  and  the  vaso-motor 
centre  is  thereby  inhibited,  the  arteries  dilate  and  the  blood  pres- 
sure falls  so  that  the  over-filled  heart  can  empty  itself. 

4.  Excito-secretory  fibres.  Stimulation  of  the  gastric  endings 
of  the  vagus  causes  not  only  gastric,  but  also  the  salivary  secre- 
tion, which  occurs  as  a  precursor  of  gastric  vomiting. 

Section  of  both  vagi  in  the  neck  causes  the  death  of  the  animal 
within  a  day  or  two  after  the  operation,  and  the  following  changes 
may  be  observed  while  it  lives :  1.  The  heart-beat  is  much  quicker 
as  shown  by  the  increased  pulse-frequency.  2.  The  rate  of  breath- 
ing is  very  much  slower.  3.  Deglutition  is  difiicult,  the  food 
easily  passing  into  the  air-passages  through  the  insensitive  larynx. 

Section  of  the  superior  laryngeal  nerves  is  followed  by  slight 
slowness  of  breathing,  loss  of  sensibility  in  the  larynx,  entrance 
of  food  into  the  air-passages,  chronic  broncho-pneumouia,  and 
death. 

Section  of  the  inferior  laryngeal  nerves  give  rise  to  the  same 


HYPOGLOSSAL  JiJERVE.  531 

final  result,  because  the  muscles  of  the  larynx  are  paralyzed, 
and  closure  of  the  glottis  is  impossible.  A  change  in  voice  fol- 
lows the  section  or  injury  of  even  one  inferior  laryngeal,  as  may 
often  be  seen  in  man  from  the  effect  of  the  pressure  of  an  aneurism. 

IX. — Hypoglossal  Nerve. 

This  nerve  appears  in  the  furrow  between  the  olivary  body 
and  the  anterior  pyramid,  on  a  line  with  the  anterior  roots  of 
the  spinal  nerves.  It  corresponds  with  the  anterior  roots  in 
function,  being  a  purely  motor  nerve.  It  bears  impulses  to  the 
muscles  of  the  tongue  and  the  other  muscles  attached  to  the  hyoid 
bone. 

Some  sensory  fibres  lie  in  its  descending  branch,  but  these  prob- 
ably are  derived  from  the  vagus  or  trifacial  nerves,  with  which  its 
branches  inosculate. 

It  is  also  said  to  contain  the  vaso-motor  fibres  of  the  tongue. 

Section  of  the  nerve  causes  paralysis  of  the  muscles  of  the 
tongue  ;  when  this  is  unilateral,  the  tongue  inclines  to  the  injured 
side,  while  being  protruded  from  the  mouth  ;  but  while  being 
drawn  in,  it  passes  to  the  sound  side.  This  is  easily  understood 
when  it  is  borne  in  mind  that  the  two  acts  depend  upon  the  in- 
trinsic muscles  of  the  tongue,  bringing  about  an  elongation  or 
shortening  of  the  organ  respectively. 


CHAPTER  XXX. 

SPECIAL  SENSES. 

It  has  been  pointed  out  that  the  sensory  nerves  receive  im- 
pressions from  without  and  carry  the  impulse  thus  excited'  more 
or  less  directly  to  certain  nerve  cells  in  the  brain  where  it  becomes 
a  sensation.  The  afferent  nerves  are,  then,  the  means  by  which 
the  mind  becomes  acquainted  with  occurrences  in  the  outer  world, 
as  well  as  the  channels  along  which  a  variety  of  stimuli  pass  to 
nerve  centres  whence  they  are  reflected  to  diflferent  organs  and 
parts,  without  causing  any  definite  sensation  in  the  nerve  cells  of 
the  sensoriura. 

The  ordinary  sensory  nerves  are  brought  into  such  relationship 
to  the  surface  that  they  are  afl^ected  by  slight  mechanical  stimuli, 
which  throw  the  nerve  fibres  into  activity,  and  send  impulses  to 
the  brain.  But  we  are  capable  of  appreciating  many  other  im- 
pressions besides  mechanical  stimulation.  We  can  distinguish 
between  degrees  of  heat  and  cold,  when  the  difference  is  far  too 
slight  to  act  as  a  direct  nerve  stimulus.  We  can  appreciate  light, 
of  which  no  degree  of  intensity  is  capable  of  exciting  a  nerve  fibre 
to  its  active  state,  or  of  stimulating  an  ordinary  nerve  cell  in  the 
least  degree.  We  recognize  the  delicate  air-vibrations  called 
sound,  which  would  have  no  eflfect  on  an  ordinary  nerve  ending. 
We  can  also  distinguish  several  tastes ;  and  finally,  we  are  con- 
scious of  the  presence  of  incomprehensibly  small  quantities  of  subtle 
odors  floating  in  the  air.  When  the  amount  of  the  substance  is 
too  small  to  be  recognized  even  by  spectrum  analysis,  which 
detects  extraordinarily  minute  quantities,  we  can  perceive  an  odor 
by  our  olfactory  organs. 

There  must  then  be  a  special  apparatus  for  the  reception  of  each 
of  these  special  impressions  in  order  that  the  nervous  system  be 
accessible  to  such  slender  influences.  In  fact  special  mechanisms 
must  exist  by  means  of  which  heat,  light,  sound,  tastes,  and  odor 


SPECIAL   SENSES.  533 

are  enabled  to  act  as  nerve  stimuli.  These  peculiar  nerve  termi- 
nals are  known  as  the  special  sense  organs,  the  physiology  of  which 
is  one  of  the  most  diflBcult  and  most  interesting  branches  of  study 
in  Biological  Science. 

The  nerves  which  carry  the  impulses  from  the  various  organs 
of  special  sense  do  not  differ  from  other  nervous  cords,  so  far  as 
their  structure  and  capabilities  are  concerned.  But  besides  their 
special  end  organs  they  are  connected  with  nerve  cells  in  the 
brain,  the  sole  duty  of  which  is  to  receive  impulses  from  one  of 
the  special  sense  organs  and  convert  the  same  into  a  special  sen- 
sation. No  matter  by  what  means  a  nerve  trunk  from  a  special 
sense  organ  be  stimulated,  the  impulse  excites  in  the  seusorium 
the  sensation  usually  arising  from  stimulation  of  the  special  organ 
to  which  it  belongs.  Thus  electric  stimulation  of  nerves  in  the 
tongue  causes  a  certain  taste ;  mechanical  or  other  stimulation  of 
the  optic  nerve-trunk  gives  rise  to  the  sensation  of  flashes  of  light, 
and  a  persistent  odor  may  be  caused  by  the  presence  of  a  bony 
growth,  pressing  upon  the  olfactory  nerve. 

The  capability  of  the  nerve  centres  connected  with  the  nerves 
of  special  sense  to  give  rise  invariably  to  a  special  sensation,  is 
called  their  specific  energy.  And  the  special  influence,  light,sound, 
etc.,  which  alone  suflSces  to  excite  the  special  peripheral  terminal, 
and  which  the  given  terminal  alone  can  convert  into  a  nerve 
stimulus,  may  be  called  its  specific  or  adequate  stimulus. 

Although  we  habitually  refer  the  sensation  to  the  surface  where 
the  stimulus  is  applied,  as  if  we  really  felt  with  our  skin,  and 
recognized  sound  sensations  with  our  ears,  etc.,  the  sensation  only 
occurs  in  the  centres  in  the  brain.  This  is  obvious  from  what 
has  been  already  said  of  the  nerve-fibres  of  the  special  sense 
organs,  namely,  that  if  a  stimulus  be  applied  to  the  nerve  trunk 
the  same  sensation  is  produced  as  if  the  specific  stimulation  had 
operated  on  the  special  nerve  terminal  from  which  these  fibres 
habitually  carried  impressions.  This  peripheral  localization  of 
sensations  is  really  accomplished  in  the  mind,  just  as  by  a  mental 
act  of  a  different  character,  the  impressions  communicated  by  the 
eye  are  projected  into  the  space  about  us  in  our  thoughts,  instead 
of  being  referred  to  the  retina,  or  thought  of  as  being  produced  in 


534  MANUAL   OF   PHYSIOLOGY. 

the  eye  itself.  This  power  of  the  centres  of  the  sensorium  to  lo- 
calize impressious  to  certain  points  of  the  skin,  and  to  project  into 
space  the  stimulation  caused  by  the  light  reflected  from  distant 
objects,  so  as  to  get  a  distinct  and  accurate  idea  of  their  position, 
is  the  result  of  experience  and  habit,  which  teach  each  individual 
that  when  a  certain  sensation  is  produced,  it  means  the  stimulation 
of  a  certain  point  of  the  skin,  and  that  the  objects  we  see  are  not 
in  our  eyes,  where  the  impulse  starts,  but  at  some  distance  from 
us.  We  learn  this  from  a  long  series  of  unconscious  experiments 
carried  on  in  our  early  youth  by  movements  of  the  eyes  with  co- 
operation of  the  hands.  Even  the  sensations  which  arise  in  the 
various  centres  of  the  sensorium,  as  the  result  of  internal  or  cen- 
tral excitations,  are  from  habit  attributed  to  external  influences, 
and  thus  we  have  various  hallucinations  and  delusions,  such  as 
seeing  objects  or  hearing  sounds  which  only  exist  in  the  brain. 

The  sensations  produced  in  our  nerve  centres  as  the  result  of 
the  afferent  impulses  coming  from  our  special  sense  organs  give 
rise  to  a  form  of  knowledge  called  perception ;  each  perception  or 
impulse  causing  an  appreciable  sensation,  helping  to  make  up  our 
knowledge  of  the  outer  world  and  of  ourselves,  for  without  this 
powder  of  perception  we  could  have  no  notion  of  our  own  existence 
and  no  ideas  of  our  surroundings ;  in  fact  we  should  be  cut  off" 
from  all  sources  of  knowledge  and  be  idiots  by  deprivation  of  all 
intelligence  from  without. 

A  complete  special  sense  apparatus  may  then  be  said  to  be 
made  up  of  the  following  parts : 

1.  A  special  nerve-ending  only  capable  of  being  excited  by  a 
special  adequate  sthnulus. 

2.  An  afferent  nerve  to  conduct  the  impulses  from  the  special 
end-organ  to  the  nerve  centre. 

3.  Nerve  cells  forming  a  centre,  which  is  capable  by  specific 
energy  of  translating  the  nerve  impulse  into  a  sensation,  and 
which  sensation  is  commonly  referred  to  some  local  point  of  the 
periphery. 

4.  Associated  nerve  centres,  capable  of  perceiving  the  sensations, 
forming  notions  thereon,  and  drawing  conclusions,  from  the  present 


SKIN   SENSATIONS. 


535 


and  past  perceptions,  as  to  the  intensity,  position,  quality,  etc.,  of 
the  external  influence. 


Skin  Sensations. 

The  sensations  arising  from  the  many  impulses  sent  from  the 
skin  come  under  the  head  of  special  sense,  and  are  commonly 
grouped  together  under  the  name  of  the  Sense  of  Touch.  This 
special  sense  may,  however,  be  resolved  into  a  number  of  specific 
sensations,  each  of  which  might  be  considered  as  a  distinct  kind 
of  feeling,  but  usually  are  regarded  as  simply  giving  different 
qualities  to  the  sensation  excited  by  the  skin.  These  sensations 
are  :  (1.)  Tactile  Sensation,  or  sensation  proper,  by  means  of  which 

Fig.  208. 


Drawing  from  a  section  of  injected  skin,  showing  three  papillae,  the  cen- 
tral one  containing  a  tactile  corpuscle  (a),  which  is  connected  with  a  med- 
ullated  nerve,  and  those  at  each  side  are  occupied  by  vessels.     (Cadiat.) 

we  appreciate  a  very  gentle  contact,  and  recognize  the  locality  of 
stimulation,  and  judge  of  the  position  and  form  of  bodies;  (2.) 
the  sense  of  pressure ;  (3.)  and  the  sense  of  temperature. 

The  variety  of  perceptions  derived  from  the  cutaneous  surface, 
and  the  large  extent  of  surface  capable  of  receiving  impressions, 
make  the  skin  the  most  indispensable  of  the  special  sense  organs, 
though  we  value  this  source  of  our  knowledge  but  little.  If  we 
could  not  place  our  hands  as  feelers  on  near  objects  to  investigate 
their  surfaces,  etc.,  we  should  lose  an  important  source  of  infor- 
mation that  has   contributed  largely   to   our  visual  judgment. 


536  MANUAL   OF   PHYSIOLOGY. 

Wc  think  we  know  by  the  look  of  a  thing  what  we  originally 
learned  by  feeling  it.  If  our  coujuuetivaj  did  not  feel,  we  should 
miss  its  prompt  warning,  and  our  voluntary  movements  could  not 
protect  our  eyes  from  many  unseen  injuries  that  normally  never 
trouble  us.  If  the  skin  were  senseless  it  would  require  constant 
mental  effort  to  hold  a  pen,  and  our  power  of  standing  and  pro- 
gressing would  be  most  seriously  impaired.  And  how  utterly  cut 
off  from  the  outer  world  should  we  be,  were  we  incapable  of  feel- 
ing heat  and  cold,  the  presence  or  absence  of  clothing,  etc. 

Nerve-Endings. 

Although  the  end-organs  of  the  nerves  of  the  skin  are  the  sim- 
plest of  all  those  belonging  to  the  apparatus  of  special  sense,  yet 
we  have  but  a  very  imperfect  knowledge  of  their  immediate  rela- 
tionships to  the  different  qualities  or  varieties  of  touch  impressions. 
We  are  familiar  with  several  different  nerve-endings  which  are 
special  terminals  adapted  for  the  reception  of  certain  kinds  of  im- 
pressions, but  what  kinds  of  stimuli  affect  the  different  terminals 
we  do  not  accurately  know.     They  may  be  thus  enumerated  : 

1.  The  Touch-corpuscles  (Meissner)  are  egg-shaped  bodies  situ- 
ated in  the  papillae  of  the  true  skin,  underlying  directly  the  epi- 
thelial cells  of  the  rete  mucosum.  They  occupy  almost  the  entire 
papilla.  The  nerve  fibres  seem  to  be  twisted  around  the  corpus- 
cle in  a  spiral  manner,  while  the  axis  cylinders  enter  the  body, 
and  the  covering  of  the  nerve  becomes  amalgamated  with  its  outer 
wall.  The  touch-corpuscles  vary  in  size  in  different  parts  of  the 
skin  ;  usually  being  larger  where  the  papillse  in  which  they  lie  are 
well  developed.  The  exact  mode  of  ending  of  the  axis  cylinder 
is  not  satisfactorily  understood. 

2.  End-hulbs  (Krause)  are  smaller  than  the  last,  and  are  less 
generally  distributed  over  the  surface  of  the  body,  being  localized 
to  certain  parts.  They  are  chiefly  found  in  the  conjuuctiva  and 
mucous  membranes  of  the  mouth  and  external  generative  organs. 
They  consist  of  a  little  vesicle  containing  fluid  in  which  the  axis 
cylinder  of  a  nerve  terminates,  the  membrane  which  forms  the 
vesicle  of  the  bulb  being  fused  with  the  sheath  of  the  nerve. 


CUTANEOUS   NERVE-ENDINGS. 


537 


Many  different  shapes  and  varieties  of  these  bodies  have  been 
described,  but  there  seem  to  be  no  very  definite  morphological  or 
physiological  distinctions  between  the  different  varieties. 

3.  Touch-cells  (Merkel)  are  found  in  the  deeper  layers  of  the 
epidermis  of  man  as  well  as  in  the  tongues  of  birds ;  they  are  large 
cells  with  distinct   nuclei   and   nucleoli.     Frequently  they  are 


Fig.  209. 


Fig.  210. 


Fig.  209. — End  bulb  from  human  conjunctiva,  treated  with  osmic  acid, 
showing  cells  of  core.  (Long worth.) — a,  Nerve  fibre  ;  h,  nucleus  of  sheath ; 
c,  nerve  fibre  within  core ;  d,  cells  of  core. 

Fig.  210. — Tactile  corpuscle  from  the  duck's  tongue,  containing  two  tac- 
tile cells,  between  which  lies  the  tactile  disk.     (Izquierdo.) 


grouped  together  in  masses  and  surrounded  by  a  kind  of  sheath 
of  connective  tissue ;  in  which  condition  they  resemble  touch-cor- 
puscles. 

4.  Free  nerve-endings  occur  on  the  surface  of  the  epithelium  of 
the  mucous  membranes,  and  are  seen  on  the  surface  of  the  cornea. 
Here  delicate,  single  strands  of  nerve-fibrils  can  be  seen  after  gold 
staining,  passing  between  the  epithelial  cells  and  ending  at  the 
surface  in  very  minute  blunted  points  or  knobs. 

Naked  nerve-fibrils  have  also  been  traced  into  the  deeper  layers 
of  the  epidermis  of  the  skin,  where  they  end  among  the  soft  cells 
of  the  mucous  layer,  either  in  branched  cell-like  bodies  (Langer- 
haus),  or  delicate  loops  (Ranvier). 

In  the  subcutaneous  fat-tissue  as  well  as  in  parts  remote  from 
the  surface  are  large  bodies,  easily  visible  to  the  naked  eye,  com- 
monly called — 

45 


638 


MANUAL  OF   PHYSIOLOGY. 


5.  Pacinian  corpuscles.  They  are  ovoid  bodies  made  up  of  a 
great  number  of  couceutrically  arranged  layers  of  material,  of 
varying  consistence,  with  a  collection  of  fluid  in  the  centre  in 
which  an  axis  cylinder  ends.  There  is  no  doubt  that  they  are  the 
terminals  of  afferent  nerves,  but  if  they  belong  to  the  sense  of  touch, 
which  is  doubtful,  it  is  unknown  to  what  special  form  of  sensation 
they  are  devoted.     From  their  comparatively  remote  relation  to 

Fig.  211. 


Drawing  of  termination  of  nerves  on  the  surface  of  the  rabbit's  cornea. 
— a.  Nerve  fibre  of  sub-epithelial  network ;  6,  Fine  fibres  entering  epithelium ; 
c,  Intra-epithelial  network.     (Klein.) 


the  skin,  lying  some  distance  beneath  it  and  not  in  it,  as  are  the 
other  endings  mentioned — they  are  probably  connected  with  the 
appreciation  of  pressure  sensations  rather  than  those  more  properly 
called  tactile. 

The  sense  of  touch  must  be  carefully  distinguished  from  ordi- 
nary sensibility  or  the  capability  of  feeling  pain,  which  is  not  a 
special  but  a  general  sensation,  and  is  received  and  transmitted  by 
different  nerve  channels.  This  we  know  from  the  facts,  that  the 
mucous  passages  in  general  can  receive  and  transmit  painful  but 


SENSE   OF    LOCALITY.  539 

not  tactile  impressions,  and  that  in  the  spinal  cord  the  sensory  and 
tactile  impulses  pass  along  distinct  tracts.  Further,  certain  nar- 
cotic poisons  destroy  ordinary  sensation  without  removing  the  sense 
of  touch.  This  effect  is  also  brought  about  by  cold,  when  for  in- 
stance the  fingers  are  benumbed,  gentle  contact  excites  tactile  im- 
pressions, while  the  ordinary  sensations  of  pain  can  only  be  aroused 
with  diflBculty  even  by  severe  pressure. 

However,  most  of  the  nerves  we  are  in  the  habit  of  calling  sen- 
sory nerves  convey  tactile  impressions,  and  speaking  generally  the 
parts  of  the  outer  skin  which  have  the  keenest  tactile  sense  are 
also  the  most  ready  to  excite  feelings  of  pain. 

The  intensity  of  the  stimulation  for  the  sense  of  touch  must  be 
kept  within  certain  limits  in  order  that  it  be  adequate,  i.e.,  capable 
of  exciting  the  specific  mental  perceptions.  If  the  stimulus  exceed 
these  limits,  only  a  general  impression,  namely  that  of  pain,  is 
produced. 

The  power  of  forming  judgments  by  feeling  an  object  differs 
very  much  in  different  parts  of  the  body,  being  generally  most 
keen  where  the  surface  is  richest  in  touch  corpuscles,  namely,  the 
palmar  aspect  of  the  hands  and  feet,  and  especially  the  finger-tips, 
the  tongue,  the  lips,  and  the  face. 

When  we  feel  a  thing  in  order  to  learn  its  properties,  we  make 
use  of  all  the  qualities  of  which  our  sense  of  touch  is  made  up. 
We  estimate  the  number  of  points  at  which  it  impinges  on  our 
finger-tip,  we  rub  it  to  judge  of  smoothness,  we  press  it  to  find  out 
its  hardness,  and  at  the  same  time  we  gain  some  knowledge  of  its 
temperature  and  power  of  absorbing  heat. 

To  get  a  clear  idea  of  our  complex  sense  of  touch  we  must  con- 
sider each  of  the  different  kinds  of  impressions  separately. 

Sense  of  Locality. 

By  this  is  meant  our  power  of  judging  the  exact  position  of 
any  point  or  points  of  contact  which  may  be  applied  to  the  skin. 
Thus,  if  the  point  of  a  pin  be  gently  laid  on  a  sensitive  part  of 
the  skin,  we  know  at  once  when  we  are  touched,  and,  if  a  second 


540  MANUAL   OF   PHYSIOLOGY. 

pin  be  applied  in  the  same  neighborhood,  we  feel  the  two  points 
of  contact  and  can  judge  of  their  distance  from  one  another  and 
their  relative  position.  When  we  feel  a  body  we  receive  im- 
pulses from  many  points  of  contact  bearing  varied  relationships 
to  each  other,  and  thus  we  become  conscious  of  a  rough  or  a  smooth 
surface. 

The  delicacy  of  the  sense  of  locality  differs  very  much  in  dif- 
ferent parts  of  the  skin.  It  is  most  accurate  in  those  parts  which 
have  been  used  as  touch  organs  during  the  slow  evolution  of  the 
animal  kingdom. 

The  method  of  testing  the  delicacy  of  the  sense  of  locality  is 
simply  to  apply  the  two  points  of  a  compass  to  the  different  parts 
of  the  skin,  and  by  varying  their  position,  experimentally,  deter- 
mine the  nearest  distance  at  which  the  two  points  give  rise  to 
distinct  sensations.  The  following  precautions  must  be  attended 
to  in  carrying  out  this  experiment.  1.  The  points  must  be  simul- 
taneously applied  or  the  two  distinct  sensations  will  be  produced 
even  at  very  close  distances.  2.  The  force  with  which  the  points 
are  applied  must  be  equal  and  minimal,  because  excessive  press- 
ure causes  a  diffusion  of  the  stimulus  and  a  blurring  of  the  tactile 
senses.  3.  Commencing  with  a  greater  and  gradually  reducing 
the  distance  of  the  points  enables  a  person  to  appx-eciate  a  less 
separation  than  if  the  smaller  distances  were  used  at  first.  4. 
The  duration  of  the  stimulus;  two  points  of  contact  being  dis- 
tinguished at  a  much  nearer  distance  if  the  points  be  allowed 
to  rest  on  the  part,  than  when  they  are  only  applied  for  a  moment. 
5.  The  temperature  and  material  of  the  points  should  be  the 
same.  6.  Moisture  of  the  surface  makes  it  more  sensitive.  7. 
Previous  or  neighboring  stimulation  takes  from  the  accuracy 
of  the  sensations  produced.  8.  The  temperature  of  the  different 
parts  of  the  skin  should  be  equal,  as  cold  impairs  its  sensibility. 

The  following  table  gives  approximately  the  nearest  distances 
at  which  some  parts,  which  may  be  taken  as  examples  of  the  most 
and  least  sensitive  regions  of  the  skin,  can  recognize  the  points  of 
contact  by  their  giving  rise  to  two  distinct  sensations: 


TACTILE  IMPRESSIONS. 


541 


Tip  of  the  tongue,  .... 

Palmar  aspect  of  the  middle  finger-tip, 

Tip  of  the  nose. 

Back  of  the  liand,   . 

Plantar  surface  of  great  toe, 

Forearm,  anterior  surface, 

Front  of  thigh, 

Over  ensiform  cartilage. 

Between  scapulae,     . 


1  mm. 

2 

4 

15 

18 
40 
55 
50 
70 


If  one  point  of  the  compass  be  applied  to  the  same  spot,  and 
the  other  moved  around  so  as  to  mark  out  in  different  directions 
the  limit  at  which  the  points  can  be  distinguished  as  separate,  we 
get  an  area  of  a  somewhat  circular  form,  for  which  the  name  sen- 
sory  circle  has  been  proposed.  It  would  be  very  convenient  to 
explain  this  on  the  simple  anatomical  basis  that  the  impressions 
of  this  area  were  carried  by  one  nerve-fibre  to  the  brain,  and  thus 
but  the  one  sensation  could  be  produced  in  the  seusorium.  But 
we  know  this  cannot  be  the  true  explanation,  because  of  the  fol- 
lowing facts:  1.  No  such  anatomical  relationship  is  known  to 
exist.  2.  By  practice  we  can  reduce  the  area  of  our  sensory 
circles  in  a  manner  that  could  not  be  explained  by  the  develop- 
ment of  new  nerve-fibres.  3.  If  the  two  points  of  the  compass  be 
placed  near  the  edges  of  two  well-determined  neighboring  sensory 
circles,  and  so  in  relation  with  the  terminals  of  two  nerve-fibres, 
they  will  not  give  distinct  impressions;  in  fact  they  require  to 
be  separated  just  as  far  as  if  they  were  applied  within  the  boundary 
of  one  of  the  circles  where  they  also  give  rise  to  the  double  per- 
ception. 

To  explain  better  the  sense  of  locality  it  has  been  supposed  that 
sensory  circles  are  made  up  of  numerous  small  areas,  forming  a 
fine  mosaic  of  totich-fields,  each  of  which  is  supplied  by  one  nerve- 
fibre,  and  that  a  certain  number  of  these  little  fields  must  intervene 
between  the  stimulating  points  of  the  compasses  in  order  that  the 
sensorium  be  able  to  recognize  the  two  impulses  as  distinct.  For, 
although  every  touch-field  is  supplied  by  a  separate  nerve-fibril 
which  carries  its  impulses  to  the  brain,  and  is  therefore  quite  sensi- 
tive, the  arrangements  in  the  sensorium  are  such  that  the  stimuli 
carried  from  two  adjoining  touch-fields  are  confused  into  oneseusa- 


542  MANUAL   OF   PHYSIOLOGY. 

tion.  Thus,  when  an  edge  is  placed  on  our  skin,  we  do  not  feel  a 
series  of  points  corresponding  to  the  individual  fields  with  which 
it  comes  in  contact,  but  the  confusion  of  the  stimuli  gives  rise  to 
an  uninterrupted  sensation,  and  we  have  a  right  perception  of  the 
object  touched. 

The  Sense  of  Pressure. 

There  seems  to  be  a  reason  for  separating  the  perception  of  dif- 
ferences in  the  degree  of  pressure  exercised  by  a  body  from  the 
simple  tactile  or  local  impression.  If  we  support  a  part  of  the 
body  so  that  no  muscular  effort  be  called  into  play  in  the  support 
of  an  increasing  series  of  weights  placed  upon  the  same  area  of 
skin,  we  can  distinguish  tolerably  accurately  between  the  different 
weights.  It  has  been  found  that  if  a  weight  of  about  30  grammes 
be  placed  on  the  skin  a  difference  of  about  1  gramme  can  be  recog- 
nized— that  is,  we  can  distinguish  between  29  and  30  grammes, 
if  they  are  applied  soon  after  one  another.  If  the  weights  em- 
ployed are  smaller,  a  less  difference  can  be  detected  ;  if  larger 
weights  are  used  the  difference  must  be  greater,  and  it  appears 
that  the  weight-difference  always  bears  the  same  proportion  to  the 
absolute  weight  used.  We  can  perceive  a  difference  between  7k 
and  7^,  14^  and  15,  29  and  30,  58  and  60,  etc.,  the  discriminating 
power  decreasing  in  proportion  as  the  absolute  degree  of  stimula- 
tion increases. 

One  of  the  reasons  why  the  sense  of  locality  is  regarded  as  dis- 
tinct from  that  of  pressure  is  that  the  latter  is  found  not  to  be 
most  keenly  developed  in  the  same  parts  where  the  impressions 
of  locality  are  most  acute.  Thus  judgment  of  pressure  can  be 
more  accurately  made  with  the  skin  of  the  forearm  than  the 
finger-tip,  which  is  nine  times  more  sensitive  than  the  former  to 
ordinary  tactile  impressions,  and  the  skin  of  the  abdomen  has 
an  accurate  sense  of  pressure  though  deficient  in  ordinary  tactile 
sensation. 

It  has  been  said  above  that  the  weights  by  which  pressure- 
sense  is  to  be  tested  should  be  applied  rapidly  one  after  the  other. 
This  facts  depends  upon  the  share  taken  in  the  mental  judgment 
by  the  function  we  call  memory.    In  a  short  time  the  recollection 


SENSATIONS  OF  TEMPERATURE.  543 

of  the  impression  passes  away  and  there  no  longer  exists  any 
sensation  with  which  the  new  stimulation  can  be  compared. 

At  best  we  can  form  but  inaperfect  judgments  of  pressure  by 
the  skin  impressions  alone.  When  we  want  to  judge  the  weight 
of  a  body  we  poise  it  in  the  free  hand,  which  is  moved  up  and 
down  so  as  to  bring  the  muscles  which  elevate  it  into  repeated 
action.  Hereby  we  call  into  action  a  totally  different  evidence, 
namely,  the  amount  of  muscle  power  required  to  raise  the  weight 
in  question,  and  we  find  we  can  arrive  at  much  more  accurate 
conclusions  by  this  means.  The  peculiar  recognition  of  how 
much  muscular  effort  is  expended  is  commonly  spoken  of  as 
muscle-sense,  which  may  arise  from  a  knowledge  of  how  much 
voluntary  impulse  is  expended  in  exciting  the  muscles  to  action, 
but  more  probably  it  depends  upon  afferent  impulses  arriving  at 
the  sensorium  from  the  muscles.  By  its  means  we  aid  the  pres- 
sure-sense in  arriving  at  accurate  conclusions  of  the  weight  of 
bodies,  so  that  in  the  free  hand  we  can  distinguish  between  39 
grm.  and  40  grm. 

Temperature  Sense. 

We  are  able  to  judge  of  the  differences  in  temperature  of  bodies 
which  come  in  contact  with  our  skin.  Since  our  sensations  have 
no  accurate  standard  for  comparison  we  are  unable  to  form  any 
exact  conception  of  the  absolute  temperature  of  the  substances 
we  feel.  The  sensation  of  heat  or  cold,  derived  from  the  skin 
itself,  without  its  coming  into  contact  with  anything  but  air  of 
moderate  temperature,  varies  with  many  circumstances,  and  be- 
cause of  these  variations  the  powers  of  judgment  of  high  or  low 
temperature  must  be  imperfect.  The  skin  feels  hot  when  its 
bloodvessels  are  full ;  it  feels  cold  when  they  are  comparatively 
empty.  An  object  whose  temperature  is  the  same  can  thus  give 
the  impression  of  being  hot  or  cold  according  as  the  skin  itself  is 
full  or  empty  of  warm  blood.  But  independent  of  any  very 
material  change  in  the  blood-supply  of  the  cutaneous  surface  of 
a  part,  any  change  in  the  temperature  of  its  surroundings  causes 
a  sensation  of  change  of  temperature,  which  is,  however,  a  purely 
relative  judgment.     Thus,  if  the  hand  be  placed  in  cold  water, 


544  MANUAL   OF   PHYSIOLOGY. 

we  have  at  first  the  sensation  of  cold  ;  to  which,  however,  the 
skin  of  the  hand  soon  becomes  accustomed  so  as  no  longer  to 
excite  the  sensation  of  cold  ;  if  now  the  hand  be  placed  in  water 
somewhat  warmer — but  not  higher  in  temperature  than  the  atmos- 
phere— we  have  a  feeling  of  warmth.  If  the  hand  be  now  placed 
in  as  hot  water  as  the  skin  can  bear,  it  feels  at  first  unpleasantly 
hot,  but  this  feeling  soon  passes  away  and  the  sensation  is  com- 
fortable. If  now  from  this  hot  water  it  be  placed  again  in  the 
water  of  the  air  temperature,  this — which  before  felt  warm — now 
feels  very  cold. 

An  important  item  in  the  estimation  of  the  temperature  of  an 
object  by  the  sensations  derived  from  the  skin  depends  upon 
whether  it  be  a  good  or  a  bad  conductor  of  heat.  Those  sub- 
stances which  are  good  conductors,  and  therefore,  when  colder 
than  the  body,  quickly  rob  the  skin  of  its  heat,  are  said  to  feel 
cold,  whilst  badly-conducting  bodies,  of  exactly  the  same  tem- 
perature, do  not  feel  cold.  It  is  then  the  rapid  loss  of  heat  that 
gives  rise  to  the  sensation  of  cold. 

The  power  of  the  skin  in  recognizing  changes  of  temperature 
is  very  accurate,  although  the  power  of  judging  of  the  absolute 
degree  of  temperature  is  very  slight. 

By  dipping  the  finger  rapidly  into  water  of  varying  tempera- 
ture it  has  been  found  that  the  skin  can  distinguish  between  tem- 
peratures which  differ  by  only  i°  Cent,  or  i°  Fahr.  The  time 
required  for  the  arrival  of  temperature  impressions  at  the  brain 
is  remarkably  long  when  compared  with  the  rate  at  which  ordi- 
nary tactile  impulses  travel.  To  judge  satisfactorily  of  the  tem- 
perature of  an  object  we  must  feel  it  for  some  time. 

There  must  be  special  nerve-endings  which  are  capable  of  re- 
ceiving heat  impressions,  because  warmth  applied  to  the  nerve 
fibres  themselves  is  not  capable  of  giving  rise  to  the  sensation  of 
heat.  Thermic  stimuli,  no  doubt,  do  affect  nerve  fibres,  but  only 
cause  the  sensation  of  pain  when  applied  to  them. 

These  nerve-endings  are  not  the  same  as  those  that  receive 
touch  and  pressure  impressions,  because  the  appreciation  of  tem- 
perature differences  is  not  most  delicately  developed  in  the  parts 
where  the  tactile  sensations  are  most  acute.     Thus  the  cheeks 


GENERAL   SENSATIONS.  545 

and  the  eyelids  are  especially  sensitive  to  changes  of  temperature, 
a  fact  known  by  people  who  want  a  ready  gauge  of  the  heat  of  a 
body — thus,  a  barber  approaches  the  curling-tongs  to  his  cheek 
to  measure  its  temperature  before  applying  it  to  the  hair  of  his 
client.  The  middle  of  the  chest,  moreover,  is  very  sensitive  to 
heat,  while  it  is  dull  in  feeling  tactile  impressions. 

The  band  is  far  from  being  the  best  gauge  of  temperature,  for 
heat  appreciation  is  not  developed  in  a  due  proportion  to  the 
keenness  of  the  tactile  sensibility.  The  larger  the  surface  ex- 
posed to  changes  of  temperature  the  more  accurate  the  judgment 
at  which  we  can  arrive — the  slightest  changes  being  at  once  rec- 
ognized when  the  entire  surface  of  the  body  is  exposed  to  them. 
The  foregoing  facts  are  well  known  to  persons  in  the  habit  of 
testing  the  temperature  of  a  warm  bath  without  the  aid  of  a 
thermometer ;  they  do  not  use  the  limited  surface  of  a  sensitive 
tactile  finger-tip,  but  plunge  the  entire  arm  into  the  water.  The 
elbow,  indeed,  is  the  common  test  used  by  nurses  in  ascertaining 
that  the  water  in  which  they  are  about  to  wash  an  infant  is  not 
too  warm  for  that  purpose. 

Great  extremes  of  heat  or  cold,  such,  in  fact,  as  would  act  as 
stimuli  to  a  nerve  fibre,  do  not  give  rise  to  sensations  of  different 
temperatures,  but  simply  excite  feelings  of  pain.  Thus,  if  one 
plunges  one's  hand  into  a  freezing  mixture  or  into  extremely  hot 
water,  it  is  difficult  to  say  at  once  whether  they  are  hot  or  cold — 
in  both  cases  pain  being  the  only  sensation  produced. 

General  Sensations. 

We  call  general  sensations  those  feelings,  pleasurable  or  other- 
wise, which  can  be  excited  in  us,  without  our  being  able  to  refer 
them  to  external  objects,  or  compare  their  sensation  with  those  of 
the  special  senses,  or  even  to  describe  their  exact  mode  of  percep- 
tion. Under  this  head  are  enumerated  Pain,  Hunger,  Thirst, 
Nausea,  Giddiness,  Shivering,  Titillation,  Fatigue,  etc. 

Of  these  only  pain  is  commonly  referred  to  any  given  part, 
and  the  attempt  to  localize  pain  with  exactness  soon  shows  how 
very  different  is  our  power  in  this  respect  in  the  case  of  pain  and 
in  the  case  of  tactile  impressions.     Thus,  when   we  strike  our 

46 


546  MANUAL  OF  PHYSIOLOGY. 

"  funny-bone  "  (the  ulnar  nerve  passing  over  the  condyle  of  the 
humerus),  by  the  tactile  impressions  of  the  skin  we  know  the 
elbow  is  the  injured  part,  but  the  locality  of  the  pain  is  not  so 
exactly  to  be  determined,  for  it  shoots  down  the  arm  to  the  little 
finger,  and  is  indefinitely  spread  over  the  region  to  which  the 
nerve  is  distributed. 

In  studying  the  laws  which  govern  the  perception  of  painful 
impressions  we  must  make  the  experiments  upon  ourselves,  since 
we  alone  can  form  conclusions  from  the  sensations  produced. 

The  best  way  to  carry  out  experiments  upon  pain  is  to  use  ex- 
tremes of  temperature,  as  we  can  thus  graduate  the  stimulation. 
The  application  of  a  liquid  over  50°  C,  or  below  2°  C,  causes 
pain.  The  suddenness  of  application  to  the  part,  and  its  dura- 
tion, and  the  extent  of  surface,  as  well  as  the  previous  tempera- 
ture, have  important  influence  in  the  amount  of  pain  produced. 

The  various  kinds  of  pain  which  we  are  all  more  or  less 
familiar  with  seem  to  be  related  in  some  way  to  their  mode  of 
production,  but  we  are  unable  to  assign  any  definite  cause  for 
these  differences  of  character.  Thus,  though  such  terms  as  shoot- 
ing, stabbing,  burning,  throbbing,  boring,  racking,  dragging 
pain,  have  a  tolerably  clear  meaning  in  general,  and  may  be  of 
diagnostic  value,  we  have  only  an  indistinct  knowledge  that 
throbbing  depends  on  excessive  vascular  distension  in  a  part,  that 
sharp  pains  are  produced  by  sudden  excitation  of  a  sensitive  part, 
and  the  dull  pains  by  the  more  permanent  stimulation  of  a  part 
less  well  supplied  with  nerves. 

Further,  pain  as  we  think  of  it  is  a  complex  mental  process, 
made  up  of  many  items,  such  as  real  sensory  impressions,  fear, 
disgust,  etc.  When  a  finger  is  being  lanced  patients  often  cry 
out  most  loudly  before  they  are  touched  with  the  knife,  and  show 
intense  feeling  when  they  look  at  the  blood  flowing  from  the 
wound. 

Hunger  and  thirst  are  peculiar  and  indefinite  sensations  which 
are  experienced  when  some  time  has  elapsed  since  food  or  drink 
has  been  taken.  The  exact  part  of  the  nervous  system  in  which 
these  impressions  arise  has  not  been  determined.  They  are,  how- 
ever, said  to  be  associated  with  peculiar  sensations  in  the  stomach 


GENERAL   SENSATIONS.  547 

and  throat  respectively.  In  the  same  way  the  venereal  appetite, 
though  associated  with  local  sensations,  cannot  be  referred  to  any 
one  part  of  the  nervous  system. 

Nausea  is  also  a  sensation  which  cannot  be  attributed  to  any 
part  of  the  nervous  centres.  It  commonly  arises  in  response  to 
afferent  impulses,  such  as  smells,  sights,  tastes,  pharyngeal,  gastric, 
or  other  visceral  irritation,  and  is  antagonistic  to  the  appetites 
just  named.  All  the  sensations  that  give  rise  to  or  precede  nau- 
sea are  associated  in  our  minds  with  disagreeable  impressions, 
and  no  doubt  mental  operations  have  much  to  do  with  its  pro- 
duction. A  child,  free  from  affectation,  may  be  heard  to  say  of 
a  castor-oil  bottle,  which,  in  itself,  is  not  ugly,  "  I  can't  bear  to 
look  at  it,  the  very  thought  of  it  makes  me  feel  sick." 

However,  even  without  any  participation  of  the  mental  func- 
tions, unavoidable  nausea  may  come  on  from  irregular  motion, 
as  that  of  a  ship,  which  often  causes  nausea  in  those  unaccus- 
tomed to  the  sea.  Certain  conditions  of  the  blood  flowing  through 
the  nerve  centres  also  cause  nausea,  as  when  emetics  are  injected 
into  the  blood. 

Giddiness,  which  consists  in  a  feeling  of  inability  to  keep  the 
normal  balance,  is  often  produced  in  connection  with  the  last  by 
irregular  movements,  but  more  surely  by  a  rotatory  motion  of  the 
body.  Other  afferent  influences  may  give  rise  to  it,  viz.,  from  the 
stomach,  in  some  cases  of  irritation  ;  from  the  eye,  when  we  look 
from  a  height ;  from  the  semicircular  canals  of  the  ear  by  rota- 
tion of  the  body ;  and  also  from  conditions  of  the  blood,  as  in 
alcoholic  toxaemia. 

Shivering  is  also  the  result  of  a  peculiar  nervous  effect  pro- 
duced by  afferent  influences  of  an  unpleasant  kind,  the  sudden 
application  of  cold  to  the  skin,  a  revolting  sight,  a  shrill  noise, 
an  intensely  nasty  taste,  and  a  very  shocking  narrative,  may 
excite  a  nervous  condition  which  makes  us  shiver. 

Titillation  follows  light  stimulation  of  certain  parts  of  the  cu- 
taneous surfaces.  It  is  a  peculiar  general  sensation,  in  modera- 
tion not  disagreeable,  and  usually  accompanied  by  a  tendency  to 
meaningless  laughter  or  reflex  movements. 


CHAPTER  XXXI. 

TASTE  AND  SMELL. 

Sense  of  Taste. 

Next  to  the  sense  of  touch,  which  is  distributed  more  or  less 
over  the  whole  cutaneous  surface,  taste  is  the  least  localized 
anatomically.  Though  confined  to  the  cavity  of  the  mouth,  its 
more  accurate  limitations  are  not  easily  fixed.  The  point,  sides, 
and  posterior  part  of  the  dorsum  of  the  tongue  can  most  accu- 
rately appreciate  tastes  ;  and  probably  parts  of  the  palate  also 
have  the  power,  but  in  a  much  less  degree.  Indeed,  though  "the 
palate"  is  often  spoken  of  as  if  it  were  the  seat  of  taste,  it  really 
enjoys  this  function  in  an  insignificant  degree  when  compared 
with  the  tongue. 

The  power  of  being  stimulated  by  various  tastes  is  not  re- 
stricted to  the  terminals  of  any  one  nerve,  but  is  shared  by  some 
of  those  of  at  least  three  trunks,  which  also  transmit  impulses 
arising  from  other  forms  of  stimulation.  The  glosso-pharyngeal 
division  of  the  eighth  pair  sends  branches  to  the  posterior  part  of 
the  tongue,  which  are  no  doubt  connected  with  the  special  taste 
organs.  The  lingual  branches  of  the  fifth — commonly  called  the 
gustatory  nerves — have  also  terminals  capable  of  being  excited 
by  taste,  and  probably  some  fibres  of  the  chorda  tympani  are  also 
employed  in  this  function. 

In  the  furrows  around  the  circuravallate  papillse,  and  also,  but 
more  sparsely,  on  the  sides  of  the  fungiform  papillas  of  the  tongue, 
are  found  peculiar  organs  called  "  taste-buds"  or  "  taste-goblets." 
They  are  imbedded  in  the  stratified  epithelium,  with  the  cells  of 
which  their  outer  layers  are  intimately  connected.  They  are  flask- 
shaped  bodies,  composed  of  a  concentric  series  of  flattened  cells 
pinched  together  at  the  base  and  at  the  free  surface,  where  they 
seem  to  inclose  a  kind  of  orifice.     Near  the  centre  the  flattened 


NERVES    OF    TASTE. 


549 


scales  are  replaced  by  short,  thread-like  elements,  so  that  the 
whole  remiuds  one  somewhat  of  the  construction  of  the  head  of 
an  artichoke. 

Fig.  212. 


Drawing  of  upper  surface  of  the  tongue,  showing  the  position  of  the 
circumvallate  papillae  (1).     (Sappey.) 


Nerves  can  be  seen  entering  these  bodies,  and  are  in  all  proba- 
bility directly  connected  with  the  modified  epithelial  cells  of  which 


650 


MANUAL   OP   PHYSIOLOGY. 


they  are  made  up.  The  relation  of  the  gl  esse -pharyngeal  nerves 
to  these  taste-buds  has  been  shown  by  the  fact  that  in  the  rabbit 
(in  which  animal  they  are  crowded  together  in  a  special  organ, 
so  as  to  be  easily  found)  they  degenerate,  and  in  a  few  months 
disappear,  after  one  of  these  nerves  has  been  cut. 

The  genuine  taste  sensations  are  very  few.     Much  of  what  we 
commonly  call  taste  depends  almost  exclusively  upon  the  smell 

Fig.  213. 


Section  through  depression  between  two  ciicuinvallate  papillse,  showing 
taste-buds.  (Cadiat.) — a,  fibrous  tissue  of  papilla;  d  and  c,  epithelial  cov- 
ering of  papilla ;  b,  taste-buds.  On  the  right,  a,  h  show  the  separate  cells 
of  a  taste-bud. 

of  the  substance,  and  we  habitually  confuse  the  impressions  de- 
rived from  these  two  senses.*  The  different  tastes  have  been 
divided  into  four,  viz.,  sweet,  sour,  bitter,  and  salt,  under  some 


*  Many  of  the  comestibles,  the  taste  of  which  we  most  prize,  have  really 
no  taste,  but  only  a  smell  which  we  habitually  confound  with  taste,  having 
mingled  the  experience  obtained  from  the  two  senses.  Thus,  if  the  draft 
of  air  be  carefully  excluded  from  the  nose,  wine,  onion,  etc.,  may  easily  be 
proved  to  have  no- taste.  Hence  the  familiar  rule  of  holding  the  nose 
adopted  in  taking  "  bad-tasting  "  medicine. 


SENSE   OF  SMELL.  551 

one  or  other  of  which  headings  all  our  tastes,  properly  so  called, 
would  naturally  fall.  Though  this  classification  has  no  just  claim 
to  being  a  chemical  one,  it  is  interesting  to  know  that  each  taste 
pretty  well  corresponds  to  a  distinct  group  of  substances  chemi- 
cally allied  one  to  the  other.  Thus,  acids  are  sour,  alkaloids 
are  bitter,  the  soluble  neutral  salts  of  the  alkalies  are  salt,  and 
poly-atomic  alcohols,  as  glycerin,  grape-sugar,  etc.,  are  commonly 
sweet. 

The  substances  most  probably  act  on  the  nerve  terminals  as 
chemical  stimuli,  because  they  must  be  in  solution  to  be  appre- 
ciated. If  solid  particles  be  placed  on  the  tongue  they  must  be 
dissolved  in  the  mouth  fluid  before  they  can  excite  the  taste 
organs. 

In  order  to  explain  the  appreciation  of  the  different  tastes  we 
may  imagine  that  there  are  different  kinds  of  terminals,  each  of 
which  is  or  is  not  influenced  by  various  substances  according  as 
they  possess  a  special  sweet,  sour,  bitter,  or  salt  energy.  From 
these  different  terminals  pass  fibres  bearing  impulses  to  certain 
central  cells,  each  of  which  is  capable  of  exciting  a  sweet,  sour, 
bitter,  or  salt  sensation,  as  the  case  may  be. 

Sense  of  Smell. 

The  numerous  delicate  nerves  which  pass  from  the  olfactory 
bulb  to  the  mucous  membrane  of  the  upper  and  part  of  the  middle 
meatus  of  the  nose  form  the  special  nerves  of  smell.  When  cer- 
tain subtle  particles  we  call  odors  come  in  contact  with  the  ter- 
minals of  these  nerves,  they  excite  impulses  which,  on  arriving  in 
the  special  centres  of  the  brain,  give  rise  to  the  impressions  of 
smell. 

Anatomically  the  relations  of  the  olfactory  region  are  well  de- 
fined. Its  mucous  membrane  is  not  covered  with  motile  cilia,  as 
is  that  of  the  rest  of  the  nasal  cavity,  and  it  is  less  vascular  and 
peculiarly  pigmented,  looking  yellow  to  the  naked  eye  when  com- 
pared with  the  neighboring  membrane.  The  epithelial  cells  are 
elongated  into  peculiar  cylinders,  between  which  lie  long  thin  rods, 
ending  on  the  surface  in  free  hair-like  processes.     The  deeper  ex- 


552 


MANUAL   OF   PHYSIOLOGY. 


tremities  of  these  rod-shaped  filaments  expand  to  surround  a  nu- 
cleus, and  are  then  continued  into  a  network  of  filaments,  into 
which  prolongations  of  the  epithelial  cells  also  seem  to  pass,  and 
in  which  the  delicate  fibrils  of  the  olfactory  nerve  can  be  traced. 
The  existence  of  direct  communication  between  the  nerves  and 
the  rod-shaped  filaments  and  the  epithelial  cells  is  satisfactorily 
established. 

The  odorous  particles  must  be  in  the  form  of  gases,  in  order  to 
be  carried  by  the  air  into  the  olfactory  region,  and  the  air  must 
be  kept  in  motion,  by  sniffing  it  in  and  out  of  the  nasal  cavity,  in 

Fig.  214. 


Section  ihiougli  llie  iiiucuiis  membrane  of  tlie  nasal  l'o>.sa  in  the  level  of 
the  olfactive  region. — o,  Epithelial  cells  and  bundles  of  nerves  ;  6,  Glands 
separated  from  each  other  by  bundles  of  nerves,  c.     (Cadiat.) 


order  to  excite  the  nerve  terminals,  which  are  not  influenced  by 
the  odors  of  air  absolutely  at  rest,  though  it  be  in  contact  with 
the  mucous  membrane  of  the  olfactory  tract. 

The  extreme  delicacy  of  appreciation  of  odors  by  the  olfactory 
nerve  terminals  is  very  remarkable.  Even  in  human  beings, 
whose  sense  of  smell  is  but  poorly  developed  when  compared  with 
that  of  animals,  an  amount  of  odorous  substance  can  be  perceived 


SENSE   OF   SMELL.  553 

which  the  finest  chemical  tests  fail  to  appreciate.  Thus  Valentin 
has  estimated  that  the  two-millionths  of  a  milligram  of  musk  is 
sufficient  to  excite  the  specific  energy  of  a  man's  olfactory  appa- 
ratus. 

No  satisfactory  classification  of  odors  has  been  made  out.  The 
common  division  into  agreeable  and  disagreeable  smells,  or  scents 
and  stinks,  is  dissimilar  in  different  individuals,  and  therefore 
cannot  have  a  physiological  basis. 

With  smell,  as  with  taste,  no  degree  of  intensity  of  stimulation 
can  be  said  to  produce  pain,  though  disgust,  nausea,  vomiting, 
and  many  other  psychical  and  nervous  operations  may  be  induced 
by  various  smells,  and  the  appetites  are  either  excited  or  annulled 
by  different  excitations  of  the  olfactory  nerves. 


CHAPTER  XXXII. 

VISION. 

Next  in  importauce  to  the  iutelligence  we  receive  from  the  skin 
is  that  which  is  conveyed  to  the  brain  from  the  outer  world  by 
the  second  pair  of  cranial,  or  the  optic  nerves. 

The  ending  of  the  optic  nerve  differs  from  any  of  those  we  met 
with  in  the  skin,  by  being  inclosed  in  a  very  specially  arranged 
organ — the  eyeball — an  apparatus  for  bending  the  rays  of  light, 
so  that  they  exactly  reach  the  delicate  sheet  of  complicated  nerve- 
ending  which  is  here  spread  out.  Nothing  but  the  blood  and 
other  tissues  of  the  eye  come  in  contact  with  the  endings  of  the 
optic  nerve,  which  is  thus  placed  out  of  the  way  of  ordinary  nerve 
stimulation.  Indeed,  we  shall  see  that  the  light,  of  which  the 
optic  nerves  convey  intelligence  to  the  brain,  is  not  properly  a 
nerve  stimulus,  being  merely  the  waving  of  an  imponderable  me- 
dium, the  existence  of  which  is  assumed.  Besides  the  special 
arrangements  in  the  eyeball  for  bringing  the  rays  of  light  to  bear 
on  the  nerve-endings,  there  must  here  be  some  extremely  delicate 
arrangement  by  which  the  ether-waves,  that  we  call  light,  can  be 
converted  into  a  nerve  stimulus,  or  in  some  way  made  to  affect 
the  nerve  terminals  in  the  retina. 

By  means  of  the  sense  of  sight  we  obtain  knowledge  of  objects 
at  a  distance  from  us,  because  all  these  objects  reflect  more  or  less 
light,  and  thus  make  different  impressions  upon  the  terminals  of 
the  optic  nerve,  which  form  the  outer  layer  of  the  retina. 

Light,  then,  is  the  adequate  stimulus  for  the  retinal  nerve- 
endings,  and  the  impulse  caused  by  light  is  the  only  impression 
the  optic  nerve  is  in  the  habit  of  carrying  to  our  sensoria,  where 
the  sensation  of  light  is  formed  and  distributed  among  the  cells 
of  the  brain  so  as  to  enable  us  to  come  to  visual  conclusions  and 
judgments.  As  already  mentioned,  no  matter  what  stimulus,  elec- 
tric, mechanical,  or  other,  be  applied  to  the  fibres  of  the  optic 


THE    TUNICS    OF    THE    EYEBALL.  5o5 

nerve,  the  sensation  produced  is  simply  light,  and  this  is  thought 
of  as  if  it  came  through  the  eye  from  the  outer  world. 
The  study  of  sight  may  then  be  divided  into  : 

1.  The  path  the  light  takes  on  its  way  through  the  eye  to  reach 
the  retina. 

2.  The  molecular  changes  in  the  retina  which  give  rise  to 
stimulation  of  the  optic  nerves. 

3.  The  sensations  arising  in  the  sensoriura  as  the  result  of  the 
molecular  changes  set  up  in  the  cerebral  nerve  cells  by  the  im- 
pulses from  the  optic  nerve. 

4.  The  visual  perceptions  and  judgments  which  our  conscious- 
ness is  capable  of  elaborating  from  the  visual  sensations. 

The  Tunics  of  the  Eyeball. 

The  organ  of  vision  of  vertebrate  animals  is  inclosed  in  a  firm 
case  of  fibrous  tissue  called  the  sclerotic  coat,  which  is  continuous 
with  the  sheath  of  the  optic  nerve,  and  is  seen  between  the  eyelids 
under  the  transparent  conjunctiva,  and  is  commonly  known  as 
the  white  of  the  eye.  It  gives  shape  and  protection  to  the  eye, 
and  though  translucent,  is  not  transparent.  In  front  a  round 
window-like  portion,  called  the  cornea,  forms  the  most  anterior 
segment  of  this  protecting  covering  of  the  eyeball.  The  cornea  is 
distinguished  from  the  sclerotic  not  only  by  its  glass-like  trans- 
parency, but  also  by  being  part  of  a  lesser  sphere  than  the  scle- 
rotic, and  thus  it  projects  a  little  more  than  the  rest  of  the  bulb. 

Closely  attached  to  the  inside  surface  of  the  sclerotic  is  a  soft, 
thin,  black  vascular  sheet  of  tissue  which  supplies  the  eyeball 
with  blood,  being  made  up  chiefly  of  bloodvessels  and  stellate, 
pigmented,  connective-tissue  cells.  Its  outer  layer  is  traversed 
by  arteries  and  veins  of  relatively  large  size,  and  its  inner  layer 
is  practically  composed  of  a  dense  network  of  close-meshed  capil- 
lary vessels.  As  the  cornea  is  approached,  the  choroid  is  pecu- 
liarly modified  and  thrown  into  folds,  called  ciliary  processes, 
fijrming  a  .series  of  vascular  projections,  which  radiate  from  the 
margin  of  the  cornea.  At  the  edge  of  the  cornea  the  choroid  is 
more  firmly  attached  to  the  sclerotic  by  a  circular  muscle  {the 


556 


MANUAL   OF   PHYSIOLOGY. 


ciliary  muacle),  and  also  by  bands  of  tissue  from  the  posterior  sur- 
face of  the  cornea  which  hold  it  in  position  ;  the  fibres  of  the  cil- 
iary muscle,  running  under  the  ciliary  processes,  radiate  from  the 
margin  of  the  cornea  towards  the  choroid,  to  which  they  are  at- 
tached.    In  a  modified  form,  known  as  the  iris,  this  vascular  and 

Fig.  215. 


Diagram  of  a  horizontal  section  through  tlie  human  eye. — 1.  Cornea;  2. 
Sclerotic ;  3.  Choroid  ;  4.  Ciliary  processes  ;  5.  Suspensory  ligament  of 
lens  ;  6.  So-called  posterior  chamber,  between  the  iris  and  the  lens :  7.  Iris  ; 
8.  Optic  nerve  ;  8'.  Entrance  of  central  artery  of  the  retina ;  8^^.  Central 
depression  of  retina  or  yellow  spot;  9.  Anterior  limit  of  the  retina;  10. 
Hyaloid  membrane ;  11.  Aqueous  chamber ;  12.  Crystalline  lens  ;  13.  Vit- 
reous humour;  14.  Circular  venous  sinus  which  lies  around  the  cornea; 
a — a,  antero-posterior,  and,  b — b,  transverse  axis  of  bulb. 


pigmented  coat  of  the  eye  leaves  the  sclerotic,  and  hangs  freely 
in  a  fluid  so  as  to  be  recognized  through  the  clear  cornea  as  a 
colored  circular  curtain,  attached  to  the  inside  of  the  periphery 
of  the  cornea,  and  having  a  central  deficiency,  which  looks  black, 


ANATOMY  OF   THE   EYE. 


557 


and  is  familiarly  known  as  the  pupil.  This  pupil  is  merely  an 
opening  in  the  iris,  which  allows  the  rays  of  light  to  pass  into  the 
interior  of  the  eyeball. 

Fig.  216. 


Pigmented  epithelium  lying 
next  to  the  choroid  coat. 

Rods  and  cones  with  their  ex- 
tremities imbedded  in  the 
epithelial  cells. 


External  nuclear  layer. 


External  granular  layer. 


Internal  nuclear  layer. 


:-fe: 


Internal  granular  layer. 


Layer  of  nerve  cells. 

Nerve-fibre  layer  in  which  the 
retinal  vessels  run  next  to 
the  vitreous  humor. 


Diagrammatic  section  of  retina,  showing  the  relation  of  the  different  layers 
in  the  posterior  part  of  the  fundus  (not  the  moMula  lutea).    (Schultze.) 


Besides  supplying  nutrition  to  the  non-vascular  central  parts 
of  the  eyeball,  the  choroid  is  useful  in  vision  by  preventing  the 
reflection  of  the  light  from  the  background  of  the  eye  in  such  a 


558  MANUAL   OF   PHYSIOLOGY. 

way  as  would  cause  irregularity  of  its  distribution,  and  thus  dazzle 
and  interfere  with  the  distinctness  of  the  image.  The  choroid 
also  is  elastic,  and  can  move  over  the  neighboring  sclerotic ;  it 
can  be  drawn  forwards  by  the  contraction  of  the  radiating  ciliary 
muscle,  which  acts  as  a  tensor  of  the  choroid  membrane. 

The  iris  has  a  special  power  of  motion,  by  means  of  which  the 
opening  in  it  can  be  made  smaller,  so  as  to  regulate  the  amount 
of  light  admitted  to  the  eye,  and  cut  off  more  or  less  of  the  rays 
which  would  pass  through  the  margin  of  the  dioptric  media. 
The  importance  of  this  will  be  better  understood  further  on. 

Within  the  choroid  coat,  and  in  immediate  contact  with  it,  is 
the  nervous  coat,  or  retina,  formed  by  the  expansion  of  the  optic 
nerve,  which  pierces  the  sclerotic  a  little  obliquely,  entering  it 
somewhat  to  the  nasal  side  of  the  axis  of  the  eye.  The  retina 
lines  all  the  back  part  of  the  eyeball,  and  stretching  forwards, 
becomes  fused  with  the  ciliary  processes,  where,  however,  the 
nervous  elements  of  the  coat  are  wanting.  The  fibrils  of  the 
optic  nerve  reach  the  inner  surface  of  the  coats  of  the  eye,  and 
lie  in  immediate  relation  to  the  transparent  medium,  which  occu- 
pies the  greater  part  of  the  bulb.  The  fibres  then  lie  internally 
to  their  terminals,  which  turn  outwards  and  are  set  against  the 
choroid  coat.  The  ultimate  nerve-endings  are  situated  in  pig- 
mented protoplasmic  cells,  which  form  the  outermost  layer  of 
the  retina. 

The  Dioptric  Media  of  the  Eyeball. 

The  transparent  substances  which  fill  the  eyeball  are,  together 
with  the  cornea,  commonly  called  the  dioptric  media.  The  aque- 
ous humo7'  lies  in  contact  with  the  posterior  surface  of  the  cornea, 
and  just  fills  the  prominence  which  is  formed  by  this  part  of  the 
eye.  It  is  in  this  fluid  that  the  movable  iris  is  stretched  and 
separates  the  aqueous  department  of  the  eye  into  an  anterior  and 
posterior  chamber.  The  vitreous  humor  occupies  much  the  larger 
share  of  the  eyeball.  It  lies  in  apposition  to  the  retina,  being 
separated  from  it  only  by  a  thin  transparent  structure,  called  the 
hyaloid  membrane,  which  incloses  the  clear  gelatinous  vitreous 
humor,  and  is  fused  with  the  ciliary  part  of  the  retina  and  oho- 


TRANSPARENT  MEDIA. 


559 


roid.  The  vitreous  humor  is  developed  from  the  young  connec- 
tive tissue  of  the  mesoblast,  and  we  find  in  the  adult  that  mucus 
is  the  most  striking  chemical  substance  in  its  texture,  though  the 


a 


Fig.  217. 


Diagram  of  lens  at  different  periods  of  life. — a,  At  birth  ;  b,  Adult ; 
c,  Old  age.     (Allen  Thomson.) 

form  elements  of  the  original  raucous  tissue  have  nearly  all  dis- 
appeared. 

The  most  important  of  the  dioptric  media  is  the  crystalline  lens. 
It  is  placed  between  the  aqueous  and  the  vitreous  humors,  just 


Fig.  218. 


Showing  early  stages  of  the  development  of  tlie  lens. — c.  Epithelial  tissue 
going  to  form  lens;  o,  Optic  cup;  a,  Epidermis.     (Cadiat.) 

behind  the  iris,  which  lies  in  contact  with  its  anterior  surface. 
It  is  like  a  strong  magnifying  glass,  biconvex  in  shape,  the  poste- 
rior surface  being  more  convex  than  the  anterior.  The  lens  is 
much  harder  than  the  vitreous  humor,  but  its  outer  layers  are 


560 


MANUAL   OF   PHYSIOLOGY. 


but  little  denser  than  a  stiff  jelly.  It  is  inclosed  by  a  firm  elastic 
capsule,  which  is  drawn  tightly  over  the  anterior  surface,  and 
influences  its  shape.  The  lens  is  held  in  its  position  by  a  thick- 
ened part  of  the  soft,  elastic  hyaloid  membrane,  called  the  «iw- 
pensory  ligament,  which  is  attached  to  the  anterior  surface  of  the 
capsule,  near  its  margin.     The  lens  and  its  capsule,  together  with 

Fig.  2iy. 


A  further  stage  of  the  development  of  the  lens.  (Cadiat.) — a,  Elongating 
epithelial  cells  forming  lens;  h,  Capsule  ;  c,  Cutaneous  tissue  becoming  con- 
junctiva; d,e,  Two  layers  of  optic  cup  forming  retina;  /,  Cell  of  mucous 
tissue  of  the  vitreous  humor ;  g,  Intercellular  substance ;  h,  Developing 
optic  nerve. 

the  vitreous  humor,  may  be  said  to  be  inclosed  in  the  hyaloid 
membrane,  which  is  thickened  and  fixed  to  the  capsule,  and  to 
the  ciliary  part  of  the  choroid.  Thus  any  tension  exercised  by 
the  suspensory  ligament  tends  to  tighten  the  anterior  part  of  the 
capsule,  and  flatten  the  anterior  surface  of  the  lens. 


STRUCTURE   OF   LENS. 


561 


The  shape  of  the  lens  varies  at  different  times  of  life,  being 
nearly  spherical  in  the  infant  and  tending  to  become  less  convex 


Fig.  220. 


Fragment  of  lens  tea.sed  out  to  show  the  separate  fibres.     (Cadiat.) 
a,  b,  and  c,  show  fibres  with  different-sized  nuclei. 


in  old  age  (v.  Fig.  217j.     The  lens  is  developed  from  the  outer 
layer  of  the  embryo  by  the  gradual  thickening  and  growing  in- 

47 


562  MANUAL   OF   PHYSIOLOGY. 

wards  of  the  epithelium,  which  meets  the  optic  cup,  and  after  a 
time  is  cut  off  from  the  parent  tissue.  The  stages  of  its  devel- 
opment may  be  followed  in  the  accompanying  woodcuts  (v.  Fig. 
218). 

The  lens  is  composed  of  a  number  of  peculiar  band-like  cells, 
derived  from  the  epithelium.  These  are  cemented  together  in 
parallel  rows,  eccentrically  arranged  in  layers.  These  bands  are 
hexagonal  in  transverse  section,  and  in  the  younger  periods  of 
life  may  be  seen  to  contain  nuclei. 

In  the  living  state  the  lens  is  perfectly  transparent,  but  after 
death  it  becomes  slightly  opaque.  The  nutriment  for  the  adult 
lens  is  derived  from  the  vessels  of  the  choroid,  which,  however, 
do  not  come  into  direct  communication  with  its  texture.  On 
this  account  the  nutrition  of  the  lens  is  not  so  perfect  as  that 
of  many  other  tissues,  and  it  is  but  imperfectly  repaired  after 
injury,  which  always  leaves  more  or  less  opacity.  Even  without 
injury,  opacity,  giving  rise  to  cataract,  sometimes  occurs  during 
life. 

Chemically  the  lens  is  made  up  of  globulin,  and  furnishes  a 
ready  source  for  obtaining  this  form  of  albumin  for  examination. 

The  Dioptrics  of  the  Eye. 

Light  travels  through  any  even  transparent  body,  such  as  the 
atmosphere,  in  a  straight  line.  But  when  it  meets  any  change  in 
density,  particularly  when  it  has  to  pass  obliquely  into  a  denser 
medium,  the  ray  is  bent  so  as  to  run  in  a  direction  more  perpen- 
dicular to  the  surface  of  the  denser  body.  The  degree  of  bending 
or  refraction  of  the  rays  depends  chiefly  on  the  difference  in  density 
of  the  two  media  and  the  angle  at  which  the  ray  strikes  the  sur- 
face of  the  more  dense. 

On  its  way  to  the  sensitive  retina,  the  light  has  to  pass  through 
the  various  transparent  media  just  named,  viz.,  the  cornea,  the 
aqueous  humor,  the  crystalline  lens,  and  the  vitreous  humor.  On 
entering  these  media,  which  have  different  densities,  the  rays  of 
light  reflected  from  any  luminous  body  become  bent  or  refracted, 
so  that  they  are  brought  to  a  focus  on  the  retina,  just  in  the  same 


REFRACTION.  563 

way  as  parallel  rays  of  light  from  the  sun  may  be  focussed  on  a 
near  object  by  means  of  an  ordinary  convex  lens. 

Only  so  much  light  reaches  the  fundus  of  the  eye  as  can  pass 
through  the  opening  in  the  iris,  so  that  a  comparatively  narrow 

Fig.  221. 


Diagram  showing  the  course  of  parallel  rays  of  liglit  from  A,  in  their  pas- 
sage through  a  biconvex  lens  l,  in  which  they  are  so  refracted  as  to  bend 
towards  and  come  to  a  focus  at  a  point  r. 

and  varying  beam  is  admitted  to  the  chamber  in  which  the  nerve- 
endings  are  spread  out  for  its  reception. 

If  we  hold  a  biconvex  lens  at  a  certain  distance  from  the  e3'e 
and  look  out  of  the  window  through  it,  we  see  an  inverted  image 
of  the  landscape.     If  we  place  a  piece  of  transparent  paper  behind 

Fig.  222. 


Diagram  showing  the  course  of  diverging  rays,  which  are  bent  to  a  point 
further  from  the  lens  than  the  parallel  rays  in  last  fig. 

the  lens,  we  can  throw  a  representation  of  the  picture  on  it,  which, 
however,  will  be  seen  to  be  inverted.  This  power  of  convex  lenses 
is  employed  in  the  instrument  used  for  taking  photographic  pict- 
ures, called  a  camera,  which  consists  of  a  box  or  chamber  into 
which  the  light  is  allowed  to  pass  through  a  convex  lens,  so  that 
an  inverted  image  of  the  objects  before  it  is  thrown  upon  a  screen 
of  ground  glass  within  the  box.     When  the  sensitive  plate  re- 


564  MANUAL   OF   PHYSIOLOGY. 

places  the  screeu,  the  light  comiog  through  the  lens  makes  the 
photographic  picture. 

Just  in  the  same  way  au  inverted  image  of  the  things  we  look 
at  is  thrown  on  the  retina  of  the  eye  by  the  refracting  media. 
This  may  be  seen  in  a  dark  room,  if  a  candle  be  placed  at  a  suitable 
distance  in  front  of  the  cornea  of  a  fresh  eye  taken  from  a  recently- 
killed  white  rabbit.  When  cleared  of  fat  and  other  opaque  tis- 
sues, the  sclerotic  is  transparent  enough  to  act  as  a  screen  upon 
which  the  inverted  candle  flame  can  be  recognized. 

Though  our  organ  of  vision  is  commonly  compared  to  a  camera 
obscura,  the  refractions  of  the  light  which  occur  in  it  are  far  more 
complex  than  those  taking  place  in  that  simple  instrument.  In 
the  latter  we  have  only  two  media — the  glass  lens  and  the  air ; 
in  the  eye,  on  the  other  hand,  we  have  several,  which  are  known 
to  have  a  distinct  refractive  influence  on  the  rays  which  pass 
through  the  pupil. 

Since  the  surfaces  of  the  cornea,  however,  are  practically  paral- 
lel, we  may  neglect  the  diflference  between  it  and  the  aqueous 
humor,  and  look  upon  the  two  as  one  medium,  having  in  front 
the  shape  of  the  anterior  surface  of  the  cornea,  and  behind,  the 
anterior  surface  of  the  lens,  so  as  to  form  a  concavo-convex  lens. 
We  thus  have  only  three  media  to  consider,  viz. :  (1)  the  aqueous 
humor  and  cornea  ;  (2)  the  lens  and  its  capsule ;  and  (3)  the 
vitreous  humor.  And  only  three  refracting  surfaces  need  be 
enumerated  viz. :  (1)  the  anterior  surface  of  the  cornea ;  (2)  the 
anterior  surface  of  the  lens ;  and  (3)  the  posterior  surface  of  the 
lens. 

These  refracting  surfaces  may  all  be  looked  upon  as  portions  of 
spheres  whose  centres  lie  in  the  same  right  line,  and  hence  may  be 
said  to  have  a  common  axis.  And  the  eye  may  be  regarded  as  an 
optic  system,  centred  around  an  axis  which  passes  through  the 
middle  point  of  the  cornea  in  front,  and  the  central  depression 
(fovea  centralis)  of  the  retina  behind.  This  is  commonly  spoken 
of  as  the  optic  axis  of  the  eye. 

The  rays  of  light  entering  the  eye  are  most  strongly  refracted 
at  the  surface  of  the  cornea,  because  they  have  to  pass  from  the 
rare  medium,  the  air,  to  the  denser  cornea  and  aqueous  humor. 


INVEESION   OF   THE   IMAGE.  566 

So  also  more  bending  of  the  rays  occurs  between  the  aqueous 
humor  and  the  anterior  surface  of  the  lens  than  between  the 
posterior  surface  of  the  lens  and  the  vitreous  humor. 

The  lens  is  not  of  the  same  density  throughout,  but  denser  in 
the  centre,  and  being  made  up  of  layers,  the  central  part  refracts 
more  than  the  outer  layers. 

The  manner  in  which  the  inversion  of  the  image  is  produced 
by  a  convex  lens  is  shown  in  the  accompanying  figure,  in  which 
the  lines  correspond  to  the  rays  passing  from  two  points  through 
the  lens.     If  the  arrow  a  a  be  taken  for  the  object,  from  either 

Fig.  223. 


Showing  tlie  course  of  the  rays  of  light  from  two  luminous  points  to  the 
retina.  The  rays  from  the  point  a,  on  passing  through  the  cornea,  lens, 
etc.,  are  collected  on  the  retina  at  b.  Those  from  a''  meet  at  6'',  and  thus 
the  lower  point  becomes  the  upper. 

extremity  of  it  rays  pass  through,  and  are  more  or  less  bent  by 
the  lens.  It  will  be  suflBcient  to  follow  the  course  of  three  rays 
from  the  head  of  the  arrow.  One  of  these  passes  through  the 
centre  of  the  lens,  and  leaves  it  in  the  same  direction  which  it 
entered,  because  the  two  surfaces  at  the  points  where  it  entered 
and  left  may  be  regarded  as  parallel,  and  so  cause  no  refraction. 
The  rays  which  do  not  pass  through  the  centre  are  bent  on  en- 
tering and  on  leaving  the  lens,  so  that  they  all  meet  at  the  same 
point  and  there  produce  an  image  of  the  head  of  the  arrow  at  b'. 
In  exactly  the  same  way  the  feather  end  of  the  arrow  is  produced 
at  b ;  the  position  of  the  image  of  the  object  is  thus  reversed  by 
the  light  rays  passing  through  the  lens. 


566  MANUAL   OP^   PHYSIOLOGY. 

lu  a  biconvex  lens,  with  the  two  surfaces  of  the  same  degree  of 
convexity,  the  central  point  through  which  the  rays  pass  without 
being  refracted  is  easily  made  out,  as  it  is  the  geometrical  centre 
of  the  lens.  This  central  point  is  spoken  of  as  the  optical  centre. 
With  systems  of  lenses  of  varying  convexity,  and  more  than  one 
in  number,  as  we  have  in  the  eye,  where  the  rays  of  light  are 
bent  at  different  surfaces,  it  is  much  more  difficult  to  determine 
the  optical  centre.  However,  by  means  of  the  measurements 
made  by  Listing,  two  points  close  together  are  known,  which  may 
be  said  to  correspond  practically  with  the  optical  centres  of  the 
eye ;  they  lie  in  the  lens,  between  its  centre  and  posterior  sur- 
face. The  path  of  the  various  rays  may  thus  be  exactly  made 
out.* 

The  rays  which  come  from  a  distant  luminous  point  and  fall 
upon  the  eye  are  refracted  by  the  cornea  and  aqueous  humor,  so 
as  to  be  made  convergent  on  their  way  to  the  lens  ;  they  are  then 
further  bent  at  the  surfaces  of  the  lens,  so  that  they  are  brought 
exactly  to  a  point  on  the  retina.  That  is  to  say,  for  distant  lumi- 
nous points,  the  retina  lies  exactly  in  the  plane  of  focus  of  the 
dioptric  media  of  the  normal  eye. 

This  convergence  of  the  rays  to  a  point  on  the  retina  is  the 
first  essential  in  order  to  be  able  to  see  clear  and  distinct  images ; 
for  if  the  rays  from  each  point  of  a  luminous  body  were  not  united 
on  the  retina  as  points,  the  effects  of  the  different  rays  from  the 
various  points  of  a  body  would  become  mixed,  and  there  would 
be  loss  of  definition  of  its  image. 

The  rays  from  any  bright  point  which  enter  the  eye  through 
the  pupil  may  be  imagined  to  form  a  luminous  cone,  the  point  of 
which  lies  at  the  retina,  and  its  base  at  the  pupil.  After  their 
union  at  the  point  of  the  cone  the  rays  would  diverge  again  if 
the  retina  were  not  there  to  receive  them. 

*.The  impossibility  of  making  clear  the  important  relationships,  nodal 
points,  and  other  constants  of  the  eye  in  a  short  text-book,  and  the  deter- 
rent effect  exerted  upon  the  mind  of  a  junior  student  by  brief  incompre- 
hensible statements,  has  induced  the  author  to  omit  this  part  of  the  subject, 
and  he  must  therefore  refer  those  who  are  anxious  to  learn  the  cardinal 
points  of  the  eye  to  the  more  advanced  text-books. 


SCHEINER'S    EXPERIMENT. 


567 


It  may  be  seen  from  the  foregoing  figure  that  if  the  retina, 
which  normally  would  lie  at  2,  were  placed  nearer  the  dioptric 
apparatus,  say  at  1,  or  further  from  it,  at  3,  it  would  not  meet  the 
exact  point  of  the  luminous  cone,  but  would  receive  the  rays  either 
before  they  came  to  a  point  or  after  they  had  diverged  from  it. 
Thus  indistinct  rings  of  light  would  be  seen  instead  of  one  lumi- 
nous point,  and  an  image  would  be  blurred  and  indefinite. 

From  this  it  follows  that  the  eye,  when  quite  passive,  can  only 
get  an  exact  image  of  bodies  which  are  placed  at  a  certain  dis- 
tance from  it,  just  as,  for  any  given  state  of  a  camera,  only  those 
bodies  in  one  plane  come  into  focus  and  give  a  clear  picture  on 


Fig.  224. 


To  illustrate  Scheiner's  experiment ;  for  explanation,  see  text. 


the  screen.  If  the  dioptric  apparatus  of  the  eye  were  rigid  and 
unalterable,  since  the  relation  of  the  retina  to  it  is  permanently 
the  same,  we  could  only  see  those  objects  clearly  which  are  at  a 
given  distance  from  the  eye.  We  know,  however,  that  we  get  a 
distinct  image  of  distant  as  well  as  of  near  objects,  and  we  can 
look  through  the  window  at  a  distant  tree,  or  we  can  adjust  our 
eyes  so  as  to  be  able  to  see  a  fly  walking  on  the  window  pane. 
However,  we  cannot  see  both  distinctly  at  the  same  moment. 
This  may  be  demonstrated  by  what  is  known  as  Scheiner's  ex- 
periment, which  is  carried  out  in  the  following  way  :  Two  pin- 
holes are  made  in  a  card  at  a  distance  from  each  other  not  wider 
than  the  diameter  of  the  pupil.  The  card  is  then  brought  close 
to  the  eye,  so  that  a  small  object — such  as  the  head  of  a  bright 
pin — can  be  seen  through  the  holes.     The   dioptric  media  being 


568  MANUAL   OF   PHYSIOLOGY. 

fixed,  moving  the  object  nearer  to  or  further  from  the  eye  would 
have  the  same  effect  as  changing  the  relation  of  the  retina  to  m  n 
or  7;  q  in  Fig.  224,  by  means  of  which  we  may  explain  the  follow- 
ing observations:  (1)  The  eye  being  fixed  upon  the  object  (of 
which  only  one  image  is  seen),  move  the  pin  rapidly  away ;  two 
objects  now  appear,  showing  that  the  rays  coming  through  the 
holes  have  met  before  they  reach  the  retina  as  at  jo  q.  (2)  Move 
the  pin  near  the  eye  ;  again  two  very  blurred  objects  are  seen,  for 
the  rays  have  not  met  when  they  strike  the  retina,  as  at  mn.  (3) 
Keeping  the  object  in  the  same  position,  alter  the  gaze,  as  if  to 
look  first  at  distant  and  then  at  near  objects ;  in  both  extremes 
two  images  are  seen.  (4)  When  the  object  is  in  exact  focus  as 
at  c,  the  closure  of  one  of  the  holes  does  not  affect  the  single 
image.  (5)  When  two  images  are  seen,  closing  the  right  hand 
hole  at  g  causes  the  right  or  left  image  to  disappear,  according  as 
the  focus  c  falls  short  of  m  n,  or  is  beyond  p  q,  the  retina.  (6) 
By  moving  the  pin's-head  nearer  the  eye  a  point  is  reached  at 
which  the  object  cannot  be  brought  to  a  focus  as  a  single  image. 
This  limit  of  near  accommodation  marks  the  near  point.  A  little 
attention  teaches  us  that  looking  at  the  near  object  requires  an 
effort  which  looking  at  the  distant  one  does  not ;  in  fact,  we  have 
to  do  something  to  see  things  near  us  distinctly.  This  act  is  the 
voluntary  adjustment  of  the  eye  which  we  call  its  accommoda- 
tion for  near  vision. 

Accommodation. 

The  difference  of  distance  for  which  we  can  adjust  our  eyes  is 
great,  so  that  our  range  of  distinct  visio7i  is  very  extensive.  As 
already  stated,  the  normal  eye  is  considered  to  be  constructed  so 
that  parallel  rays  of  light,  i.e.,  those  coming  from  practically  in- 
finite distance,  are  brought  to  a  focus  on  the  retina.  This  is  why 
we  see  the  stars — which  are  practically  infinitely  remote  from  us 
— as  mere  luminous  points.  It  is  therefore  impossible  to  fix  a 
"  far  limit "  to  our  power  of  distant  vision.  The  nearer  an  object 
is  brought  to  our  eyes,  however,  the  more  effort  is  required  to  see 
it  distinctly,  until  at  last  a  point  is  reached  where  we  cannot  get 
a  clear  outline,  no  matter  how  we  "  strain  our  eyes."     For  a  nor- 


MECHANISM   OF   ACCOMMODATION.  569 

mal  eye,  called  the  emmetropic  eye,  this  "  near  limit''  is  about  12 
cm.,  or  5  inches,  but  it  varies  in  different  individuals. 

For  objects  that  are  over  10  metres  distant  very  little  change 
in  the  eye  is  required  in  order  to  see  them  distinctly,  and  the 
nearer  the  object  approaches  the  more  frequently  the  adjustment 
of  the  eye  has  to  be  altered  in  order  to  see  it  clearly.  But  at 
every  part  of  the  range  of  distinct  vision  objects  at  different  dis- 
tances can  be  seen  without  moving  the  adjustment.  The  range 
of  this  power  is  measured  on  the  line  of  vision,  and  called  the 
focal  depth.  In  the  distance  we  can  take  in  a  greater  depth  of 
landscape,  and  this  without  effort  or  fatigue ;  but  when  looking 
at  near  objects  we  must  constantly  accommodate  our  eyes  afresh 
in  accordance  with  the  shallowness  of  our  focal  depth. 

The  raechanisrus  by  means  of  which  the  accommodation  of  the 
eye  is  accomplished,  differ  from  anything  that  can  be  applied  to 
an  artificial  optical  instrument,  and  are  much  more  perfect. 

The  changes  which  have  been  observed  to  take  place  are :  (1) 
The  iris  contracts  so  that  the  pupil  becomes  smaller;  (2)  the  cen- 
tre of  the  anterior  surface  of  the  crystalline  lens  moves  slightly 
forwards,  pushing  before  it  the  pupillary  margin  of  the  iris,  and 
becomes  more  convex  ;  (S)  the  posterior  surface  of  the  lens  also 
becomes  more  convex  but  without  changing  its  position. 

These  changes  can  be  seen  in  the  accompanying  diagram,  show- 
ing a  section  of  the  lens,  cornea,  and  ciliary  region  (Fig.  225), 
iu  the  left-hand  side  of  which  the  lens  is  drawn  in  the  position 
it  assumes  when  accommodated  for  near  objects.  These  move- 
ments can  be  seen  to  take  place  in  life  by  observing  the  changes 
in  relative  positions,  etc.,  of  the  reflections  of  a  candle  flame 
thrown  from  the  cornea  and  the  two  surfaces  of  the  lens.  On 
the  cornea  is  seen  a  bright  upright  flame  ;  next  comes  a  large 
diffused  reflection  from  the  anterior  surface  of  the  lens,  and  at  the 
other  side  of  this  a  small  inverted  image  of  the  flame  reflected 
from  the  posterior  surface  of  the  lens.  When  the  adjustment  is 
changed  by  looking  from  a  far  to  a  near  object,  the  image  on  the 
front  of  the  lens  becomes  smaller  and  moves  towards  the  centre 
of  the  pupil.  The  image  on  the  back  of  the  lens  also  becomes 
smallerbutdoes  not  change  its  position.  Theexactaraountof  raove- 

48 


570 


MANUAL   OF    PHYSIOLOGY. 


ment  has  been  accurately  measured  by  a  special  instrument  called 
an  ophthalmometer,  and  the  motions  can  be  made  more  obvious 
by  means  of  the  phakoscope,  in  which  a  dark  box  and  prisms  are 
placed  before  the  observed  eye,  and  each  image  is  made  double, 
so  that  the  change  in  position  of  its  two  parts  may  be  more  ob- 
vious than  a  mere  change  of  size. 

The  alteration  in  the  shape  of  the  lens  is  accomplished  by  the 
action  of  the  muscular  ring  already  named,  which  radiates  from 
the  edge  of  the  cornea  to  the  ciliary  region  of  the  choroid  coat, 

Fig.  225. 


Diagram  showing  the  changes  in  the  lens  during  accommodation.  The 
muscle  on  the  right  is  supposed  to  be  passive  as  in  looking  at  distant  objects, 
the  ligament  (l)  is  therefore  tight,  and  compresses  the  anterior  surface  of 
the  lens  (a)  so  as  to  flatten  it.  On  the  left  the  ciliary  muscle  (mj  is  con- 
tracting so  as  to  relax  the  ligament,  which  allows  the  lens  to  become  more 
convex.     This  contraction  occurs  when  looking  at  near  objects. 


where  it  is  attached.  The  junction  of  the  cornea  and  sclerotic 
being  its  fixed  point,  when  the  ciliary  muscle  contracts  it  draws 
the  choroid  coat  and  the  connections  of  the  suspensory  ligament 
of  the  lens  slightly  forwards.  Under  ordinary  circumstances,  the 
eye  being  at  rest,  the  suspensory  ligament  is  tense  and  exerts  a 
radial  traction  on  the  anterior  part  of  the  capsule  of  the  lens, 
and  thus  tends  to  stretch  it  flat ;  this  affects  the  shape  of  the  soft 
lens  and  reduces  its  convexity.  When  the  ciliary  muscle  shortens 
it  draws  forwards  the  attachment  of  the  suspensory  ligament, 
relaxes  it,  and  removes  the  tension  of  the  capsule,  so  that  the 
unconstrained  elastic  lens  bulges  into  its  natural  form.  The  pos- 
terior surface  cannot  extend  backwards,  because  there  it  is  in  con- 


DEFECTS   OF   ACCOMMODATION.  571 

tact  with  the  vitreous  humor,  which,  if,  anything,  is  held  more 
firmly  against  it  by  the  increased  tension  of  the  hyaloid  membrane 
during  the  contraction  of  the  ciliary  muscle. 

The  act  of  accommodation  is  a  voluntary  one,  the  nerve  bear- 
ing the  impulse  to  the  ciliary  and  iris  muscles,  coming  from  the 
third  nerve  by  the  ciliary  branches  of  the  lenticular  ganglion. 
The  local  application  of  the  alkaloid  of  the  belladonna  plant 
(atropin)  causes  paralysis  of  the  ciliary  muscle  and  wide  dilata- 
tion of  the  pupil  ;  and  the  alkaloid  of  the  calabar  bean  (physos- 
tigmatin)  produces  contraction  of  the  muscle  of  accommodation 
and  extreme  contraction  of  the  pupil. 

Defects  of  Acoommodatjon. 

Myopia. — It  has  been  said  that  the  "  near  limit "  of  distinct 
vision  differs  in  many  persons  from  the  twelve  centimetres  of  the 
normal  emmetropic  eye,  and  it  is  further  found  that  the  power  of 
accommodation  varies  very  much  in  different  individuals.  Thus 
in  "  short-sighted ''  people,  who  have  myopic  eyes,  i.e.,  in  which  dis- 
tant parallel  rays  fall  short  of  the  retina,  the  near  limit  may  only 
be  half  the  normal,  i.e.,  five  centimetres,  and  the  far  limit,  which 
is  normally  indefinite,  is  found  to  be  within  a  comparatively  short 
distance  of  the  eye.  They,  therefore,  cannot  see  distant  objects 
clearly,  since  the  rays  are  focussed  before  the  retina  is  reached, 
and  then  diverging,  cause  diffusion  circles  and  a  blurred  picture. 
The  work  of  their  accommodation  is  also  much  more  laborious, 
since  they  can  only  see  in  that  part  of  the  range  of  accommoda- 
tion where  the  adjustment  has  to  be  altered  for  slight  variations 
of  distance.  The  defect  can  be  made  much  less  distressing  by 
the  use  of  concave  glasses,  which  make  parallel  rays  strike  the 
cornea  as  divergent  ones,  and  thus  allow  them  to  be  focussed  on 
the  retina. 

Hypermetropia. — Another  abnormality  is  "  long  sight."  In  the 
hypermetropic  eye,  parallel  rays  of  light  are  brought  to  a  focus 
at  a  point  beyond  the  retina,  so  that  divergent  or  parallel  rays 
cause  diffusion  circles  and  a  blurred  image.  This  may  be  cor- 
rected by  means  of  convex  glasses  which  make  the  rays  conver- 


572  MANUAL    OF    PHYSIOLOGY. 

gent  before  they  strike  the  corneal  surface,  and  thus  enable  them 
to  be  sooner  brought  to  a  focus  by  the  dioptric  media  of  the  eye. 
Presbyopia  is  the  name  given  to  a  change  in  the  perfectness  of 
accommodation  frequently  accompanying  old  age.  The  lens 
probably  gets  less  elastic  and  the  ciliary  muscle  weaker,  so  that 
the  change  in  form  required  to  see  near  objects  is  more  difficult 
or  impossible  to  attain.  Biconvex  lenses  help  to  overcome  the 
difficulty. 

Fig.  226. 


Showing  the  course  of  the  rays  of  light  from  two  luminous  points  to  the 
retina.  The  rays  from  the  point  a,  on  passing  through  the  cornea,  lens, 
etc.,  are  collected  on  the  retina  at  b.  Those  from  a^  meet  at  6',  and  thus 
the  lower  point  becomes  the  upper. 

Defects  of  Dioptric  Apparatus. 

In  common  with  all  dioptrical  instruments  the  eye  has  certain 
optical  defects  which  tend  to  interfere  with  the  exact  definition 
of  the  image. 

Chromatic  aberration  is  due  to  the  breaking  up  of  white  light 
into  colored  rays  owing  to  the  different  colored  lights,  of  which 
ordinary  light  is  composed,  possessing  different  degrees  of  refran- 
gibility.  We  know  this  in  the  spectrum  and  in  the  colored  rings 
always  seen  in  the  marginal  part  of  a  biconvex  lens  made  of  one 
kind  of  glass,  which  acts  like  a  prism.  It  can  be  corrected  by 
making  lenses  of  two  kinds  of  glass,  one  of  which  counteracts 
the  dispersion  caused  by  the  other.  Optical  instruments  may 
thus  be  made  achromatic.  This  defect  is  minimized  by  the  iris, 
which  cuts  off  the  marginal  rays  in  which  it  is  most  apt  to  occur. 
Possibly  the  different  density  of  the  dioptric  media  may  have  a 


DEFECTS    OF    DIOPTRIC   MEDIA.  573 

correcting  effect  on  the  chromatisra  of  the  eye.  Further  correc- 
tion takes  place  in  the  nerve  centres  which  receive  the  sensation, 
for  just  as  we  mentally  reinvert  the  image,  we  fail  to  see  the 
color.  At  any  rate  the  chromatic  aberration  is  so  slight  that  it 
needs  certain  artifices  to  make  it  observable. 

Spherical  aberration  depends  upon  the  fact  that  luminous  rays, 
on  passing  through  a  convex  lens,  strike  the  various  parts  of  its 
surface  at  different  angles,  and  hence  are  differently  refracted. 
The  rays  striking  the  margin  of  the  lens  are  more  bent  than  those 
passing  through  the  centre,  and  hence  the  former  come  sooner  to 
a  focus.  Thus  a  luminous  point  gives  rise  to  a  diffused  figure, 
which  is  circular  in  perfectly  centred  dioptric  systems,  or  stellate 
in  our  eyes  where  the  centring  of  the  lenses  is  not  absolutely 
accurate.  Spherical  aberration  causes  us  no  inconvenience,  as 
the  iris  only  allows  the  central  rays  to  pass,  upon  which  it  can 
produce  no  noticeable  influence. 

Another  optical  defect  in  our  eyes  is  astigmatism,  depending 
upon  some  irregularity  of  the  curvature  of  the  cornea,  which  may 
be  bent  more  horizontally  than  vertically,  or  vice  versa.  In  either 
of  these  cases  the  light  in  the  vertical  and  horizontal  planes  will 
be  differently  refracted,  so  that  lines  drawn  in  the  two  directions 
will  require  different  adjustments  to  see  them  distinctly.  This 
may  be  at  once  recognized  if  we  gaze  with  one  eye  at  the  centre 
from  which  many  sharply-defined  lines  radiate,  when  only  certain 
ones  can  be  seen  distinctly,  unless  we  move  the  eye  or  change  its 
accommodation.  When  the  excessive  curvature  extends  evenly 
over  the  whole  diameter  of  the  cornea,  it  gives  rise  to  what  is 
called  regular  astigmatism,  and  when  the  unevenness  is  localized 
to  one  spot  of  the  corneal  surface  it  is  called  irregular  astigmatism. 

The  astigmatism  which  may  be  called  physiological  is  not  no- 
ticed by  the  individual,  but  pathological  astigmatism  often  occurs 
and  requires  cylindrical  glasses  to  correct  it. 

Entoptic  images  are  those  which  depend  on  the  presence  of  some 
opacity  or  difference  in  density  in  the  transparent  media  of  the 
eye  itself.  They  look  like  variously  shaped  specks  moving  over 
the  field  of  vision.  They  are  only  remarkable  when  we  look  at 
an  evenly  colored  object  or  through  a  pin-hole  in  a  black  card. 


574 


MANUAL   OF    PHYSIOLOGY. 


In  using  the  microscope   they  often  annoy  the  unpracticed  ob- 
server. 

The  Iris. 

It  has  already  been  mentioned  that  the  motions  of  the  iris  alter 
the  size  of  the  pupillary  opening  through  which  the  rays  of  light 
must  pass,  and  while  it  regulates  the  amount  of  light  admitted, 
it  always  cuts  off  a  large  amount  of  the  marginal  rays,  acting 
like  the  diaphragm  of  an  optical  instrument.     The  great  import- 

FiG.  227. 


Section  throngli  the  ciliary  region,  showing  the  rehition  of  the  iris  (/)  to 
the  choroid  and  the  ciliary  ninscle  (a),  which  arises  frona  the  margin  of  the 
cornea  at  (e),  and  passes  towards  the  choroid  to  tlie  right,  where  it  separates 
the  latter  from  the  sclerotic. 

ance  of  not  allowing  the  rays  which  would  traverse  the  margin 
of  the  lens  to  enter  the  eyeball  can  be  understood  after  what  has 
been  said  of  spherical  aberration.  But  the  iris  also  moves  so  as 
to  contract  the  pupil  when  the  eye  is  adjusted  for  near  vision, 
independently  of  the  intensity  of  the  light  by  which  the  object 
is  illuminated.  This  action  is  of  great  advantage  in  viewing 
near  objects,  because  the  more  convex  the  lens  becomes,  the  more 
injurious  are  the  marginal  rays.  If  the  iris  did  not  thus  contract 
in  near  vision,  the  nearer  we  brought  an  object  to  our  eye  the 
greater  would  be  the  tendency  to  indistinctness  caused  by  spher- 
ical aberration. 

The  iris  consists  of  a  framework  of  delicate  connective  tissue, 
like  that  of  the  choroid  coat,  containing  many  bloodvessels.     On 


MUSCLES   OF   THE   IRIS.  575 

its  posterior  surface  is  a  dense  layer  of  pigment  cells  called  the 
uvea,  which  gives  the  eye  its  color.  The  motions  of  contracting 
and  dilating  the  pupil  are  carried  out  by  smooth  muscle  fibres. 
The  act  of  contracting  the  pupil  is  performed  by  a  very  definite 
set  of  fibres  forming  the  sphincter  which  surrounds  the  margin 
of  the  pupil,  while  other  fibres  are  said  to  radiate  from  the  pupil 
to  the  attached  margin  of  the  iris.  The  sphincter  muscle  seems 
always  to  be  more  or  less  in  action,  because  if  it  be  paralyzed 
the  action  of  the  dilating  forces  becomes  obvious.  But  the  mus- 
cular character  of  the  dilator  has  been  doubted,  from  the  fact 
that  the  fibres  have  not  been  satisfactorily  demonstrated.  Cer- 
tainly the  sphincter  seems  to  be  the  stronger  of  the  two,  for  strong 
electric  stimulation  causes  contraction  of  the  pupil,  and  shortly 
after  death  the  pupils  dilate.  We  must  assume  that  the  power 
of  the  sphincter  dies  more  quickly  than  that  of  the  dilator,  or 
relaxes  because  it  has  lost  the  stimulus  reflected  from  the  fragile 
retina. 

The  nerves  supplying  the  dilator  muscle  seem  to  be  derived 
from  the  sympathetic,  for  when  the  sympathetic  in  the  neck  is 
cut,  the'  pupil  remains  permanently  contracted.  These  fibres  are 
supposed  to  take  origin  in  the  gray  matter  of  the  cervical  spinal 
cord.  The  sympathetic  also  supplies  the  muscles  in  the  walls  of 
the  vessels,  and  thus  controls  the  amount  of  blood  going  to  the 
iris.  Though  the  variation  in  blood  supply  may  cooperate  in 
causing  dilatation,  it  cannot  be  the  only  cause,  as  the  widening 
of  the  pupil  may  be  caused  in  a  bloodless  eye. 

The  nerve  mechanism  by  which  the  sphincter  muscle  is  made 
to  contract  is  quite  distinct,  and  more  definitely  understood.  Its 
contraction  is  a  reflex  act,  the  stimulus  of  which  starts  in  the 
retina  and  travels  along  the  optic  nerve  as  an  aflferent  channel  to 
the  corpora  quadrigeraina,  where  there  is  one  centre  governing 
the  contractions  of  both  irides.  The  efferent  impulses  are  sent 
along  the  third  nerve  to  the  lenticular  ganglion,  and  thence  by 
the  short  ciliary  nerves  to  the  eyeball. 

When  we  accommodate  for  near  objects  three  muscles  act  in 
unison,  so  we  say  their  movements  are  "  associated  "  with  one 
another.     The  voluntary  effort  that  causes  the  ciliary  muscle  to 


576  MANUAL   OF   PHYSIOLOGY. 

relax  the  suspensory  ligameut,  makes  the  sphincter  of  the  iris 
contract,  and  also  stimulates  the  internal  rectus  to  move  the  eye 
inwards.  The  voluntary  nerve  centre  must  be  in  intimate  rela- 
tion to  the  reflex  centre,  which  keeps  up  the  tonic  action  of  the 
sphincter  iridis. 

We  have  then  central  nerve  governors  for  the  ciliary  and  iris 
movements.  The  ciliary  muscle  and  sphincter  of  the  pupil  are 
together  stimulated  by  the  will,  and  the  sphincter  alone  is  excited 
by  means  of  a  centre  which  reflects  the  stimulus  arriving  from 
the  retina  by  the  optic  nerve  along  the  branches  of  the  third 
nerve.  The  dilator  of  the  pupil,  if  a  muscle,  is  also  kept  in  gentle 
tonic  action  by  the  impulses  sent  from  the  spinal  marrow,  via 
the  sympathetic,  with  the  vaso-motor  impulses ;  but  some  think 
that  the  blood  supply  and  tissue  elasticity  explain  the  dilatation. 

From  the  facts  (1)  that  reflex  contraction  of  the  pupil  may  be 
produced  by  stimulating  the  retina,  even  when  the  eye  is  cut  off 
from  the  brain  centres,  and  (2)  that  the  local  effect  of  atropia  in 
dilating,  and  calabar  bean  in  narrowing  the  pupil,  seem  in  a 
measure  independent  of  the  central  nerve  organs,  it  has  been 
concluded  that  there  must  be  some  local  nerve  mechanism  in  the 
eye  itself  capable  of  reflecting  nerve  stimuli  and  being  affected 
by  these  poisons. 

The  student  must  never  lose  sight  of  the  circumstances  under 
which  the  pupils  contract,  uamely  : 

1.  The  application  of  strong  light  even  to  one  retina,  causes 
reflex  stimulation  of  the  ciliary  nerves. 

2.  Stimulation  of  the  nasal  and  ophthalmic  branches  of  the 
fifth  afferent  nerve  excites  the  sphincter. 

3.  Contraction  of  the  pupil  is  "associated''  with  accommoda- 
tion for  near  objects. 

4.  Similar  "  associated  "  contraction,  accompanies  inward  move- 
ment of  the  eyeball. 

5.  During  sleep,  or  as  the  result  of  vaso-raotor  disturbances  in 
the  brain  (anaemia),  the  pupil  contracts. 

6.  Under  the  influence  of  physostigmatiu,  nicotin,  and  morphia. 

7.  From  any  stimulation  of  the  optic  or  third  nerves  or  the 
corpora  quadrigemina. 


THE    OPHTHALMOSCOPE. 


577 


The  circurastauces  iu  which  the  pupils  are  found  to  be  dilated 
are  equally  important  from  a  practical  point  of  view,  namely  : 

1.  In  the  dark  or  with  insensitive  retinte. 

2.  Irritation  of  the  cervical  sympathetic. 

3.  Under  the  influence  of  atropin,  daturin,  etc. 

4.  In  asphyxia  or  dyspnoea  from  venosity  of  the  blood. 

5.  Painful  sensations  from  the  skin,  etc. 

The  Ophthalmoscope. 

When  we  look  into  the  eye  the  pupil  appears  quite  black,  be- 
cause the  illumination  of  the  eye-chamber  is  not  sufficient  to  show 


Ophthalmoscopic  view  of  fundus  of  eye,  in  which  the  central  artery  {g 
and  c)  and  the  corresponding  veins  (A  and  d)  are  seen  coursing  through  the 
retina  from  the  optic  disk  {A). 

its  parts  from  the  outside,  when  the  light  is  strong.  In  the  same 
way  when  we  try  to  look  into  a  room  in  the  daytime  through  the 
window,  we  see  nothing  in  the  depth  of  the  room,  because  the  light 
outside  is  so  much  stronger  than  that  within  the  room.  If,  how- 
ever, we  look  in  at  night,  when  the  room  is  lighted  up  while  it  is 


578  MANUAL   OF   PHYSIOLOGY. 

dark  outside,  we  can  see  every  object  clearly.  So  if  we  illuminate 
the  inside  of  the  eye  by  any  means  we  shall  be  able  to  see  the  de- 
tails of  the  inside  of  the  eye-chamber. 

This  means  is  supplied  by  the  ophthalmoscope,  which  reflects  the 
light  from  a  lamp  into  the  chamber  of  the  eye  so  as  to  illuminate 
it  completely,  and  when  the  surroundings  are  not  too  bright,  the 
fundus  of  the  eye  can  be  clearly  seen  and  investigated.  A  lens 
is  usually  interposed  between  the  eyes  of  the  observer  and  the  ob- 
served, in  order  not  only  to  illuminate  but  also  to  magnify  the 
fundus  and  enable  the  observer  to  see  all  the  details  of  the  parts. 
With  this  instrument  a  round  whitish  part  is  seen  a  little  to  the 
nasal  side  of  the  axis  of  the  eye,  where  the  nerve  pierces  the  dark 
choroid  coat.  This  is  called  the  optic  disk.  The  fundus  now, 
when  lighted  up,  does  not  look  black,  but  is  of  a  lurid  red  color, 
owing  to  the  great  vascularity  of  the  choroid  coat.  Over  this  red 
field  are  seen  a  number  of  bloodvessels,  which  start  from  the  cen- 
tre of  the  optic  disk,  and  radiating  over  the  fundus  send  branches 
to  the  most  anterior  parts  that  can  be  seen.  These  are  the 
branches  of  the  vessel  which  runs  in  the  centre  of  the  nerve.  In 
the  very  axis  of  the  eye  a  peculiar  depression,  free  from  branches 
of  the  bloodvessels,  can  be  seen.  This  central  depression  (fovea 
centralis)  differs  a  little  in  color  from  the  neighboring  parts  during 
life,  and  turns  yellow  at  death,  and  hence  has  been  called  the 
"  yellow  spot."  The  retina  is  so  transparent  that  we  cannot  see 
it  with  the  ophthalmoscope,  but  the  radiating  vessels  (central  arte- 
ries and  veins  of  the  retina)  lie  in  it  and  belong  to  the  nervous 
structure  only. 

The  opthalmoscope  has  proved  of  inestimable  value  not  only 
to  the  ophthalmologist  but  also  to  the  physician,  as  a  means  of 
arriving  at  an  accurate  knowledge  of  disease.  Hence  it  has  be- 
come more  a  pathological  than  a  physiological  instrument. 

Light  Impressions. 

The  retina  is  the  part  of  the  eye  by  which  the  physical  motion 
— light  is  changed  into  the  physiological  phenomena  known  as 
nerve  impulse,  by  means  of  which  the  impression  of  light  is  excited 
in  the  brain.     In  reaching  the  retina  the  light  is  not  in  any  way 


STRUCTUEE    AND    FUNCTION   OF   THE    RETINA.  579 

altered  from  the  light  with  which  physicists  experiment,  but  at 
the  retina  this  physical  motion  is  stopped.  The  optic  nerves  no 
more  convey  the  light  waves  from  the  eye  to  the  brain  than  the 
tactile  nerves  carry  the  objects  that  stimulate  their  endings,  but 
only  send  a  nerve  impulse  which  the  retina,  on  its  exposure  to 
the  light,  excites  in  the  optic  nerve.  As  already  stated,  any  form 
of  stimulation  will  cause  the  same  kind  of  impulse  to  pass  to  the 
brain  and  there  set  up  the  same  sensation  of  light.  Thus  we  are 
told  by  persons  who  have  had  their  optic  nerves  cut  that  the  sec- 
tion was  accompanied  by  the  sensation  of  a  flash  of  light.  Any 
violent  injury  of  the  eyeball  causes  a  flash  of  light  to  be  experi- 
enced. This  fact  has  long  since  been  arrived  at  in  a  practical 
manner,  for  a  blow  implicating  the  eyeball  is  vulgarly  said  to 
"  make  one  see  stars."  Also,  without  violent  injury,  if  we  close 
the  eyes  and  turn  them  to  the  left  side  and  then  press  with  the 
point  of  a  pencil  on  the  right  side  of  the  eyeball  through  the  lid, 
we  have  a  sensation  of  a  point  or  ring  of  light  on  the  opposite  side 
of  the  eyeball.  Thus  we  say  that  the  specific  energy  of  the  optic 
nerves  excites  a  sensation  of  light,  and  the  adequate  stimulus  of 
the  organ  of  vision  is  light.  The  first  question  that  arises  is,  what 
part  of  the  retina  does  this  important  work  of  stimulating  the 
optic  nerve  when  light  impinges  on  its  terminals? 

The  Function  of  the  Ketina. 

The  retina  is  not  a  simple  sheet  of  nerve  fibrils  or  of  nerve  cells, 
but  a  most  complex  peripheral  apparatus,  which  to  histologists 
has  offered  an  endless  field  of  study.  This  complex  body  is  spread 
all  over  the  fundus  of  the  eye  except  at  the  optic  disk,  where  the 
nerve  pierces  all  the  coats  of  the  eye ;  here  there  is  nothing  else 
but  nerve  fibres  and  hence  no  retina  properly  so-called. 

The  structure  of  the  retina  varies  in  different  parts,  but  it  may 
be  said  to  be  composed  in  the  main  of  the  following  layers.  The 
exceptions  will  be  mentioned  afterwards,  viz. :  (compare  Fig.  216) 

Lying  next  to  the  hyaloid  membrane  is  the  layer  o{  nerve  fibres 
which  radiate  from  the  optic  disk  to  the  ora  serrata  near  the  cili- 
ary region.     The  fibres  spread  evenly  over  the  fundus  except  at 


580 


MANUAL   OF   PHYSIOLOGY. 


the  central  poiut  (fovea  centralis),  which  they  avoid  by  passing 
on  each  side  of  it. 

Next  to  the  fibres  comes  a  layer  of  nerve  cells,  these  seem  com- 
monly to  have  one  pole  connected  with  a  fibre  from  the  optic 


Fig.  229. 


Showing  the  course  of  the  fibres  of  the  optic  nerve  N,  as  they  pass  along 
the  inner  surface  of  retina  R,  to  meet  the  ganglion  cells,  whence  special 
communications  pass  outwards  to  the  layer  of  rods  and  cones  in  the  pigment 
layer  p,  next  the  choroid  c. 

nerve,  while  from  the  other  side  two  or  three  poles  send  processes 
into  the  other  layers  of  the  retina. 

Then  comes  an  indistinct  layer  made  up  of  granular  material 
and  two  layers  of  peculiar  nuclear  bodies,  with  a  layer  of  granular 
material  between  them. 

Outside  of  these,  and  separated  by  a  fine  limiting  membrane,  is 
the  most  important  layer  of  the  retina.  It  consists  of  a  layer  of 
rods  and  cones  which  are  connected  with  those  parts  of  the  retina 
already  named,  and  seems  to  project  into  the  protoplasm  of  pig- 


THE    BLIND   SPOT.  581 

mented  cells  of  epithelial  character,  which,  on  their  outer  face, 
show  a  striking  hexagonal  outline. 

A  nerve  fibril  then  may  be  said  to  have  the  following  course : 
entering  with  the  other  fibrils  at  the  porus  opticus,  it  reaches  the 
immediate  vicinity  of  the  hyaloid  membrane,  and  runs  a  certain 
distance  in  contact  with  that  membrane,  it  then  turns  outwards 
towards  the  choroid  and  enters  a  nerve  cell.  From  the  nerve 
cell  pass  on  a  couple  of  filaments  which  pierce  the  various  granu- 
lar and  nuclear  layers — where  they  probably  freely  inosculate 
with  the  fibrils  from  other  cells — and  finally  terminate  in  a  rod 
or  a  cone.  The  rods  and  cones  may  then  be  regarded  as  the  ulti- 
mate terminals  of  the  nerves,  and  they  lie  in  the  active  proto- 
plasm of  the  peculiar,  pigmented  epithelium  cells. 

It  is  this  outer  layer,  consisting  of  rods  and  cones  lodged  in 
epithelial  protoplasm,  which  is  the  really  effective  part  of  the 
retina.     Of  this  we  have  the  following  evidence: 

1.  The  fact  that  the  rods  and  cones  must  be  regarded  as  the 
real  anatomical  nerve  terminals  of  the  optic  nerve. 

2.  Where  the  optic  nerve  enters  the  eyeball  and  the  nerve  fibres 
are  fully  exposed  to  the  light,  there  are  no  rods  and  cones.  This 
part,  the  optic  disk,  cannot  appreciate  light,  and  hence  is  called 
the  "  blind  spot."  This  fact  shows  that  the  nerve  fibres  are  quite 
insensitive  to  light,  and  that  we  must  look  to  the  terminals  for  its 
appreciation.  The  existence  of  the  blind  spot  can  be  demonstrated 
as  follows.     Shut  the  left  eye,  and  hold  the  left  thumb,  at  ordinary 


reading  distance,  in  front  of  the  right  eye.  While  the  eye  is  fixed 
on  the  left  thumb  bring  the  right  thumb  within  about  four  inches 
of  it  and  move  it  slowly  an  inch  or  so  from  side  to  side.  A  little 
practice  soon  finds  a  place  when  the  right  thumb  uail  disappears. 
It  also  can  be  demonstrated  by  steadily  fixing  the  right  eye  on  the 


582  MANUAL   OF   PHYSIOLOGY. 

small  letter  "a"  and  moving  the  page  to  or  from  the  eye  very 
slowly;  a  distance  may  thus  be  reached  when  the  large  letter  "A" 
is  quite  lost.  On  approaching  the  page — the  eye  still  fixed  on 
"a" — "A"  reappears  from  the  inner  side;  on  withdrawing  the 
page  it  comes  into  view  from  the  outer  side.  This  blind  spot 
is  not  noticed  in  ordinary  vision  as  we  have  habitually  over- 
come the  deficiency  since  infancy  by  our  judgments  being  de- 
rived from  two  eyes.  By  rapid  movements  one  eye  hides  the 
deficiency,  as  seen  when  attempting  the  experiment  just  described. 

3.  The  fact  that  wlien  the  eye  is  illuminated  in  a  peculiar  way 
we  can  see  the  shadow  of  the  bloodvessels  which  lie  in  the  inner 
layers  thrown  upon  the  outer  layer  of  the  retina,  also  shows  the 
latter  to  be  the  sensitive  one.  This  phenomenon,  known  as  Pur- 
kinje's  figures,  can  be  demonstrated  as  follows.  Close  the  left 
eye  in  a  dark  room,  with  an  evenly  dull-colored  wall,  and  while 
you  stare  fixedly  at  the  wall  hold  a  candle  so  that  the  light  can 
reach  the  fundus  of  the  eye  from  the  side.  With  a  little  practice 
the  least  motion  of  the  candle  will  bring  out  an  arborescen*; 
figure  on  the  wall,  which  exactly  corresponds  to  the  retinal  ves- 
sels. It  may  also  be  seen  by  arranging  a  microscope  so  as  to  show 
a  bright  light — on  looking  into  the  instrument  and  either  moving 
it  or  the  head  slightly  but  constantly — the  shadows  of  the  retinal 
vessels  will  be  clearly  seen  as  though  they  were  under  the  instru- 
ment. 

4.  The  yellow  spot,  where  the  retina  is  chiefly  made  up  of  the 
cone  layer,  is  very  much  the  most  sensitive  part  of  the  retina, 
and,  nearer  the  ora  serrata,  where  the  rods  and  cones  are  but 
badly  developed,  sight  is  least  acute. 

As  in  the  perception  of  two  points  of  contact,  so  we  find  the 
retina  ceases  to  be  able  to  distinguish  the  difference  between  two 
luminous  points,  if  they  be  brought  to  a  focus  at  a  distance  of 
less  than  .002  mm.  from  one  another.  This  distance  nearly  corre- 
sponds to  the  diameter  of  the  cones,  and  it  is  supposed  that  the 
rays  from  two  luminous  points  must  come  upon  two  different  cones 
in  order  to  be  visible  as  two  distinct  objects.  The  cones  are,  how- 
ever, very  irregularly  distributed  over  the  retina,  being  packed 
closely  together  at  the  yellow  spot,  and  scattered  more  and  more 


CONDITIONS   AFFECTING   VISION. 


583 


widely  apart  as  one  passes  to  the  peripheral  parts  of  the  retina. 
It  is  only  at  the  yellow  spot  in  fact  that  the  cones,  which  here 
are  very  thin,  are  so  close  together  as  .002  mm.,  so  that  this  esti- 
mation of  the  size  of  visual  areas  only  could  hold  good  of  the 
yellow  spot,  and  towards  the  peripheral  parts  the  power  of  dis- 
crimination must  be  much  less  keen.  This  is  found  to  be  the  case, 
for  in  ordinary  vision  everything  seen  clearly  with  a  sharp  out- 
line must  be  brought  upon  the  yellow  spot.     This  is  spoken  of  as 


-  %/|fli^! 


Fig.  230. 


<5_ 


Siiiiiiil^ 


Section  of  the  retina  at  the  yellow  spot,  showing  the  great  number  of  cones 
(a)  at  this  point,  and  the  thinness  of  the  other  layer.     (Cadiat.) 

"direct  vision."  The  images  falling  on  the  other  parts  of  the 
retina  are  but  dim  and  indistinct  outlines,  and  these  are  spoken 
of  as  "indirect  vision." 

The  stimulus  need  only  be  applied  for  a  very  short  time  in  order 
to  cause  a  distinct  sensation,  for  we  can  distinctly  see  a  single  elec- 
tric spark ;  it  need  only  be  applied  to  an  extremely  small  part  of 
the  retina,  as  we  can,  by  direct  vision,  see  a  very  minute  speck  of 
light,  and  a  very  feeble  ray  suffices  to  stimulate  the  retina.  The 
amount  of  stimulation  produced  depends  upon  (1)  the  intensity  of 
the  light,  i.e.,  the  amount  of  light  received  in  a  given  area  ;  (2) 
the  duration  of  its  application  ;  and  (3)  the  extent  of  retina  to 
which   it  is  applied  ;  (4)  the  part  of  the  retina  stimulated  ;  (5) 


584  MANUAL   OF    PHYSIOLOGY. 

the  darker  the  background  the  weaker  the  illumination  we  can 
distinguish,  i.e.,  the  greater  the  stimulating  effect  of  a  weak  light ; 
(6)  by  fatigue  the  retina  loses  its  power  of  appreciating  light,  and 
more  stimulus  is  required  to  produce  a  given  effect.  On  waking, 
the  daylight  is  at  first  dazzling,  but  soon  the  retina  can  bear  the 
stimulus.  An  increase  of  intensity  of  light  does  not  cause  an 
exactly  proportional  increase  of  stimulation,  for  we  find  the  more 
the  light  is  intensified  the  less  we  are  able  to  notice  a  fresh  incre- 
ment of  light  until  a  degree  of  intensity  is  arrived  at,  when  no 
further  addition  can  be  detected,  and  the  light  becomes  blinding. 
The  less  the  absolute  intensity  of  two  lights  the  better  can  we  dis- 
tinguish any  difference  that  may  exist  between  them. 

The  effect  lasts  for  a  noticeable  time  after  the  stimulus  has  been 
removed,  particularly  if  the  light  be  very  intense.  This  can  well 
be  seen  when  a  brilliant  point  is  observed  in  rapid  motion  ;  instead 
of  a  point  a  streak  of  light  is  seen.  Thus  falling  stars  leave  a 
line  of  light  after  them  caused  by  the  persistence  of  the  stimulus, 
and  a  luminous  body  rapidly  rotated  gives  the  impression  of  a 
circle  of  fire  and  not  of  a  moving  point. 

When  the  stimulus  is  very  intense,  such  as  is  caused  by  an  elec- 
tric light  or  when  we  look  at  a  bright  object  like  the  globe  of  a 
lamp  steadily  for  some  time,  then  the  effect  persists  for  a  very  con- 
siderable time,  and  even  after  the  eyes  are  shut  we  see  a  distinct 
image  of  the  object.  This  is  called  the  positive  after  image.  If 
the  retina  be  exposed  to  a  bright  light  until  it  be  fatigued,  and 
then  suddenly  turning  we  gaze  at  a  white  wall,  the  bright  part  of 
the  positive  after  image  is  replaced  by  a  dark  figure  which  is 
termed,  the  negative  after  image. 

A  strong  stimulus  applied  to  the  retina  spreads  from  the  part 
upon  which  the  bright  image  falls  to  the  parts  in  its  immediate 
neighborhood,  so  that  the  bright  object  looks  larger.  This  phe- 
nomenon is  called  irradiation.  It  helps  to  explain  many  of  the 
peculiarities  of  vision. 

The  question  now  arises,  how  does  the  retina,  or  rather  its  layer 
of  rods  and  cones,  convert  light  into  a  nerve  stimulus?  It  would 
appear  quite  out  of  the  question  that  the  456  to  700  billions  of 
waves  of  light  per  second  could  mechanically  excite  the  nerve 


EPITHELIAL   CELLS    OF   THE    RETINA.  585 

terminals  as  the  waves  of  sound  excite  the  endings  of  the  auditory 
nerve.  But  we  know  that  light  has  a  very  distinct  action  on 
many  chemical  combinations,  such  as  reducing  salts  of  silver  and 
gold,  etc.  We,  therefore,  imagine  that  the  light  waves  set  up,  in 
the  outer  layer  of  the  retina,  certain  intermolecular  motions  or 
chemical  interchanges,  the  result  of  which  is  that  the  nerve  fibres 
are  stimulated  to  activity  and  transmit  an  impulse  to  the  brain. 
In  the  outer  layer  of  the  retina  the  light  may  be  said  to  produce 
a  change  in  the  retina  which  in  some  respects  may  be  compared 
to  that  which  occurs  on  the  sensitive  photographic  plate,  when  ex- 
posed, in  a  camera.  In  some  respects  only,  however,  because 
there  is  this  great  difference,  that,  while  the  chemical  change  on 
the  sensitive  plate  persists  so  as  to  give  rise  to  a  photograph,  in 
the  eye,  on  the  other  hand,  it  only  lasts  during  the  brief  moment 
during  which  we  can  recognize  the  positive  after  image.  The 
chemical  explosion  in  muscle  may  be  compared  to  the  explosion 
of  gunpowder,  in  giving  rise  to  force,  but  not  in  the  result  to  the 
materials.  For  in  muscle  as  the  chemical  change  which  causes 
the  contraction  is  rapidly  repaired,  so  in  the  retina  a  new  sensitive 
plate  is  at  once  produced  by  the  restoration  of  the  normal  con- 
dition of  the  molecules. 

The  idea  that  the  layer  of  rods  and  cones  undergoes  some  chem- 
ical change  on  exposure  to  light  which  suffices  to  induce  excita- 
tion of  the  optic  nerve,  has  received  great  support  from  the 
observation  that  a  color  of  a  red  or  purplish  hue  exists  in  the 
outer  part  of  the  rods  and  that  this  changes  when  exposed  to  the 
light.  But  this  co-called  visual  purple  cannot  have  a  very  in- 
separable connection  with  vision,  since  it  is  absent  where  the 
retina  is  most  sensitive,  i.e.,  the  fovea  centralis,  where  there  are 
no  rods,  and  further,  frogs  with  blanched  eyes  seem  to  see  quite 
well. 

The  peculiar  pigmented  epithelial  cells  of  the  retina  have  been 
observed  to  change  their  shape  slightly,  and  definitely  to  alter  the 
position  of  the  pigment  granules  they  contain  when  exposed  to 
light.  When  we  remember  how  sensitive  to  light  the  protoplasm 
of  many  unicellular  infusoria  is,  we  cannot  be  surprised  that  the 

49 


586 


MANUAL   OF   PHYSIOLOGY. 


protoplasm  of  the  retinal  epithelium  is  affected  by  it.  Moreover 
in  the  pigment  cells  of  the  frog's  skin  we  are  familiar  with  a 
change  in  shape  and  in  the  arrangement  of  their  pigment  granules 
in  response  to  different  light  stimuli.  We  know  that  in  the 
nervous  centres  nerve  impulses  are  commonly  originated  by  proto- 
plasm under  the  influences  of  slight  changes  in  temperature  or 
nutrition.  It  would  be  hardly  too  much  to  assume,  then,  that 
the  retinal  epithelium  has  some  important  share  in  the  trans- 
formation of  light  into  a  nerve  stimulus.  The  arguments  point- 
ing to  the  rods  and  cones  as  the  essential  part  of  the  retina  apply 
equally  well  to  the  pigmented  epithelium,  for  they  are  so  dove- 
tailed one  into  the  other  that  they  form  but  one  layer.     They  are 


Fig.  231. 


Epithelial  cells  of  the  retina.— a,  seen  from  the  outer  surface ;  J,  seen 
from  the  side  as  in  a  section  of  the  retina ;  c,  shows  some  rods  projecting 
into  the  pigmented  protoplasm. 

not  known  to  be  connected  with  the  nerve  fibres,  but  even  sup- 
posing they  be  not  in  any  way  connected  with  the  nerves  they 
might  still  be  influenced  by  the  light,  and  by  some  kind  of  motion 
communicate  the  effect  to  the  contiguous  sensitive  nerve  termi- 
nals, which  are  elaborately  adapted  to  appreciate  subtle  differen- 
tiations of  stimulus. 

Color  Perceptions. 

If  a  beam  of  pure  white  sunlight  be  allowed  to  pass  through 
an  angular  piece  of  glass  or  prism,  it  is  decomposed  into  a  num- 
ber of  colors,  which  may  be  seen  by  looking  through  the  prism, 
or  may  be  thrown  on  a  screen,  like  that  of  a  camera.  These 
colors,  which  look  like  a  thin  slice  of  a  rainbow,  are  together 


COLOR  PERCEPTIONS.  587 

called  the  spectrum.  The  white  solar  light  is  thus  shown  to  be  a 
compouud  of  rays  of  several  colors  which  possess  different  de- 
grees of  refrangibility,  and  hence  are  separated  on  their  way 
through  the  prism.  The  violet  rays  are  the  most  bent,  and  the 
red  the  least,  so  that  these  form  the  two  extremes  of  the  visible 
spectrum.  The  difference  of  color  depends  upon  the  different 
lengths  of  the  waves,  the  vibrations  of  violet  (667  billions  per 
sec.)  being  much  more  rapid  than  those  of  red  (456  billions  per 
sec).  Beyond  the  visible  spectrum  at  the  red  end  there  are  other 
rays  which,  though  they  look  black  to  the  eye,  are  capable  of 
transmitting  heat.  This  thermic  power  is  best  developed  in  these 
ultra-red  rays,  and  fades  gradually  towards  the  middle  of  the 
spectrum.  Outside  the  violet  are  ultra-violet  rays,  which,  though 
non-exciting  to  the  retina,  are  very  active  in  inducing  many 
chemical  changes.  Only  those  other  vibrations  which  have  a 
medium  length  can  stimulate  the  retina. 

If  two  different  colors  be  mixed  before  reaching  the  retina,  or 
be  applied  to  it  in  very  rapid  succession  one  after  the  other,  an 
impression  is  produced  which  differs  from  both  the  colors  when 
looked  at  separately;  thus  violet  and  red  give  the  impression  of 
purple,  a  color  not  in  the  spectrum.  If  all  the  colors  of  the  spec- 
trum in  the  same  proportion  and  with  the  same  brightness  fall 
upon  the  retina,  the  result  is  white  light.  This  we  know  from 
the  common  experience  of  ordinary  white  light,  which  is  really 
a  mixture  of  all  the  colors  of  the  spectrum,  and  we  can  see  it 
with  a  "  color  top"  painted  to  imitate  the  colors  of  the  spectrum. 
When  the  top  is  spinning  the  colors  meet  the  eye  in  such  rapid 
succession  that  the  stimulus  of  each  falls  on  the  retina  before  that 
of  the  others  has  faded  away,  and  thus  many  colors  are  practi- 
cally applied  to  the  retina  at  the  same  time,  and  the  top  looks 
nearly  white. 

It  has  been  found  that  certain  pairs  of  colors  taken  from  the 
spectrum  when  mixed  in  a  certain  proportion  produce  white. 
These  are  complementary  to  one  another.  The  complementary 
colors  are : 

Red  and  peacock-blue.  Yellow  and  indigo, 

Orange  and  deep-blue,  Greenish-yellow  and  violet. 


588 


MANUAL   OF   PHY8IOLOGY. 


If  colors  which  lie  nearer  to  each  other  in  the  spectrum  than 
these  complementary  colors  be  mixed,  the  result  is  some  color 
which  is  to  be  found  in  the  spectrum  between  the  two  mixed. 

The  perception  of  the  vast  variety  of  shades  of  color  that  we 
can  distinguish  can  only  be  explained  by  means  of  this  color- 
mixing.  We  assume  that  there  are  three  primary  colors  which 
overlap  one  another  in  the  spectrum  so  as  to  produce  all  the  va- 
rious tints.  These  are  red,  greeyi,  and  violet ;  the  arrangement  of 
which  may  be  thus  diagrammatically  explained  (Fig.  232). 

We  must  further  assume  that  there  are  in  the  retina  three 
special  sets  of  nerve  terminals,  each  of  which  can  only  be  stimu- 
lated by  red,  green,  or  violet  respectively,  and  the  innumerable 


Diagram  of  the  three  Primary  Sensations:  1  =  red  ;  2  =  green  ;  3  =  vi- 
olet.— The  letters  below  are  the  initials  of  the  colors  of  the  spectrum.  The 
height  of  the  shaded  part  gives  extent  to  which  the  several  primary  sensa- 
tions are  excited  by  different  kinds  of  light  in  the  spectrum. 

shades  of  color,  we  see,  depend  upon  mixtures  of  different  strengths 
of  these  primary  colors,  so  as  to  produce  different  degrees  of  stimu- 
lation of  each  set  of  nerve  terminals. 

The  view  that  such  special  nerve  apparatus  do  exist  for  red, 
green,  and  violet,  is  supported  by  the  fact  that  the  most  anterior 
or  marginal  part  of  the  retina  is  incapable  of  being  stimulated 
by  red  objects  which,  therefore,  look  black  when  only  seen  by 
this  part  of  the  retina.  This  inability  to  see  red  may  extend 
over  the  whole  retina,  as  is  found  in  some  persons  who  may  be 


MENTAL   OPERATIONS    IN    VISION.  589 

said  to  be  red-bliud.  Moreover,  if  we  investigate  our  negative 
after  images,  after  looking  for  a  long  time  at  a  red  object,  we 
find  them  to  be  greenish -blue;  that  is  to  say,  the  nervous  mech- 
anism for  receiving  red  impressions  is  fatigued,  and  consequently 
those  of  its  complementary  color  are  easily  stimulated. 

Mental  Operations  in  Vision. 

Our  visual  sensations  enable  us  to  perceive  the  existence,  the 
position,  and  the  form  of  the  various  objects  around  us.  For  the 
perfection  of  a  visual  perception  much  more  is  necessary  than 
the  mere  perfection  of  the  dioptric  media  of  the  eye,  and  of  the 
retinal  nerve-mechanisms.  Besides  the  changes  produced  in  the 
retina  by  the  light,  by  means  of  which  the  optic  nerve  is  stimu- 
lated, and  the  excitations  produced,  by  the  impulses  passing  along 
the  nerve,  in  the  nerve  cells  of  the  seeing  centre,  there  must  be 
further  a  psychical  action  in  other  cells  of  the  cortex  of  the  brain. 
This  psychical  action  of  the  brain  consists  of  a  series  of  conclu- 
sions drawn  from  the  experiences  gained  by  our  visual  and  other 
sensations. 

Our  ideas  of  external  objects  are  not  in  exact  accord  with  the 
image  produced  on  the  retina  and  transmitted  to  the  brain,  but 
are  the  result  of  a  kind  of  argument  carried  on  unconsciously  in 
our  minds.  Thus  when  no  light  reaches  the  retina,  we  say  (with- 
out what  we  call  thought)  that  it  is  dark ;  our  retina  being  un- 
stimulated, no  impulse  is  communicated,  and  the  sensation  of 
blackness  arises  in  our  sensorium.  When  luminous  rays  are  re- 
flected to  the  retina  from  various  objects  around  us,  the  physio- 
logical impulse  starts  from  the  eye,  but  in  the  brain,  by  uncon- 
scious psychical  activity,  it  is  referred  in  our  minds  to  the  objects 
around  us,  so  that  mentally  we  project  into  the  outer  world  what 
really  occurs  in  the  eye.  So  also,  from  habit,  we  reinvert  in  our 
minds  the  image  which  is  thrown  on  the  retina  by  the  lens,  upside- 
down,  and  so  unconscious  are  we  of  the  psychical  act  that  we 
find  it  hard  to  believe  that  our  eyes  really  see  everything  in- 
verted, and  our  minds  have  to  reinstate  them  in  the  upright 
position. 


590  MANUAL   OF   PHYSIOLOGY. 

One  of  the  most  iraportaut  means  employed  to  enable  us. to 
form  accurate  visual  perceptions  is  the  varied  motion  which  the 
eyeballs  are  capable  of  performing. 

Movements  op  the  Eyeballs. 

The  eyeballs  may  be  regarded  as  spherical  bodies,  lying  in  loosely 
fitted  sockets  of  connective  tissue  padded  with  fat,  in  which  they 
can  move  or  revolve  freely  in  all  directions,  in  a  limited  degree. 
The  muscles  which  act  directly  on  the  eyeball  are  six  in  number. 
Four  recti  muscles,  which  pass  from  the  back  of  the  orbit  and 
are  attached  to  the  eyeball,  one  at  each  side  and  one  above  and 
below,  not  far  from  the  cornea,  can  move  the  front  of  the  eye  to 
the  right  or  left,  and  up  or  down  respectively ;  and  two  oblique 
muscles  which  pass  nearly  horizontally  outwards,  and  a  little 
backwards,  and  are  attached  to  the  upper  and  under  surface  of 
the  eyeball  respectively.  These  muscles  can  slightly  rotate  the 
eye  on  its  antero-posterior  axis,  the  upper  one  drawing  the  upper 
part  of  the  eyeball  inwards,  and  the  lower  one,  as  its  antagonist, 
drawing  the  lower  part  inwards,  so  as  to  rotate  the  eyeball  in  an 
opposite  direction  on  the  same  axis. 

The  internal  and  external  recti  draw  the  centre  of  the  cornea 
towards  or  from  the  median  line  respectively,  directly  opposing 
one  another. 

On  account  of  the  direction  of  the  superior  and  inferior  recti 
being  different  from  that  of  the  axis  of  the  eyeball,  they  draw 
the  outer  edge  of  the  cornea,  not  its  centre,  up  and  down  respec- 
tively, and  at  the  same  time  tend  to  give  the  eyeball  a  slight  ro- 
tation in  the  same  direction  as  the  corresponding  oblique  mus- 
cles. The  tendency  to  rotation  is  counteracted  by  the  antago- 
nistic oblique  muscle  when  simple  elevation  or  depression  is  per- 
formed. 

Thus  pure  abduction  or  adduction  only  requires  the  unaided 
action  of  the  internal  or  external  recti,  while  direct  depression 
of  the  eye  requires  the  combined  action  of  the  inferior  rectus 
and  superior  oblique,  and  direct  elevation  requires  the  superior 
rectus  and  inferior  oblique  to  act  together.     The  various  oblique 


ACTION   OF   THE   ORBITAL   MUSCLES. 


591 


movements  are  accomplished  by  various  combined  coordinations 
of  movements  of  the  different  muscles. 

The  diagram  shows  the  directions  towards  which  the  different 
muscles  tend  to  draw  the  centre  of  the  cornea  from  the  straight 
position. 

From  this  it  is  obvious  that  the  commonest  movements  of  the 
eye  require  the  cooperation  of  different  muscles. 

Fig.  233. 


r.  iht 


r.ext. 


Diagram  of  the  direction  of  the  action  of  the  muscles  of  the  eyeball, 
which  is  shown  by  the  dark  lines.  The  axis  of  the  rotation  caused  by  the 
oblique  and  upper  and  lower  recti  are  shown  by  the  dotted  lines.  The 
inner  and  outer  recti  rotate  the  ball  on  its  vertical  axis,  which  is  cut  across. 
The  abbreviated  names  of  the  muscles  are  affixed  to  the  lines. 


In  the  ordinary  movements  of  the  two  eyes  more  than  this  is 
necessary.  The  two  eyes  must  move  in  the  same  direction  at  the 
same  time,  now  to  the  right,  now  to  the  left,  so  that  while  the 
external  rectus  moves  the  right  eye  to  the  right  side,  the  internal 
rectus  moves  the  other  eye  in  the  same  direction.  We  say  then 
that  the  coordination  of  the  movements  of  the  muscles  of  the 


592  MANUAL   OF   PHYSIOLOGY. 

eyeball  is  so  arranged,  that  the  contractions  of  the  external  and 
internal  recti  of  opposite  sides  always  occur  in  association,  and 
we  call  these  "  associated  movements."  This  association  of  move- 
ment has  been  acquired  by  the  habit  of  voluntarily  directing  the 
two  eyes  at  the  same  object,  and  has  gradually  become  involun- 
tary, for  few  persons  have  the  power  of  exerting  voluntary  con- 
trol over  the  muscles  of  one  or  other  eye  alone. 

.  Binocular  Vision. 

When  we  look  at  an  object  with  both  eyes  we  have  a  separate 
image  thrown  upon  each  retina,  and  therefore  two  sets  of  im- 
pulses are  sent  to  the  sensorium,  one  from  the  right  and  one  from 
the  left  eye.  Yet  we  are  only  conscious  of  the  occurrence  of  one 
stimulation.  The  reason  of  this  is,  that  experience  has  taught 
us  that  similar  images  thrown  upon  some  certain  parts  of  the  two 
retinae  correspond  to  the  same  object,  and  in  our  minds  we  fuse 
the  sensations  caused  by  the  two  images  so  that  they  produce  but 
one  idea. 

These  points  of  the  retina  which  are  thus  habitually  stimulated 
by  the  same  objects  are  called  "  corresponding  points." 

Besides  being  of  great  use  in  making  up  for  such  deficien- 
cies as  the  blind  spots  (which  are  not  corresponding  points) 
binocular  vision  is  useful  for  the  following  purposes : 

To  judge  of  distance.  When  using  one  eye  only,  some  knowl- 
edge of  distance  may  be  gathered  by  the  force  employed  to  ac- 
commodate, but  a  much  more  accurate  judgment  can  be  made 
when  both  eyes  are  used  and  the  muscular  sense  of  the  ocular 
muscles,  employed  in  converging  the  eyeballs  for  near  objects, 
can  be  used  as  evidence  of  their  distance. 

In  judging  of  size,  in  the  same  way,  with  one  eye,  we  can  only 
have  an  idea  of  the  apparent  size  of  an  object,  which  will  vary 
with  its  distance.  With  a  knowledge  of  the  apparent  size  and 
of  the  distance  such  as  is  gained  by  binocular  vision,  we  can 
come  to  a  fairly  accurate  conclusion  as  to  the  size  of  an  object. 

To  judge  of  the  relative  distances  of  objects  so  as  to  see  depth 
in  the  picture  before  our  eyes,  binocular  vision  is  necessary.     If 


BINOCULAE   VISION.  593 

one  eye  alone  is  used  we  see  a  flat  picture  without  having  an 
accurate  idea  of  the  relative  distances  of  the  different  things  we 
see.  With  each  eye,  however,  we  get  a  slightly  different  view  of 
each  object,  and  thus  we  are  helped  to  conclude  as  to  their  exact 
distances  and  shape,  so  as  to  be  able  to  arrive  at  fairly  correct 
judgments  as  to  their  exact  form,  etc. 


60 


CHAPTER  XXXIII. 

HEARING. 

Just  as  impulses  travelling  along  the  optic  nerves  can  only 
give  rise,  in  the  sensorium,  to  impressions  of  light,  so  impulses 
communicated  to  the  portio  mollis  of  the  seventh  pair  of  cranial 
nerves  can  only  excite  impressions  of  sound,  and  any  stimulation 
of  that  nerve  gives  rise  to  sound  sensations. 

The  peripheral  end  of  the  special  nerve  of  hearing  is  distrib- 
uted to  an  organ  of  very  peculiar  construction  situated  in  the 
internal  ear,  which,'  from  its  complexity,  has  been  called  the 
labyrinth.  The  nerve-ending  is  spread  out  between  layers  of  fluid, 
so  that  it  must  be  aflfected  by  very  gentle  forms  of  stimulation  ; 
and,  when  we  know  its  delicacy,  we  can  hardly  be  surprised  that 
even  sound  vibrations  suflBce  to  stimulate  this  terminal  to  trans- 
mit a  nerve  impulse  to  the  brain.  But  the  organs  of  hearing  of 
mammalia  and  man  are  so  deeply  placed  in  the  petrous  part  of 
the  temporal  bone  that  a  special  mechanism  has  to  be  adopted  to 
convey  the  sound  with  sufficient  intensity  from  the  air  to  the  fine 
nerve-terminals.  These  beautiful  contrivances  make  up  a  com- 
plex piece  of  anatomy  which  will  be  briefly  referred  to  presently. 

Sound. 

Before  attempting  to  describe  the  complex  mechanisms  by 
means  of  which  the  sound  is  conveyed  to  the  nerve-endings,  some 
notion  must  be  formed  of  what  sound  is  from  a  merely  physical 
standpoint.  Without  the  sense  of  hearing  one  cannot  form  any 
idea  of  sound,  and  here  the  knowledge  of  sound  ends  with  many 
people,  since  they  only  think  of  it  as  something  they  can  hear. 
A  physicist,  however,  regards  sound  in  a  very  different  way.  He 
knows  that  it  is  caused  by  a  kind  of  motion  known  as  the  vibra- 
tions of  elastic  bodies,  such  as  a  tense  string,  a  metal  rod,  or  an 


SOUND.  595 

elastic  membrane.  These  vibrations,  being  communicated  to  the 
air,  are  conveyed  by  it  to  our  nerve-endings,  where  they  set  up  a 
nerve-impulse.  The  impulse  is  transmitted  along  the  nerve  to 
the  brain,  and  there  gives  rise  to  the  sensation  with  which  we  are 
familiar  as  sound. 

The  vibrations  of  the  air  are  wave-like  movements  depending 
upon  a  series  of  changes  of  density  in  the  gases,  the  particles  of 
which  move  towards  or  from  one  another,  and  transmit  the  mo- 
tion to  their  neighbors,  so  as  to  propagate  the  sound  wave.  To 
demonstrate  these  vibrations  a  special  apparatus  must  be  used. 

When  a  tuning  fork  is  struck  it  is  thrown  into  vibration,  and 
a  sound  is  given  forth.  But  the  vibrations  are  often  so  rapid  and 
so  small  that  the  motion  of  the  tuning  fork  cannot  be  appreciated 
by  the  eye.  But  if  a  fine  point  be  attached  to  one  prong  of  the 
tuning  fork — or,  indeed,  any  elastic  body,  such  as  a  bar  of  metal — 
and  this  point  be  brought  into  contact  with  a  moving  smoked  sur- 
face, such  as  has  been  already  described  for  similar  records,  a  little 
wavy  line  is  drawn,  showing  that  the  vibrating  fork  moves  up  and 
down  at  an  even  and  regular  rate.  Each  up  and  down  stroke 
indicates  a  vibration.  The  length  of  the  wave,  as  drawn  on  the 
evenly-moving  surface  of  the  recorder,  shows  the  amount  of  time 
occupied  by  each  vibration.  This  is  always  found  to  be  the  same, 
for  a  tuning  fork  of  a  given  pitch,  and  thus  the  recording  fork  is 
in  constant  use  by  the  physiologist  as  an  exact  measure  of  small 
intervals  of  time.  The  pitch  of  the  note,  then,  depends  upon  the 
rate  or  period  of  vibration,  a  note  or  tone  of  a  certain  pitch  being 
simply  a  sound  caused  by  so  many  vibrations  per  second.  The 
quicker  the  vibration  the  higher  the  note,  and  the  slower  the 
deeper,  until,  at  the  rate  of  about  thirty  per  second,  no  sound  is 
any  longer  audible.  Whether  a  note  be  produced  by  a  metal 
fork,  a  tense  string,  or  any  other  vibrating  body,  if  the  number 
of  vibrations  per  second  be  the  same,  the  note  must  have  the  same 
pitch. 

The  elevation  of  each  vibration  as  seen  in  the  tracing  made  by 
a  recording  fork  is  different  at  different  times.  When  the  fork 
is  first  .struck  the  waves  are  high  and  well  marked,  and  the  ex- 
cursions of  the  recording  prong  can  be  seen  to  become  less  and 


596  MANUAL    OF    PIIY8IOLOGY. 

less  extensive  as  the  fork  gradually  ceases  to  vibrate,  and  th€ 
sound  becomes  faint ;  or,  in  other  words,  as  the  sound  produced 
becomes  less  loud,  the  vibrations  are  smaller,  and  the  amount  of 
excursion  made  by  the  vibrating  body  is  commonly  spoken  of  as 
the  amplitude  of  the  vibration,  and  upon  it  alone  depends  the 
loudness  of  the  sound.  Thus  the  pitch  of  a  tone  bears  no  relation 
to  the  amplitude  of  the  waves  of  the  vibration,  but  depends  upon 
their  rate ;  while  its  loudness  is  quite  independent  of  the  period 
occupied  by  the  vibrations,  but  is  in  proportion  to  the  extent  or 
amplitude  of  the  waves. 

So  far  only  tones  or  musical  notes  have  been  mentioned.  They 
are  produced  by  vibrations  occurring  at  perfectly  regular  periods. 
The  simpler  and  more  regular  the  vibrations,  the  purer  the  tone. 
But  the  great  majority  of  the  sounds  we  are  accustomed  to  hear 
are  not  pure  tones,  but  are  the  result  of  an  association  of  vibra- 
tions bearing  more  or  less  relation  one  to  the  other.  When  the 
variety  of  vibrations  is  very  great,  their  intervals  irregular  and 
out  of  proportion,  they  give  rise  to  a  discordant  sound  devoid  of 
musical  tone,  which  is  commonly  called  a  noise.  But  so  long  as 
such  commensurability  exists  in  the  rate  of  the  vibrations  as  to 
produce  a  sound  not  disagreeable  to  the  sense  of  hearing,  it  may 
be  called  a  note. 

By  the  use  of  a  series  of  different  resonators,  each  of  which  is 
capable  of  magnifying  a  certain  tone,  it  can  be  shown  that  the 
clearest  and  purest  notes  of  our  musical  instruments  are  far  from 
being  simple  tones,  but  are  really  compounds  of  one  prominent 
note  or  fundamental  tone,  modified  by  the  addition  of  numerous 
over-tones  or  harmonics.  If  one  blows  forcibly  across  an  orifice 
leading  to  a  space  in  which  a  small  amount  of  air  is  confined,  such 
as  the  barrel  of  a  key  or  the  mouth  of  a  short-necked  flask  or 
bottle,  either  a  clear  shrill  or  dull  booming  sound  is  heard,  which 
varies  in  pitch  according  to  the  proportions  of  the  air-containing 
cavity.  This  dull  note  is  a  simple  tone.  It  is  devoid  of  charac- 
ter, and  in  this  respect  differs  greatly  from  the  notes  produced  by 
a  musical  instrument.  The  notes  of  every  instrument  have  certain 
characters  or  qualities  which  enable  even  the  most  unpracticed 
ear  to  distinguish  them  from  one  another. 


QUALITIES   OF   SOUND.  597 

This  peculiar  quality,  which  is  independent  of  the  pitch  (i.e., 
rate  of  vibration)  or  the  intensity  {i.e.,  amplitude  of  wave),  is 
called  the  color  or  timbre  of  the  note.  It  depends  on  the  number, 
the  variety,  and  the  relative  intensity  of  the  over-tones  or  har- 
monics, which  accompany  the  notes.  So  that  really  the  timbre 
or  quality  of  a  note,  and  therefore  the  special  characters  of  the 
different  musical  instruments,  is  produced  by  their  impurity,  or 
the  complexity  of  the  over-tones  which  aid  in  producing  them. 

All  elastic  bodies  can  vibrate,  and  therefore  are  more  or  less 
capable  of  conducting  sounds.  Sound  vibrations  can  be  trans- 
mitted from  one  body  to  another  placed  in  contact  with  it.  From 
a  hard  material  the  waves  are  readily  communicated  to  the  air, 
and  this  is  the  ordinary  medium  by  means  of  which  sound  is  trans- 
mitted and  finally  arrives  at  our  organs  of  hearing.  The  old  ex- 
periment of  placing  a  small  bell  under  the  glass  of  an  air-pump 
and  making  the  tongue  strike  it  after  the  air  has  been  removed, 
shows  that  the  medium  of  the  air  is  essential  for  the  transmission 
of  the  sound  vibrations. 

The  transmission  of  waves  of  sound  from  the  air  to  more  dense 
materials,  such  as  those  which  surround  our  auditory  nerve-ter- 
minals, takes  place  with  much  greater  difiiculty  than  that  from 
a  solid  to  the  air,  and  we  find  a  variety  of  contrivances  by  which 
the  gentle  waves,  arriving  at  the  ear  by  the  air,  are  collected  and 
intensified  on  their  way  to  the  labyrinth. 

But  the  medium  of  the  air  is  not  necessary  in  order  that  sound 
may  reach  the  internal  ear.  Nor  is  the  route  through  the  outer 
canal,  and  the  drum  and  its  membrane,  the  only  one  by  which 
the  vibrations  can  arrive  at  the  cochlea.  The  solid  bone  which 
surrounds  the  labyrinth  is  in  direct  communication  with  all  the 
bones  of  the  head,  and  the  sound  can  travel  along  these  bones 
and  reach  the  nerve-endings.  This  can  easily  be  proved  by  placing 
the  handle  of  a  vibrating  tuning  fork  against  the  forehead,  or 
better  still,  against  the  incisor  teeth.  The  sound,  although  pre- 
viously hardly  audible,  at  once  becomes  quite  distinct,  or  even 
appears  loud. 

This  direct  conduction  through  the  bones  of  the  head  is,  under 
normal  conditions,  of  little  use  to  man  ;  but  attempts  have  been 


6i)8  MANUAL    OF    PHYSIOLOGY. 

made,  in  cases  where  the  ordinary  auditory  passages  were  rendered 
inefficient  by  disease,  to  gather  the  vibrations  on  a  vibrating  plate 
or  tympanum,  and  apply  this  to  the  teeth.  This  direct  conduction 
of  sound  is,  however,  very  valuable  in  determining  the  seat  of  the 
disease  in  cases  of  deafness.  So  long  as  a  clear  sensation  of  sound 
reaches  the  brain  through  the  bones  of  the  head,  one  knows  that 
the  important  nerve-endings  and  their  central  connections  are 
unimpaired,  and  can  then  conclude  that  the  disease  lies  in  the 
mechanical  conducting  parts  of  tlie  hearing  organ. 

In  fishes,  where  the  labyrinth  is  the  only  part  of  the  auditory 
apparatus  which  exists,  it  is  imbedded  in  the  cranium,  and  the 
sound  waves  arrive  through  the  medium  of  water,  and  are  directly 
conveyed  to  the  nerve-endings  by  the  bones  of  the  head.  An  air- 
containing  tympanum  would  be  a  great  impediment  to  the  hearing 
of  these  animals. 

Conduction  of  Sound-vibrations  through  the  Outer  Ear. 

The  parts  of  the  ear  through  which  the  sound  commonly  passes 
before  it  reaches  the  nerve,  are  naturally  separated  into  three, 
viz.,  (1)  the  external  ear  and  the  auditory  canal ;  (2)  the  middle 
ear,  tympanum  or  drum,  which  is  shut  off  from  the  latter  by  the 
tympanic  membrane ;  and  (3)  the  fluid  of  the  labyrinth. 

In  man,  the  ear  muscles  are  so  poorly  developed  that  he  can 
hardly  move  the  external  ear  or  pinna  perceptibly,  and  the  part 
commonly  called  the  ear  is  of  little  use.  We  know  this,  because 
the  outer  ear  may  be  quite  removed  without  materially  affecting 
the  power  of  hearing.  Birds  hear  well  without  any  outer  ear,  and 
the  sound  reflected  from  the  pinna  may  be  excluded  by  placing  a 
little  tube  in  the  auditory  canal  without  reducing  the  intensity  of 
the  sound.  But  the  movable  ears  of  many  animals  are,  no  doubt, 
useful  in  helping  them  to  ascertain  the  direction  from  which  a 
sound  arrives,  by  catching  more  of  the  vibrations  coming  to  their 
orifice.  That  the  external  ear  may  be  of  some  use,  even  to  man, 
one  is  led  to  believe,  by  the  natural  readiness  with  which  a  person 
with  dull  hearing  supplements  it  by  means  of  his  hand.  But  in 
this  act  the  ear  is  commonly  pushed  away  from  the  head  to  an 


MEMBRANA    TYMPANI.  599 

angle  of  about  forty-five  degrees,  and  thus  its  projection  is  con- 
siderably increased. 

The  auditory  canal  is  a  crooked  and  irregular  passage,  getting 
rather  wider  as  it  approaches  the  tympanic  cavity.  It  is  usually 
the  seat  of  some  short,  stifi"  hairs,  which  help  to  prevent  the  en- 
trance of  foreign  matters.  It  is  supplied  with  a  peculiar  modifi- 
cation of  sweat  glands,  which  secrete  a  waxy  material  that  helps 
to  keep  the  walls  of  the  canal  and  the  outside  of  the  membrane 
moist  and  soft.  Upon  the  more  ordinary  sound-vibrations,  how- 
ever, the  auditory  canal  has  little  or  no  effect.  The  elastic  column 
of  air  in  any  circumscribed  space  resounds  more  readily  to  some 
one  certain  tone,  which  varies  according  to  the  capacity  of  the 
space ;  thus  are  formed  resonators  of  different  pitch.  Just  as  dif- 
ferent tubes  or  key  barrels  have  different  notes  when  blown  into, 
so  the  auditory  canal  has  a  note  of  its  own,  and  if  the  canal  be 
short,  the  note  is  one  of  a  very  high  pitch.  When  a  tone  of  the 
same  pitch  as  that  to  which  the  canal  is  tuned  strikes  the  ear,  it 
is  unpleasantly  magnified,  and  it  is  said  that  such  sounds  are  those 
which  we  commonly  call  shrill  and  disagreeable. 

The  end  of  the  auditory  canal  is  closed  by  the  membrana  tym- 
pani,  which  slopes  obliquely  from  above  downwards  and  inwards, 
by  which  means  its  size  is  greater  than  if  it  were  directly  across 
the  canal.  This  membrane  is  not  flat,  but  the  central  point  is 
drawn  in  by  the  handle  of  the  malleus,  which  is  firmly  attached 
to  it  along  its  entire  length.  The  membrane  is  thus  held  in  the 
shape  of  a  very  blunt  cone,  somewhat  like  a  Japanese  umbrella, 
the  apex  of  which  points  inwards  towards  the  cavity  of  the  drum. 
The  peculiar  form  of  the  membrane  of  the  drum  is  of  great  impor- 
tance for  distinct  hearing. 

As  every  confined  volume  of  air  has  a  certain  tone  of  its  own 
to  which  it  resonates  more  readily  than  to  others,  so  a  membrane 
of  a  given  size  and  tension  has  a  certain  self-tone,  the  vibration 
periods  of  which  it  follows  with  great  ease.  This  tone  varies  with 
the  tension,  as  may  be  seen  in  a  common  drum,  the  note  of  which 
can  be  changed  with  the  tension  of  its  parchment — the  tenser  the 
membrane,  the  higher  the  pitch.  If  the  membrane  of  the  drum 
of  our  ears  were  thus  set  to  one  tone,  our  hearing  would  be  most 


600 


MANUAL   OF    PHYSIOLOGY. 


imperfect  and  unpleasant,  for  we  should  be  wearied  by  the  reit- 
eration and  persistence  of  the  note  to  which  the  tympanic  mem- 
brane was  tuned.  But  this  does  not  occur ;  the  tympanic  mem- 
brane has  no  self-tone,  and  no  succession  of  vibrations  follows  the 
first  effect  of  the  sound  waves. 


Fig.  234. 


e.au.m 


Diagram  of  the  tyiiipauuiu,  snowing  the  relation  of  the  ossicles  to  the 
tympanic  membrane  and  tlie  internal  ear.  The  tympanum  is  cut  through 
nearly  transversely,  and  the  cavity  viewed  from  tlie  front  (left  ear).  (Schiifer.) 
— m.t,  Membrane  of  the  drum,  to  which  tlie  handle  of  the  malleus  is  attached 
at  u.  m,  head  of  malleus,  which  is  held  in  position  by  its  suspensory  ligament 
sd.m.,  and  external  ligament  l.e.m;  i.,  long  process  of  incus  connecting  mal- 
leus and  St.  stapes,  the  base  of  which  closes  oval  opening  of  the  vestibule. 
e.au.m,  external  auditory  meatus,  i.au.m,  internal  auditory  meatus,  where 
the  two  parts  of  the  auditory  nerve  enter,  a  and  h. 

The  existence  of  any  special  note  of  its  own  is  prevented  by 
its  conical  shape,  which  is  partly  due  to  the  traction  of  the  handle 
of  the  malleus.  If  a  stretched  membrane,  such  as  that  of  a  drum, 
be  drawn  out  at  its  centre  so  that  it  is  no  longer  a  flat  surface, 
then  its  tension  is  different  at  the  centre  and  the  periphery,  being, 


CONDUCTION   THROUGH   THE   TYMPANUM.  601 

of  course,  greatest  at  the  point  at  which  it  is  drawn  upon,  and 
gradually  decreasing  towards  the  margin.  Since  the  existence 
of  a  tone  of  a  definite  pitch  depends  upon  a  certain  degree  of 
tightness  of  the  membrane,  if  no  two  parts  of  the  membrane 
have  exactly  the  same  degree  of  tightness,  then  of  course  no  one 
tone  can  be  more  conspicuous  than  another.  This  is  the  case 
with  the  tympanic  membrane. 

The  independent  vibrations  of  the  membrane  are  further  pre- 
vented by  the  tympanic  ossicles.  These  little  bones  do  not  really 
vibrate,  but  move  back  and  forwards  in  time  to  the  sound-vibra- 
tion. If  a  body,  not  capable  of  vibrating  with  the  membrane 
of  a  common  drum,  be  attached  to  it,  the  drum  would  not  sound. 
A  touch  of  the  finger  of  the  musician  to  the  membrane  sujBaces 
to  check  the  sound  produced  by  a  drum.  The  handle  of  the 
malleus,  which  is  joined  to  the  other  bones,  being  fixed  to  the 
membrane,  acts  in  this  way  as  a  damper,  and  checks  the  continu- 
ance of  any  special  vibration  in  the  membrane  of  the  drum. 

The  small  muscle  attached  to  the  malleus  so  as  to  draw  it  to- 
wards the  cavity  of  the  drum  is  called  the  tensor  tympani. 

Conduction  of  Sound-vibrations  through  the  Tympanum. 

The  motions  occurring  in  the  membrane  of  the  drum  are  con- 
veyed across  the  tympanic  cavity  by  means  of  the  three  small 
bones  known  as  the  malleus,  the  incus,  and  the  stapes.  Together, 
these  ossicles  form  an  angular  or  two-armed  lever,  one  end  of 
which  (the  handle  of  the  malleus)  is  attached  to  the  centre  of  the 
tympanic  membrane,  and  the  other  (the  long  limb  of  the  incus) 
which  is  the  shorter  arm  of  the  lever,  pushes  the  stapes  against 
the  little  secondary  tympanic  membrane,  which  fits  the  oval  open- 
ing leading  into  the  vestibule.  The  stirrup  bone  acts  as  a  kind 
of  adaptable  extremity  to  this  inner  arm  of  the  lever,  being  ad- 
herent to  the  membrane  of  the  vestibule  and  jointed  to  the  long 
arm  of  the  incus.  This  little  angular  lever  works  round  an  axis 
which  passes  from  before  backwards  through  the  head  of  the 
malleus,  and  lies  above  the  membrane  of  the  drum  ;  the  two 
points  which  act  as  the  bearings  or  pivots  of  the  motion  being 


602  MANUAI.   OF    PHYSIOLOGY. 

the  slender  process  of  the  malleus  in  front,  and  the  short  linib  of 
the  incus  behind. 

When  the  tympanic  membrane  vibrates  in  response  to  the 
sound  waves  of  the  air,  it  moves  in  and  out,  and  the  handle  of 
the  hammer  bone  must  move  in  and  out  with  it.  The  body  of 
the  incus,  being  fixed  by  a  firm  joint  to  the  head  of  the  malleus, 
must  follow  these  movements,  and  thus  they  cause  the  little  oval 
foot-piece  of  the  stirrup  to  press  in  or  to  draw  out  the  membrane 
which  separates  the  tympanum  from  the  vestibule.  Thus  the 
vibrations  of  the  air  communicated  to  the  tympanic  membrane 
are  conveyed  across  the  cavity  of  the  drum  to  the  liquid  in  the 
labyrinth. 

A  small  muscle — the  stapedius — is  attached  to  the  stapes  near 
its  junction  with  the  incus,  and  pulls  upon  it  in  such  a  direction 
that  the  bone  is  drawn  out  of  the  line  of  motion.  This  action, 
possibly,  has  the  eflfect  of  reducing  the  efiect  of  the  more  ample 
vibrations  of  the  tympanic  membrane,  which  might  have  too 
great  an  efiect  upon  the  liquid  in  the  labyrinth. 

The  tympanum  is  connected  with  the  pharynx  by  means  of  the 
Eustachian  tube,  which,  though  habitually  closed,  is  opened  for 
a  moment  by  svvallowing  and  other  motions  of  the  pharynx.  On 
these  occasions  air  can  pass  in  or  out  of  the  tympanum  easily,  so 
that  the  pressure  on  the  two  sides  of  the  membrane  of  the  drum 
is  equalized.  When  there  is  too  much  or  too  little  air  in  the 
tympanic  cavity,  the  tympanic  movements  are  impeded.  This 
difficulty  is  felt  when  one  has  a  bad  cold;  the  tube  is  occluded 
by  the  inflammatory  swelling  of  the  mucous  membrane.  Or 
when  one  performs  what  is  known  as  Valsalva's  experiment,  i.e., 
to  hold  the  nose  and  violently  puff  air  into  it;  when  the  tubes 
are  blown  open,  too  much  air  is  often  retained  in  the  tympanum, 
80  that  the  pressure  from  within  is  higher  than  that  from  without, 
and  hearing  becomes  dull.  If,  now,  the  act  of  swallowing  be 
performed,  the  feeling  of  tension  leaves  the  ears  and  hearing  be- 
comes as  acute  as  before. 

The  Eustachian  tube  also  acts  as  a  way  of  escape  for  any  fluid 
that  may  be  secreted  by  the  epithelial  lining  of  the  tympanic 
cavity.     This  fluid  is  so  minute,  that  the  periodic  opening  of  the 


CONDUCTION   THROUGH    THE    LABYRINTH.  603 

tube  suffices,  under  ordinary  circumstances  for  its  complete  escape. 
When  increased  by  disease,  however,  it  may  collect  in  the  tym- 
panum and  require  catheterization. 

If  the  tube  were  permanently  open,  we  should  suffer  from  two 
great  disadvantages.  In  the  first  place,  at  every  breath,  even 
during  ordinary  respiration,  some  little  change  in  tension  of  the 
air  contained  in  the  cavity  of  the  drum  would  occur  and  impair 
the  hearing ;  and  secondly,  the  vibrations  of  the  air  in  the  phar- 
ynx, produced  by  the  voice,  would  enter  the  drum  directly,  and 
give  rise  to  an  exaggerated  shouting  noise. 

Conduction  through  the  Labyrinth. 

Every  motion  of  the  oval  base  of  the  little  stirrup  bone  causes 
a  wave  to  pass  along  the  liquid  in  the  labyrinth.  The  bony  case 
of  the  internal  ear  being  firm,  and  its  contained  liquid — like 


Diagram  of  the  membranous  labyrinth. — a,  b,  c,  semicircular  canals  open- 
ing into  the  ventricle  d;  e,  the  saccule  from  which  the  uniting  canal  leads 
into  the  membranous  canal  of  the  cochlea,  g.    (Cleland.) 

most  liquids — quite  incompressible,  the  wave  travels  through  all 
the  parts  of  the  internal  ear.  Through  the  cochlea  it  can  arrive 
at  the  yielding  membrane  covering  in  the  round  opening,  which 
separates  the  cavities  of  the  tympanum  and  the  cochlea.  To 
pass  from  the  oval  vestibular  opening  which  is  closed  by  the 
stapes,  to  the  inner  tympanic  membrane  which  closes  the  scala 
tympani  of  the  cochlea,  the  waves  have  a  very  complex  route. 
From  the  liquid  lying  around  the  membranous  labyrinth — the 


604  MANUAL   OF   PHYSIOLOGY. 

perilymph — the  waves  pass  up  the  fluid  in  the  vestibular  spiral 
of  the  cochlea,  and  arriving  at  its  summit,  they  descend  by  the 
tympanic  spiral  to  the  round  opening.  In  this  course  they  pass 
at  first  over  and  then  under  tiie  fluid  contained  in  the  membran- 
ous canal  of  the  cochlea — endolymph — in  which  the  special  nerve 
terminations  of  the  cochlea  are  placed. 

For  the  construction  of  the  labyrinth  the  student  is  referred 
to  the  text-books  of  anatomy,  as  space  only  admits  of  a  brief 
account  of  the  special  arrangements  of  the  nerve-ending  being 
given. 

The  nervous  mechanisms  which  are  most  important  for  the  ap- 
preciation of  tones  are  those  situated  in  the  cochlea. 

The  endings  of  the  nerves  which  are  found  in  the  membranous 
sacs  in  the  vestibule  are  connected  with  peculiar  epitheloid  cells, 
to  which  are  attached  fine  bristle-like  processes.  These  processes 
lie  in  the  endolymph,  and  are  related  to  calcareous  masses  called 
otoliths.  Waves  in  this  endolymph  possibly  bring  the  otoliths 
into  collision  with  the  hairs,  and  thus  give  a  stimulus  to  the 
nerve-endings.  Thus  noises  may  be  heard,  but  no  fine  impres- 
sions of  tones  can  be  explained.  The  exact  use  of  the  nerves 
going  to  the  other  parts  of  the  labyrinth,  namely,  the  ampullce 
of  the  semicircular  canals,  is  somewhat  doubtful,  and  possibly  not 
immediately  connected  with  hearing.*  The  coils  of  the  snail- 
shell-like  cochlea  are,  throughout  their  entire  length,  even  in  the 
dried  state,  partially  divided  into  two  by  a  kind  of  shelf  project- 
ing from  its  central  axis  into  the  spiral  cavity.  This  is  called 
the  osseous  spiral  lamina.  In  the  fresh  state  the  separation  of  the 
spiral  canal  into  an  upper  (vestibular)  and  a  lower  (tympanic) 
coil  is  completed  by  a  membranous  partition,  which  stretches 
from  the  bony  spiral  lamina  to  the  opposed  side  of  the  spiral 
canal.  This  is  called  the  membranous  spiral  lamina,  and  forms 
the  base  upon  which  the  special  nerve-endings  of  the  organ  of 
hearing  are  spread  out.  An  extremely  delicate  membrane  called 
the  membrane  of  Eeissner  stretches  from   the  upper  side  of  the 

*  Compare  equilibration,  in  connection  with  which  they  will  be  described, 
p.  639. 


THE   COCHLEA. 

Fig.  236. 


605 


Transverse  section  through  the  membranous  canal  of  the  cochlea.  (Cadiat.) 
— a,  Striated  zone  of  basilar  membrane ;  b,  Pectinate  zone  of  the  basilar 
membrane;  c,  Perforated  zone  of  basilar  membrane  through  which  the 
nerves  pass ;  d,  Nerve  fibres  from  spiral  ganglion ;  e,  Spiral  ganglion ;  /, 
Limbns ;  r/,  Reissner's  membrane ;  A,  Tectorial  membrane ;  i,  Internal  rod 
of  Corti ;  m,  External  rod  of  Corti ;  o,  p,  p,  Special  cells  receiving  nerve 
terminals;  9,  Epithelial  cells  covering  the  basilar  membrane;  s,  Nerve 
fibres  ;  I,  Spiral  ligament. 


606  MANUAL   OF    rilYSIOLOGY. 

spiral  partition  obliquely  upwards  to  the  outer  wall  of  the  spiral 
cavity,  so  as  to  cover  the  special  organ,  and  shut  off  that  part  of 
the  vestibular  coil  which  lies  over  the  membranous  spiral  lamina. 
This  canal  of  the  cochlea  is  triangular  in  section,  and  is  separate 
from  the  rest  of  the  spiral  cavity.  Its  floor  is  made  up  chiefly 
of  the  membranous  spiral  lamina,  particularly  the  part  called 
the  basilar  membrane,  while  the  oblique  roof  is  composed  of  only 
the  thin  membrane  of  Reissner.  This  space  follows  the  turns  of 
the  cochlea,  lying  between  the  vestibular  coil  and  that  leading 
to  the  tympanum,  and  it  is  filled  with  a  fluid  (endolyraph)  which 
is  quite  separate  and  distinct  from  that  in  the  vestibular  or  tym- 
panic coils  of  the  cochlea  (perilymph). 

The  cochlear  division  of  the  auditory  nerve  passes  into  little 
tunnels  in  the  central  bony  column  around  which  the  coils  of  the 
cochlea  turn,  and  it  gives  off"  a  sj)iral  series  of  branches  which 
run  through  the  osseous  spiral  lamina  to  reach  the  membranous 
portion.  A  collection  of  ganglion  cells  connected  with  the  radi- 
ating nerve-fibres  is  found  lying  in  a  spiral  canal  in  the  osseous 
lamina.  Passing  through  the  bony  spiral  the  nerves  reach  the 
basilar  membrane,  which,  as  before  mentioned,  forms  a  great  part 
of  the  membranous  spiral  lamina,  and  upon  which  the  organ  of 
Corti  is  placed. 

The  organ  of  Corti,  placed  within  the  membranous  canal  of  the 
cochlea,  is  made  up  of  a  series  of  peculiarly  curved  bars  or  fibres, 
called  the  rods  of  Corti,  and  some  remarkable  cells  provided  with 
short,  bristle-like  processes.  The  rods  of  Corti  are  fixed  by  their 
broad  bases  upon  the  basilar  membrane,  and  unite  above  in  such 
a  way  that  the  outer  and  inner  rods  together  form  a  bow  or  arch. 
The  spiral  series  of  rods  thus  propped  up  against  each  other  leave 
a  small  space  or  tunnel  under  them,  which  runs  the  entire  length 
of  the  basilar  membrane.  Beside  these  rods  of  Corti  are  placed 
rows  of  cells  of  an  epithelial  type  into  which  the  nerve-endings 
pass.  From  the  upper  surface  of  these  cells,  on  a  level  with  the 
apex  or  junction  of  the  rods,  a  number  of  hair-like  processes  pro- 
ject. A  delicate  reticulated  membrane  lies  over  the  rods  and  the 
cells,  and  seems  to  be  lightly  attached  to  their  surface,  while  the 
hairs  pass  through  its  meshes. 


AUDITORY   SENSATIONS.  607 

The  basilar  membrane  is  made  up  of  fibrous  bands  held  together 
by  a  delicate  membrane.  The  fibres  pass  transversely  across  the 
spiral  canal  of  the  cochlea,  so  as  to  subtend  the  bases  of  the  outer 
aud  inner  rods.  The  basilar  membrane  gradually  becomes  wider 
as  it  passes  from  the  base  to  the  summit  of  the  cochlea.  The 
length  of  the  rods  also  increases  towards  the  summit  of  the  organ, 
their  bases  being  more  widely  separated  from  one  another  aud 
their  point  of  junction  nearer  to  the  basilar  membrane,  so  as  to 
form  a  lower  and  wider  tunnel.  The  entire  number  of  rods  of 
Corti  has  been  estimated  at  3000. 

Stimulation  of  the  Auditory  Nerve. 

The  stimulation  of  the  nerve  of  hearing  by  sound-vibrations  of 
the  air  is  less  diflBcult  to  understand  than  the  excitation  of  the 
optic  nerve  by  light-waves  which  are  conveyed  by  an  imponderable 
medium.  The  motions  of  the  membrane  of  the  drum,  being  con- 
veyed in  the  manner  already  indicated  to  the  liquids  within  the 
internal  ear,  pass  first  over  and  then  under  the  cells  connected 
with  the  nerve  terminals,  which  are  placed  on  the  elastic  basilar 
membrane.  The  transverse  fibres  are  set  in  motion  by  the  waves 
in  the  fluid,  and  as  they  vibrate  they  communicate  the  motion  to 
the  rods  of  Corti.  The  bases  of  the  inner  rods,  being  fixed  at  the 
inner  margin  of  the  basilar  membrane,  can  move  but  little,  and 
the  bases  of  the  outer  rods  being  placed  near  the  middle  of  the 
fibres  of  the  membrane,  where  the  motion  of  the  vibrations  is  most 
extensive,  a  slight  change  in  their  relative  positions,  and  a  con- 
sequent movement  of  the  apex  of  the  bow,  must  take  place.  This 
movement  at  the  apex  of  the  bow,  where  the  rods  join,  is  commu- 
nicated by  the  medium  of  the  reticular  membrane  to  the  hairs  in 
the  special  auditory  cells,  thence  to  the  nerves,  where  an  excita- 
tion is  produced  which  gives  rise  to  the  transmission  of  an  impulse 
to  the  brain. 

But  we  can  distinguish  differences  of  (1)  loudness,  of  (2)  pitch, 
and  of  rS)  quality  in  sound. 

Bince  the  loudness  depends  simply  on  the  amplitude  of  the  vi- 
bration, we  can  have  no  difficulty  in  understanding  how  varieties 


608  MANUAL   OF    PHYBIOLOGY. 

in  it  can  be  appreciated,  since  the  more  ample  the  vibration  the 
more  marked  the  motion,  and,  therefore,  the  more  intense  the 
stimulation  of  the  nerve  terminals.  What  we  call  the  loudness 
of  a  sound,  then,  simply  means  greater  or  less  intensity  of  stimula- 
tion of  the  nerve. 

The  comprehension  of  the  perception  of  differences  of  pitch  pre- 
sents greater  difficulty.  As  already  mentioned,  this  depends  on 
the  rate  or  period  of  vibrations.  We  know  that  most  bodies 
capable  of  producing  sound  vibrations  have  a  proper  tone,  i.e., 
that  which  they  produce  when  struck.  When  the  proper  tone  of 
a  body  capable  of  vibrating  is  sounded  in  its  immediate  neighbor- 
hood, this  is  also  set  vibrating  through  the  medium  of  the  air.  If 
a  clear  tone  be  sung  loudly  over  the  strings  of  a  piano  a  kind  of 
sympathetic  echo  will  be  heard  to  come  from  the  cords,  on  account 
of  the  strings  corresponding  to  the  notes  sounded  being  thrown 
into  sympathetic  vibrations.  Now,  in  the  basilar  membrane  we 
have  practically  a  series  of  strings  of  different  length — since  the 
membrane  gets  wider  as  it  passes  from  belo.w  upwards  to  the 
summit  of  the  cochlea — and  therefore  a  great  variety  of  proper 
tones.  With  a  high  note,  then,  a  fibre  of  one  part  of  the  mem- 
brane will  readily  fall  into  vibration,  and  with  a  low  note  a  fibre 
of  another  part.  Different  nerve  fibrils  are  in  relation  to  these 
different  parts,  and  thus  we  may  conclude  that  tones  of  different 
pitches  stimulate  distinct  nerve-terminals,  and  are  conveyed  to 
the  brain  by  separate  nerve-channels.  Impulses  arriving  at  cer- 
tain brain-cells  then  give  rise  to  the  idea  of  high  tones,  and  im- 
pulses coming  to  others  cause  the  impression  of  lowtones.  Thereare 
about  a  sufficient  number  of  fibres  in  the  basilar  membrane  for 
all  the  notes  we  can  hear,  viz.,  from  about  30  to  4000  waves  in  the 
second. 

The  rods  of  Corti  cannot  be  the  vibrating  agents,  because  they 
are  too  few  in  number,  and  they  are  absent  in  birds,  which  appre- 
ciate and  reproduce  various  notes.  Further,  the  rods  are  not 
elastic,  and  therefore  not  well  suited  to  vibrate.  It  may  therefore 
be  concluded  that  they  only  act  as  levers  which  convey  the  vibra- 
tions of  the  fibres  of  the  basilar  membrane  to  the  nerve-endings 
in  the  auditory  cells. 


AUDITORY  SENSATIONS.  609 

The  explanation  of  our  wonderful  appreciation  of  the  delicate 
shades  of  quality  of  tones  is  more  difficult.  Even  persons  with 
indifferently  good  ears,  as  musicians  say,  and  no  special  musical 
education,  can  at  once  distinguish  between  the  quality  of  the  same 
note  when  sounded  on  a  violin,  a  piano,  and  a  flute.  In  examin- 
ing the  resound  from  a  piano  when  a  note  is  sung  against  its 
strings  it  becomes  obvious  that  with  ever  so  pure  a  tone  a  great 
number  of  strings  are  set  vibrating.  It  will  be  found  that  not 
only  the  string  which  sounds  the  note  vibrates,  but  also  all  those 
strings  that  have  a  certain  simple  numerical  relation  to  its  number 
of  vibrations.  In  fact,  all  its  over-tones  or  harmonics  are  also 
sounded.  Isow,  in  the  cochlea  we  suppose  the  same  takes  place 
with  the  fibres  of  the  basilar  membrane.  Not  only  does  the  one 
fibre  whose  proper  tone  is  sounded  vibrate  in  response,  but  also 
all  those  fibres  which  represent  the  many  and  varied  over-tones 
or  harmonics  of  the  fundamental  tone  that  reach  the  ear.  It  has 
already  been  pointed  out  that  quality  of  a  note  depends  on  the 
relative  number,  force,  and  arrangement  of  the  harmonics  which 
invariably  accompany  any  musical  note  possessing  a  definite  char- 
acter. 

When  such  a  note,  then,  arrives  at  the  auditory  nerve-terminals, 
one  of  these  is  strongly  stimulated  by  the  wave  of  the  funda- 
mental tone,  and  many  others  are  stimulated  by  the  different 
over-tones.  Thus,  a  complexity  of  impulses,  corresponding  to  a 
mixture  of  tones  of  varying  intricacy,  is  transmitted  to  the  brain- 
cells,  where  it  gives  rise  to  the  impression  of  the  quality  which  we 
by  experience  associate  with  that  of  a  violin,  flute  or  piano,  as 
the  case  may  be. 

With  regard  to  the  judgment  of  the  distance  of  sounds,  it  need 
only  be  remarked  that  they  chiefly  depend  on  former  experience 
of  the  habitual  quality  and  intensity  of  the  sounds.  A  faint  sound 
with  the  same  quality  that  we  familiarly  attribute  to  loud  sounds 
seems  to  us  to  be  far  away.  Thus,  sounds  reaching  our  labyrinths 
by  the  cranial  bones  appear  distant,  and  ventriloquists  deceive 
us  by  imitating  the  character  of  distant  sounds. 

In  man  the  direction  from  which  sounds  come  is  chiefly  judged 

51 


610  MANUAL   OF   PHYSIOLOGY. 

b}  the  difference  of  distinctness  with  which  they  are  heard  by  one 
or  other  ear.  When  we  cannot  form  any  idea  of  whence  a  sound 
comes  we  usually  turn  our  heads  one  way  or  the  other  in  order  to 
present  one  ear  more  directly  to  the  origin  of  the  sound.  When 
a  sound  is  either  directly  behind  or  before  us  we  cannot  judge 
which  position  it  really  comes  from,  unless  the  head  be  slightly 
turned  to  one  side  or  to  the  other. 


CHAPTER  XXXIV. 


CENTEAL  NERVOUS  ORGANS. 


Fig.  237. 


The  more  important  properties  of  the  peripheral  nerves  and 
their  terminals  have  been  discussed  in  the  previous  pages.  The 
central  part  of  the  nervous  system,  which  remains  to  be  consid- 
ered, consists  of  the  spinal  marrow  and  the  brain.  These  parts 
are  composed  of  a  soft  texture,  the  elements  of  which  are  held 
ogether  by  a  peculiar  and  very  deli- 
cate form  of  connective  tissue,  known 
as  Neuroglia.  With  the  naked  eye 
the  central  nervous  organs  can  be  seeu 
to  be  made  up  of  two  distinct  kinds 
of  substance :  (1)  a  white  substance, 
which  is  found  by  the  microscope  to 
be  composed  of  nerve  fibres,  with  a 
medullary  sheath,  and  (2)  a  gray  sub- 
stance, consisting  of  a  dense  feltwork 
of  naked  axis  cylinders,  with  numer- 
ous ganglion  cells  interspersed  be- 
tween them  in  various  quantities  and 
relationships. 

In  the  brain  the  gray  substance  is 
distributed  chiefly  on  the  surface, 
forming  a  kind  of  gray  cortex,  which 
follows  all  the  irregularities  of  the 
convolutions.  In  the  spinal  cord  the 
gray  matter  is  situated  inside,  and  the  white  outside.  The  gray 
substance  of  the  cord  forms  separate  columns  on  either  side, 
which  run  its  entire  length,  but  is  thicker  in  the  cervical  and 
lumbar  regions.  These  gray  columns,  together  with  their  con- 
nection with  the  roots  of  the  spinal  nerves,  divide  the  white  sub- 
stance of  the  cord  into  separate  columns, 


Transverse  section  of  nerve 
fibres,  showing  the  axis  cylin- 
ders cut  across,  and  looking 
like  dots  surrounded  by  a  clear 
zone,  which  is  the  medullary 
sheath.  Fine  connective  tis- 
sue separates  the  fibres  into 
bundles. 


612  MANUAL   OF    PHYSIOLOGY. 

As  already  pointed  out,  the  nerve  fibres  are  simply  conducting 
channels  for  the  transmission  of  nerve  impulses  from  one  cell  or 
terminal  to  another. 

The  nerve  or  ganglion  cells  are  remarkable  for  their  large  size, 
their  large,  clear  nucleus,  distinct  nucleolus,  and  fibrillated  pro- 
toplasm.    They  have,  at  least,  one— more  commonly  several — 

Fig.  238. 


Multipolar  cells  from  the  anterior  gray  column  of  the  spinal  cord  of  the 

dog-fish  (a)  lying  in  a  texture  of  fibrils;  (6)  prolongation  from  cells;  (c) 
nerve-fibres  cut  across.     (Cadiat.) 

processes  connected  with  them,  and  are  commonly  called  uni-, 
bi-,  or  multi-polar  cells.  Often  one  of  these  processes  is  more 
distinct  and  more  definite  than  the  others,  and  does  not  subdivide 
into  branches  like  them.  It  appears  to  be  connected  directly 
with  the  axis  cylinder  of  a  medullated  nerve  fibre. 

The  nerve-cells  can  conduct  the  nerve  impulses  which  reach 
them  by  any  of  their  attached  poles,  and  they  can  transmit  these 
impulses  on  to  other  cells  by  means  of  their  protoplasmic  strands 
of  intercommunication.  They  thus  frequently  seem  to  direct  an 
impulse  coming  by  a  sensory  or  afferent  nerve  from  the  surface 
back  again  by  an  eflierent  nerve  to  some  texture  or  organ  in  the 
neighborhood.     Thuss,  the  slightest  stimulation  of  the  conjunctiva 


SPINAL   CORD.  613 

causes  immediate  and  involuntary  winking  of  the  eyelid.  This 
kind  of  transmission  of  an  impulse  in  a  direction  differing  from 
that  by  which  it  arrived  at  the  nerve-cell  is  called  reflection,  and 
motions  such  as  that  just  alluded  to  are  called  reflex  acts.  Col- 
lections of  cells,  whose  duty  seems  to  be  habitually  to  receive 
impulses  from  the  periphery  and  to  change  their  direction,  are 
called  reflex  centres. 

Some  groups  of  nerve-cells  send  forth  impulses,  either  con- 
stantly or  periodically,  without  receiving  any  nerve  impulse  from 
the  surface.     Such  centres  are  called  au- 
tomatic, since  they  appear  to  act  inde-  ^^^-  '^^^■ 
pendently   of  influences   from  without. 
The   only  source  of  energy  these  cells 
have,  is  the  warmth  and  nutrient  mate- 
rial carried  to  them  by  the  blood  flowing 
in  their  immediate  neighborhood.     The 
vaso-motor  centres  are  good  examples  of 
automatic  centres,  in  which  the  constant       s.  Sensory  receiving  or- 
or  tonic  character  of  action   predomi-     S^"  ^^*^  attached  affer- 

nates.   The  respiratory  centre  is  one  from     ®"  "^''^^      ^^-    g.    en- 
,  .  ,  ,  .      .  ,  ,       ,      .        tral     organs  —  ganglion 

which  automatic  impulses  are  rhythmi-     ^^jj^    m.  Peripheral  or- 
cally  discharged  by  a  special  regulating     gan  and  efferent  nerve, 
apparatus. 

Besides  having  the  power  of  conducting,  reflecting,  and  origi- 
nating impulses,  we  must  attribute  to  the  activity  of  the  nerve- 
cells  of  the  brain  the  various  mental  phenomena,  such  as  feeling, 
thought,  volition,  memory,  etc.,  which  form  of  activity  may  be 
excited  either  by  impulses  arriving  from  without,  or  from  the 
spontaneous  (automatic)  action  of  the  cells  of  the  cerebral  cortex. 

The  Spinal  Cord. 

Being  the  great  bond  of  connection  between  the  brain  and  the 
majority  of  the  peripheral  nerves,  the  spinal  cord  is  obviously  a 
conducting  apparatus  of  the  very  first  importance,  and  from  the 
quantity  of  nerve-cells  lying  in  its  gray  matter,  it  must  also  enjoy 
the  function  of  a  governing  organ  or  nerve  centre. 


614 


MANUAL   OP    PHYSIOLOGY. 


These  two  great  duties  of  the  spinal  marrow  had  better  be 
considered  separately: 

I. — Spinal  Cord  as  a  Conductor. 

From  the  anatomical  investigation  it  may  be  seen  that  there 
must  be  some  special  method  of  conducting  impulses  along  the 
spinal  marrow,  and  that  it  is  not  merely  a  collection  of  the  nerves 


Fig.  240. 


ji.m.f 


Diagram  illustrating  the  course  probably  taken  by  the  fibres  of  the  nerve- 
roots  on  entering  the  spinal  cord.  (Schafer.) — a.m.f.,  Anterior  median  fis- 
sure ;  p.m.f.,  Posterior  median  fissure ;  c.c,  Central  canal ;  s.r.,  Substantia 
gelatinosa  of  Rolando ;  a.o.,  Funiculi  of  anterior  root  of  a  nerve ;  p.,  Funic- 
ulus of  posterior  root  of  a  nerve.  By  following  the  fibres  1, 2,  3,  etc.,  their 
course  through  the  gray  matter  of  the  spinal  cord  may  be  traced. 

or  an  aggregation  of  the  fibres  that  spring  from  it.  In  the  first 
place,  these  nerves,  if  all  bundled  together,  would  be  much  larger 
than  the  cord,  even  at  its  thickest  part ;  and,  further,  it  does  not 
taper  evenly  towards  its  lower  extremity,  as  it  should  were  each 
succeeding  pair  of  roots  a  direct  loss  to  its  thickness. 


SPINAL  CORD   AS   A   CONDUCTOR.  615 

The  posterior  roots  of  the  spinal  nerves  pass  through  the  white 
substance  to  reach  the  posterior  gray  column,  where  they  break 
up  into  numerous  fine  twigs,  which  are  distributed  to  neighboring 
parts  of  the  gray  network  of  fibrils,  in  which  they  are  lost  with- 
out their  union  with  the  cells  being  obvious  or  immediate.  The 
fibres  of  anterior  roots  traverse  the  superficial  white  part  of  the 
cord  on  their  way  to  reach  the  anterior  gray  columns,  into  the 
cells  of  which  they  can  be  directly  traced.  The  numerous  pro- 
cesses from  these  cells  then  pass  into  the  fibrillar  network  which 
lies  between  the  cells  and  makes  up  the  great  mass  of  the  gray 
substance.  By  means  of  two  sets  of  fibres  (one  lying  in  the  lat- 
eral white  column  on  the  same  side,  and  another,  which  crosses 
at  once  to  the  other  side  of  the  cord)  these  cells  are  kept  in  com- 
munication with  the  parts  of  the  cord  above.  The  medullated 
nerve-fibres  of  the  cord  then  are  not  directly  continuous  with  those 
of  the  roots  of  the  spinal  nerves,  but  seem  only  to  have  the  func- 
tion of  connecting  the  diflferent  regions  or  districts  of  the  cord 
with  one  another,  and  with  the  brain,  and  they  thus  establish  a 
near  relation  between  the  cells  in  the  lumbar,  dorsal,  and  cervical 
regions  of  the  spinal  cord,  with  the  medulla  oblongata,  etc. 

Histology  thus  leads  us  to  expect  that  the  essential  parts  of  the 
cord  are — (1)  innumerable  fibrils  in  the  gray  matter,  and  (2) 
series  of  groups  of  cells  all  intimately  connected  with  one  an- 
other, with  the  cells  in  the  masses  of  gray  matter  at  the  base  of 
the  brain  (cerebellum),  and  with  the  fibres  in  the  anterior  and 
posterior  spinal  roots,  by  which  they  are  related  to  sporadic  gan- 
glia and  the  various  tissues  and  organs.  The  white  fibres  of  the 
cord  are  then,  probably,  only  used  for  the  more  rapid  conveyance 
of  impulses  from  one  group  of  cells  to  some  others  lying  in  a 
distant  region  of  the  cord  itself,  while  the  main  conducting  work 
is  accomplished  by  the  fibrils  of  the  gray  matter. 

Experiments  have  taught  us  the  following  facts :  1.  Section  of 
the  cord  causes  loss  of  both  sensation  and  motion  in  the  part 
behind — speaking  of  a  lower  animal — the  point  of  section  (Galen). 
2.  Section  of  one  side  of  the  cord  is  followed  by  loss  of  sensation 
on  the  same  side  with  increased  sensitiveness  and  loss  of  motion 
(recovering  slowly)  on  the  side  opposite  to  the  injury.     3.  Divi- 


616  MANUAL   OF   PHYSIOLOGY. 

sion  of  the  cord  in  the  median  line  gives  rise  to  impairment  of 
feeling  in  a  badly-defined  part  of  the  surface,  but  no  loss  of  mo- 
tion. 4.  Section  of  the  posterior  white  columns  gives  rise  to  the 
loss  of  perception  of  tactile,  temperature,  and  muscle  sense,  but 
the  sensation  of  pain  can  still  be  felt.  A  partial  section  of  these 
columns  is  followed  by  a  local  loss  of  touch  in  a  part  of  the  skin 
of  corresponding  extent.  This  lesion  is  complete,  as  if  the  im- 
pulses were  transmitted  directly  by  definite  fibres  in  the  cord 
from  each  region  of  the  skin.  5.  Section  of  the  antero -lateral 
white  column  causes  loss  of  voluntary  power  in  a  corresponding 
part  of  the  same  side  of  the  body.  If  the  gray  matter  be  per- 
fect, the  power  of  motion  is  soon  restored,  showing  that  the  gray 
matter  can  take  on  a  function  habitually  performed  by  the  medul- 
lated  fibres.  The  respiratory  and  vaso-motor  impulses  appear  to 
be  conveyed  in  the  lateral  white  columns.  6.  If  the  gray  matter 
and  the  posterior  white  columns  be  quite  cut  across,  the  impulses 
giving  rise  in  the  brain  to  common  sensation  (pain),  as  well  as 
tactile  impulses,  are  no  longer  carried  to  the  centres,  which  shows 
that  the  impulses  causing  common  sensation  travel  exclusively  in 
the  gray  substance.  7.  Section  of  one  side  of  the  gray  matter 
has,  however  little  effect.  The  passage  of  painful  impressions  is 
not  quite  lost,  even  if  both  sides  be  cut  at  different  regions  of  the 
cord.  The  dulness  of  feeling  is  moreover  general  below  the  in- 
jury; no  one  spot  of  skin  being  quite  anaesthetic.  A  kind  of 
delay  in  transmission  occurring  from  the  "  blocks,"  as  if  constant 
decussation  of  the  fibrils  exists,  but  no  direct  or  special  channels 
of  communication.  If  the  posterior  white  column  be  left  intact, 
the  skin  may  transmit  tactile  impulses,  but  not  painful  ones.  This 
remarkable  condition,  called  "analgesia,"  sometimes  occurs  in  the 
human  being  when  partially  under  the  influence  of  chloroform. 

The  Spinal  Cord  as  a  Collection  of  Nerve  Centres. 

The  various  groups  of  cells  in  the  spinal  cord  are  in  more  or 
less  direct  union  with  the  roots  of  the  nerves,  and  the  conducting 
fibrils  of  the  cord  itself,  so  that  they  participate  in  the  transmis- 
sion of  the  impulses  to  and  from  the  centres  situated  in  the  brain. 


SPINAL  CORD  AS   NERVE  CENTRE.  617 

In  the  transmission  of  these  impulses  the  cells  seem  to  have  a 
certain  directing  and  controlling  influence  which  deserves  special 
attention,  as  it  gives  us  the  key  to  the  more  complex  mechanisms 
of  the  higher  centres.  Although  the  various  powers  exerted  by 
the  cells  of  the  spinal  cord  are  so  intimately  associated  together 
as  to  be  practically  inseparable,  it  is  found  convenient  to  consider 
their  functions  under  distinct  headings.  The  following  division 
of  their  duties  will  be  found  useful  in  working  out  their  general 
function,  viz. : 

1.  Their  influence  on  aflferent  impulses. 

2.  Their  influence  over  efferent  impulses. 

3.  Their  automatic  or  independent  power  of  originating  im- 
pulses. 

When  an  impulse — the  result  of  some  slight  stimulus,  such  as 
the  prick  of  a  pin — arrives  at  the  cells  of  the  spinal  cord,  it  is 
at  once  communicated  to  the  other  cells  in  the  immediate  neigh- 
borhood, and  reaching  some  cells  connected  with  motor  nerves  it 
gives  rise  to  a  movement  of  the  muscles  of  the  neighborhood 
from  which  the  impulse  first  started.  At  the  same  time  impulses 
travel  to  the  brain,  and  there  give  rise  to  a  consciousness  of  the 
various  events  taking  place,  i.e.,  a  local  stimulation  and  a  local 
movement.  The  action  of  the  cells  of  the  cord  takes  place  with- 
out the  aid  of  the  will,  and  generally  occurs  before  one  is  con- 
scious of  it ;  it  is  therefore  called  an  involuntary  act,  and  on 
account  of  being  a  throwing  back  of  the  impulse,  these  kinds  of 
movements  are  called  reflex  acts. 

Reflex  action  forms  one  of  the  commonest  duties  of  the  cells 
of  the  spinal  cord.  Even  the  gentlest  stimulation  commonly 
gives  rise  to  a  complex  movement,  the  execution  of  which  requires 
many  muscles  to  act  together  with,  as  it  were,  a  common  object. 
Thus  the  sudden  prick  of  a  pin  in  the  finger  will  cause  a  person 
to  withdraw  the  hand  quickly  from  the  irritating  object.  If  a 
greater  or  more  prolonged  stimulus  be  applied,  much  more  ex- 
tensive movements  occur,  and  these  are  in  the  same  way  accom- 
plished by  the  well-arranged  cooperation  of  many  muscles,  acting 
together  in  a  such  a  way  that  a  definite  and  familiar  action  is  per- 
formed.    For  example,  if  the  burning  head  of  a  match  adhere 

52 


618  MANUAL  OF   PHYSIOLOGY. 

Fio.  241. 


Diagram  illustrating  the  course  taken  by  the  fibres  in  the  spinal  cord. 
(After  Fick.) — a,  b,  and  c  represent  oblique  views  of  three  transverse  sec- 
tions of  the  cord,  the  tissue  between  which  is  supposed  to  be  transparent. 
The  outline  of  the  gray  substance  is  marked  with  a  line  which  incloses  the 
ganglion  cells.  At  the  lowest  section  (c)  sensory  nerve  fibres  (a)  enter  by 
the  posterior  root,  and,  after  connection  with  ganglion  cells  of  the  gray  mat- 
ter, communicate  with  the  posterior  white  column,  through  which  some 
passes  directly  to  the  brain,  as  shown  by  the  direction  of  the  arrow-head 
pointing  to  (6).  This  is  the  route  which  offers  least  resistance  to  an  impulse 
travelling  to  the  brain  through  the  cord.  Hence  it  is  that  traversed  by  weak 
peripheral  (tactile)  stimuli.  By  the  same  posterior  root  arrive  impulses  at 
the  cord  which  may  traverse  the  finer,  more  irregular  and  resistant  fibrils 
of  the  gray  matter — shown  by  the  fine  lines.  Through  these  channels  pain- 
ful sensations  are  carried.  From  many  parts  of  the  gray  matter  of  the  cord 
ganglion  cells  may  dispatch  impulses  by  the  motor  root  (d).  Hence  many 
reflex  actions  are  arranged.  When  an  impulse  comes  directly  from  the 
brain  (voluntary  centres)  it  adopts  the  direct  route  (c),  which  passes  through 
the  white  substance  of  the  anterior  columns  before  it  excites  the  motor  gan- 
glion cells  of  the  cord  to  coordinated  activity. 


COORDINATION.  619 

under  the  thumb-nail,  more  than  a  mere  withdrawal  of  the  hand 
takes  place.  The  entire  arm  is  violently  shaken  with  the  ob- 
vious purpose  of  shaking  off  the  offending  stimulus  before  the  will 
has  time  to  come  into  operation.  Here,  then,  we  have  a  complex 
form  of  coordination  of  purposeful  muscular  movement,  as  the 
immediate  result  of  an  impulse  coming  from  the  skin,  and  this 
coordination  is  the  result  of  mutual  relationships  existing  be- 
tween the  cells  of  the  cord  employed  in  transmitting  both  sen- 
sory and  motor  impulses.  Not  only  is  the  movement  a  regular 
and  coordinated  act,  but  in  many  cases,  as  has  just  been  men- 
tioned, it  is  performed  with  a  definite  purpose,  as  if  it  were  the 
result  of  thought,  but  since  there  need  be  no  consciousness  it 
cannot  be  mental.  All  these  points  may  be  easily  studied  on  a 
frog  killed  about  an  hour  beforehand  by  decapitation.  If  a  frag- 
ment of  blotting-paper,  moistened  in  weak  acid,  be  placed  on  the 
skin  of  the  body  of  such  an  animal,  in  a  position  not  easily 
reached  by  the  foot,  a  most  complex  series  of  movements  follows, 
first  with  one  leg  then  the  other,  or  with  both.  The  muscular 
action  is  both  elaborately  coordinated  and  purposeful ;  indeed, 
the  movements  of  the  headless  animal  might  be  called  ingenious, 
and  usually  result  in  the  removal  of  the  offending  paper. 

If  the  degree  of  the  stimulation  be  carefully  regulated,  it  will  be 
found  that  the  results  obtained  by  peripheral  stimulation  depend 
on  (a)  the  strength  of  the  stimulus,  and  the  length  of  time  for 
which  it  is  applied  ;  (b)  the  degree  of  excitability  of  the  cells  of 
the  cord ;  (c)  the  readiness  with  which  the  impulses  pass  along 
the  thin  conducting  channels  to  the  gray  matter,  and  (d)  the 
functional  activity  of  the  muscles  which  act  as  the  indicators  of 
the  reflex  eflfects. 

By  graduating  the  strength  of  the  solution  of  acid  with  which 
a  square  millimetre  of  blotting-paper  is  saturated  before  it  is 
placed  on  a  frog's  foot,  the  following  results  are  obtained  :  When 
very  weak  acid  is  employed,  only  slight  local  and  unilateral 
movement  is  caused.  If  steeped  in  stronger  acid,  the  same  sized 
paper  produces  a  series  of  reflex  movements,  spreading  to  several 
muscles  on  both  sides  of  the  body.  If  the  stimulus  be  further 
strengthened,  the  movements  become  violent  and  more  extended 


620  MANUAL   OF    l'iIV81()L(JG Y. 

until  the  whole  body  is  tossed  about  by  coordinated  motions. 
The  movements  seem  to  spread  from  the  local  nerve-cells  to  their 
neighbors,  and  then  to  reach  those  governing  the  corresponding 
muscles  of  the  other  side,  in  which,  however,  they  are  always  less 
marked  than  in  those  of  the  side  stimulated.  This  spreading  of 
movement  from  one  set  of  muscles  to  another,  as  the  strength  of 
the  stimulus  is  increased,  of  course,  must  be  preceded  by  a  spread- 
ing of  the  impulse  from  one  group  of  nerve-cells  in  the  cord  to 
another  by  a  kind  of  radiation  from  the  focus  of  excitation. 

Very  slight  stimulation,  though  not  suflBcient  to  produce  imme- 
diate response,  may,  after  a  time,  give  rise  to  definite  reflex  action, 
as  if  the  weak  impulses  arriving  at  the  nerve-cells  in  the  cord  were 
stored  up  until  their  sum  sufficed  to  produce  a  definite  reflex 
movement.  This  may  also — indeed,  much  better — be  seen  in  ani- 
mals whose  nerve  centres  are  intact,  for  the  cells  of  more  remote 
parts  exercise  a  kind  of  checking  influence  on  those  in  the  region 
receiving  the  stimulus,  and  thus  the  accumulative  action  (summa- 
tion) comes  more  commonly  and  more  eflfectively  into  play.  This 
is  seen  in  the  human  subject  where  slight  visceral  stimulations 
exist  for  a  long  time.  In  some  of  these  cases,  even  without  any 
really  sensory  appreciation  of  any  local  excitation,  an  amount  of 
energy  may  be  accumulated  along  the  gray  tract  of  the  cord  from 
the  prolonged  income  of  impulses,  that  will  bring  on  the  most  ex- 
tensive forms  of  reflex  muscular  movement,  and  give  rise  to  serious 
results.  These  movements  are  generally  difterent  from  the  regular 
coordinated  motion  resulting  at  once  from  an  adequate  skin  stim- 
ulation, and  have  usually  a  tendency  to  assume  a  convulsive  form. 
As  an  example  of  this  may  be  named  the  convulsions  that  com- 
monly occur  in  young  children,  from  the  prolonged  irritation  of 
intestinal  worms,  or  during  the  painful  period  of  dentition.  Epi- 
lepsy and  hydrophobia  may  possibly  be  explained  in  the  same 
way. 

In  certain  conditions  of  the  nervous  system  these  irregular 
movements  or  spasms  (convulsions)  can  be  excited  much  more 
readily  than  is  normally  the  case.  As  most  striking  among  these, 
may  be  named  poisoning  with  the  alkaloid  of  nux  vomica  (strych- 
nia) and  the  state  of  the  blood  which  is  produced  by  cessation  of 


REFLEX   ACTION.  621 

the  respiratory  function  (asphyxia).  These  toxic  conditions  of 
the  blood  bring  about  a  peculiar  excitable  condition  of  the  cells 
or  conducting  fibres  of  the  spinal  cord,  in  which  impulses  seem  to 
pass  with  unwonted  facility  from  one  part  to  another,  and  give 
rise  to  an  excessive  degree  of  action  even  in  response  to  normal 
stimulations.  A  frog  poisoned  with  strychnia  is  thrown  into  gen- 
eral spasm  of  the  entire  body  by  even  the  least  touch,  which  nor- 
mally would  only  cause  it  to  withdraw  the  limb  slowly. 

On  the  other  hand,  there  are  many  poisons  which  seem  to  have 
the  effect  of  dulling  the  reflex  powers  of  the  cord  centres,  among 
these  are  opium,  chloroform,  chloral,  digitalin,  etc.  And  the 
condition  of  the  blood  which  may  be  brought  about  by  too  rapid 
respiratory  movements  (apnoea),  has  also  the  effect  of  lowering 
the  excitability  of  the  spinal  nerve-cells,  and  slowing  respiration. 

The  great  majority  of  reflex  actions  may  be  prevented  or  con- 
trolled by  the  will,  and  the  masses  of  cells  in  the  basal  ganglia 
and  medulla  seem  habitually  to  exert  a  checking  or  inhibitory 
action  on  the  reflex  actions  of  the  spinal  cord.  It  is  in  this  way 
that  we  account  for  the  well-known  facts  that  in  a  frog  which  has 
not  been  decapitated  it  is  impossible  to  induce  the  ordinary  regu- 
lar reflex  movements,  and  that  a  human  being,  when  asleep,  shows 
well-marked  reflex  action  in  response  to  a  slight  stimulus  that 
would  be  quite  ineffectual  when  he  is  awake.  We  know,  too, 
that  for  some  little  time  after  pithing  a  frog  one  cannot  count  on 
constant  or  regular  results,  because  the  act  of  section  of  the  upper 
part  of  the  spinal  cord  acts  as  a  stimulus  to  those  channels  which 
habitually  bear  impulses  from  the  brain,  and,  by  exciting  them, 
has  the  same  inhibitory  effect.  Further,  it  has  been  said  that 
artificial  stimulation  of  the  corpora  quadrigemina  and  medulla 
have  the  effect  of  completely  checking  the  reflex  action  of  the 
cord. 

It  is  not  only  impulses  coming  from  the  higher  centres  that  are 
capable  of  inhibiting  reflex  activity.  If,  while  the  cord  is  era- 
ployed  in  reflex  action,  in  response  to  gentle  cutaneous  stimula- 
tion, a  large  sensory  nerve  trunk  be  stimulated  with  an  interrupted 
electric  current,  the  reflex  action  ceases.  In  short,  it  may  be 
accepted  that  strong  impulses   arriving  at  the   cord  from   any 


622  MANUAL   OF    PHYSIOLOGY. 

direction,  have  the  effect  of  inhibiting  the  action  of  its  reflecting 
cells. 

The  theory  of  reflex  action  lies  at  the  bottom  of  all  nervous 
activities,  and  it  is  therefore  useful  to  attempt  to  work  out  the 
details  of  the  mechanisms  by  means  of  which  it  is  carried  on. 
The  simplest  scheme  of  the  channels  traversed  by  the  impulses  is 
given  in  the  diagram  (Fig.  241),  in  which  the  arrow-heads  show 
the  direction  of  the  afferent  impulse  passing  along  the  posterior 
root  of  the  cord  to  reach  the  cell  in  the  posterior  gray  column, 
thence  through  the  fine  gray  network,  it  passes  to  a  cell  in  the 
anterior  column,  to  reach  the  efferent  fibre,  and  through  the  an- 
terior motor  root  of  the  nerve  on  its  way  to  the  muscle.  It  has 
been  suggested  that  the  impulse  meets  with  considerable  resistance 
in  passing  through  the  protoplasm  of  the  cells,  and  that  owing  to 
this  resistance  the  effect  of  a  slight  stimulus  remains  localized, 
while  a  more  powerful  irritation  gives  rise  to  impulses  that  can 
overcome  the  resistance,  and  thus  spread  to  a  greater  number  of 
cells,  even  reaching  those  situated  in  a  remote  district.  Thus  the 
coordination  in  the  cord  would  be  simply  dependent  on  the  ina- 
bility of  the  impulses  to  affect  cells  other  than  those  in  their  im- 
mediate neighborhood,  and  the  relation  between  the  strength  of 
stimulus  and  the  effect  would  in  this  manner  be  easy  of  explana- 
tion. It  has  also  been  suggested  that  this  resistance  is  increased 
by  impulses  arriving  at  the  cells  from  a  different  direction,  and 
the  checking  or  inhibitory  action  of  the  higher  centres,  or  intense 
peripheral  excitation  of  another  part,  impedes  the  spreading  of 
the  impulses  to  be  reflected,  and  a  lesser  result  is  obtained. 

But  this  view  of  resistance  to  and  interference  with  the  trans- 
mission of  impulses  in  the  nerve-cells  hardly  explains  all  the  phe- 
nomena observed  even  in  the  reflex  action  of  the  spinal  cord  and 
the  various  modifications  it  can  undergo  with  varying  conditions. 
It  will,  however,  help  us  in  formulating  the  mechanism  if  we 
suppose  the  resistance  in  the  gray  part  of  the  nerve  centres  to 
be  much  greater  than  in  the  ordinary  nerve  channels,  and  that 
throughout  it  the  ways  are  so  infinitely  numerous  that  we  can 
imagine  every  individual  nerve-cell  to  be  in  communication  with 
every  other  nerve-cell  by  some  path  possible  of  being  traversed. 


REFLEX  AcrrioN.  623 

But  these  paths  have  to  be  made  passable  by  use ;  the  oftener  an 
impulse  traverses  a  given  route  the  more  adapted  such  a  route 
becomes  for  future  traffic.     Thus,  by  practice  or  the  habit  of  using 

Fig.  242. 


Diagram  of  the  paths  taken  by  tlie  impulses  in  the  brain  and  cord. — mm, 
motor  channels;  s  s,  sensory  channels ;  rr.,  cranial  nerves. 

certain  sets  of  muscles,  we  constantly  freshen  certain  channels  of 
intercommunication  between  the  various  cells  of  the  cord,  and  thus 


624  MANUAL   OF   PHYSIOLOGY. 

make  beaten  tracks,  along  which  impulses  can  pass  without  hin- 
drance. The  nerve-paths  along  which  the  impulses,  producing 
certain  movements  commonly  performed  by  every  individual,  have 
to  pass,  are  no  doubt  prepared  by  the  long  practice  of  our  ances- 
tors, and  the  power  of  performing  these  actions  is  transmitted  to 
us  ready  for  immediate  application.  Other  paths  required  for  the 
production  of  unusual  combinations  of  movements  have  to  be 
worked  out  by  the  individual,  and  much  of  the  difficulty  of  learn- 
ing any  trade  depends  on  the  necessity  of  making  impulses  readily 
traverse  some  definite  directions,  so  as  to  excite  certain  groups  of 
cells  to  act  synchronously  and  set  certain  sets  of  muscles  in  accu- 
rately  coordinated  motion.  Indeed,  the  delicacy  of  manipulation 
required  by  some  trades  cannot  be  attained  in  the  lifetime  of  one 
individual ;  thus,  it  is  said  to  take  three  generations  to  make  a 
perfect  glass-blower;  the  grandson  having  the  benefit  of  the  he- 
reditary tendency  to  accomplish  certain  coordinations  acquired 
by  the  life-long  habit  of  the  parents. 

The  reflex  convulsions  that  occur  in  poisoning  with  strychnia, 
or  as  the  result  of  some  constant  but  slight  stimulation,  may  be 
explained  as  follows: 

We  know  that,  besides  the  resistant  thin  paths  of  connection 
between  the  cells  of  the  cord,  there  also  exist  medullated  fibres — 
short  cuts,  as  it  were — for  impulses  to  travel  from  one  part  of  the 
cord  to  another,  for  the  various  cell  groups  are  in  communication 
with  those  situated  in  the  other  regions,  by  means  of  fibres  that 
lie  in  the  white  columns.  Now,  if  we  suppose  the  ordinary  reflex 
traffic  of  the  cord-cells  to  be  carried  on  without  the  assistance  of 
these  direct  lines  of  communication,  we  must  assume  that  there  is 
some  special  means  of  shutting  these  fibres  out  of  the  working  of 
the  reflex  machine.  Such  special  mechanisms  do,  in  all  proba- 
bility, exist,  and  are  in  relationship  with,  or  under  the  command 
of  the  inhibitory  cells  of  the  higher  centres.  We  may  then  sup- 
pose that  strychnia  removes  the  power  of  these  special  agents,  and 
the  impulses  finding  the  direct  ways  from  one  part  of  the  cord  to 
another  open,  take  these  routes,  and  are  simultaneously  and  irregu- 
larly difiused  throughout  all  the  cell  territories  (independent  of 


SPECIAL   REFLEX   CENTRES.  625 

the  ordinary  paths  they  have  been  educated  to  follow),  and  thus 
convulsive  movements  are  excited  in  many  parts  of  the  body. 

In  like  manner  we  can  imagine  that  the  unremitting  activity 
necessary  to  keep  in  check  the  impulses  arriving  from  a  constant 
source  of  stimulation,  such  as  intestinal  worms,  eventually  fatigues 
the  active  elements  in  this  inhibitory  mechanism,  and  then — often 
suddenly — the  force  of  the  accumulated  irritation,  no  longer  re- 
strained by  the  checking  influence,  rushes  along  the  direct  chan- 
nels to  all  parts  of  the  cord,  and  simultaneously  exciting  them 
brings  many  discordant  muscles  into  spasmodic  action. 

The  reflection  of  an  impulse  from  a  sensory  nerve,  through  the 
cells  of  the  spinal  cord  to  a  motor  nerve,  occupies  a  measurable 
length  of  time,  which  has  been  estimated  at  about  -j^  of  a  second. 
The  time  required  for  the  performance  of  a  reflex  act  varies  con- 
siderably in  the  same  individual  under  different  conditions;  of 
these,  high  temperature  and  intense  stimulation  shorten  the  time, 
and  fatigue  or  cold  lengthen  it. 

Special  Reflex  Centres. 

Many  of  the  groups  of  nerve-cells  in  the  cord  are  employed  in 
executing  definite  familiar  acts  essential  to  the  animal  economy 
and  more  or  less  independent  of  the  will.  Many  of  these  acts  are 
very  complex,  and  require  the  coordinated  action  of  certain  sets 
of  muscles  and  the  inactivity  of  others.  Such  groups  of  nerve- 
cells  have  been  called  special  centres,  and  many  of  them  have 
already  been  referred  to  in  the  preceding  chapters,  where  a  fuller 
consideration  of  them  may  be  found.     The  more  important  are : 

1.  A  centre  for  securing  the  retention  of  the  urine  by  the  tonic 
contraction  of  the  sphincter  muscle  of  the  bladder.  This  group 
of  nerve-cells  is  probably  kept  in  action  by  impulses  arriving  from 
the  bladder  by  the  afferent  nerves,  passing  from  its  walls  to  the 
spinal  cord.  The  more  distended  the  bladder  becomes,  the  more 
powerful  the  stimulus  sent  to  the  cord,  and  therefore  the  more 
firmly  the  sphincter  is  made  to  contract. 

2.  Nearly  related  to  the  former  is  the  centre  which  presides 
over  the  evacuation  of  the  bladder.     This  is  excited  by  impulses 


626  MANUAI^   OF    PHYSIOLOGY. 

arriving  from  the  urethra,  near  the  neck  of  the  bladder.  It  then 
seta  the  detrusor  muscle  iu  action,  while  the  sphincter  is  relaxed 
by  voluntary  inhibition. 

3.  The  ejaculation  of  the  semen  may  also  be  said  to  be  accom- 
plished by  a  special  spinal  centre,  capable  of  controlling  certain 
special  movements,  in  which  involuntary  muscles  play  an  im- 
portant part. 

4.  In  parturition  a  number  of  motions  are  called  into  play  (as 
well  as  the  uterine  contraction)  which  are  so  regularly  coordinated 
as  to  entitle  us  to  suppose  that  they  are  arranged  by  a  special 
centre  in  the  spinal  cord. 

5.  The  act  of  defecation  is  accomplished  by  means  of  a  spinal 
centre  also.  The  action  of  this  centre  might  (like  that  presiding 
over  the  urinary  bladder)  be  divided  into  two  parts — retention 
and  evacuation — in  which  volition  and  intestinal  peristalsis  play 
a  very  important  part. 

Automatism. 

Besides  being  excited  to  action  by  impulses  coming  from  the 
brain — volition — and  from  the  surface — reflection — the  groups  of 
cells  in  the  spinal  cord  may  act  without  any  obvious  incoming  im- 
pulse ;  that  is  to  say,  some  of  the  cells  are  capable  of  spontaneous 
activity.  Such  groups  of  nerve-cells  are  commonly  called  auto- 
matic centres ;  the  more  important  of  those  found  in  mammalia 
may  be  classified  as  follows : 

1.  Vaso-motor  centres  :  Though  the  central  point  from  which 
the  contraction  of  the  bloodvessels  is  controlled  is  situated  in  the 
medulla,  there  is  no  doubt  that  even  in  mammalia  centres  are  dis- 
tributed throughout  the  gray  matter  of  the  spinal  marrow,  which 
are  capable  of  keeping  up  the  arterial  tone  in  the  regions  to  which 
they  correspond.  As  evidence  of  this  may  be  mentioned  the  fact 
that  the  dilatation  of  the  arteries,  which  follows  the  severance  of 
the  lumbar  part  of  the  cord  from  the  medulla,  only  lasts  a  few 
days,  after  which  the  vessels  again  contract  in  the  usual  tonic 
manner.  The  arterial  tonus  only  disappears  completely  and 
permanently  when  the  spinal  cord  is  destroyed.  Thus,  it  would 
appear — although  habitually  the  vessels  of  all  the  body  are  regu- 


AUTOMATIC   ACTION.  627 

lated  by  a  centre  in  the  medulla,  nearly  related  to  the  cardiac 
centre — that  every  vascular  region  has  a  nervous  mechanism  of 
its  own  in  the  cord,  which  suffices  to  keep  up  the  tonic  contraction 
of  the  muscular  coat  of  its  vessels  as  soon  as  the  necessary  new 
lines  of  intercommunication  have  been  opened  between  the  differ- 
ent districts  most  nearly  related  to  it. 

2.  Sweating  centres :  Though  closely  related  to  the  preced- 
ing, the  centres  which  preside  over  the  secretion  of  sweat  in  the 
lower  part  of  the  body  and  hinder  extremities  must,  for  many 
reasons  which  cannot  now  be  mentioned,  be  regarded  as  separate 
centres. 

3.  Many  sets  of  smooth  muscle-fibres  appear  to  be  kept  in  a 
state  of  tonic  (automatic)  contraction  by  means  of  centres  in  the 
cord.  Thus,  in  the  lower  part  of  the  cervical  cord  is  a  group  of 
nerve-cells  which  keep  the  dilator  muscle  of  the  iris  partially  con- 
tracted ;  a  narrowing  of  the  pupil  has  been  described  as  following 
injury  of  this  region. 

4.  The  gray  matter  of  the  cord  is  also  said  to  keep  the  skeletal 
muscles  in  a  state  of  slight  tonic  contraction,  either  from  auto- 
matic or  reflex  stimulation. 

On  account  of  the  elaborate  and  purposeful  reflex  movements 
performed  by  decapitated  frogs  or  eels,  it  has  been  suggested 
that  in  the  lower  vertebrates  the  spinal  cord  is  capable  of  sensation 
and  volition — mental  activity — but  to  follow  this  assumption,  we 
should  have  to  modify  our  ideas  of  volition  and  sensation,  for 
which  consciousness  is  commonly  taken  to  be  a  necessary  factor. 
It  is,  however,  important  to  note  that  the  lower  we  come  in  the 
scale  of  vertebrate  animals  the  less  powerful  are  the  mental  facul- 
ties, and  the  more  important  are  the  functions  that  the  spinal 
marrow  presides  over. 


CHAPTER  XXXV. 

THE  MEDULLA  OBLONGATA. 

The  "oblong  marrow"  is  the  direct  continuatiou  of  the  spinal 
marrow,  and  contains  the  different  items  in  the  construction  of 
the  latter,  prolonged  upwards  and  mingled  with  some  additional 
gray  masses.  The  exact  relationship  of  the  different  parts  of  the 
medulla  to  those  of  the  spinal  cord  may  be  best  understood  if  we 
suppose  the  latter,  when  it  reaches  its  upper  limit,  to  be  split 
vertically  on  its  posterior  side  down  to  the  central  canal,  and  the 
lateral  masses  so  separated  from  one  another  that  the  central  gray 
part  of  the  spinal  cord  becomes  spread  out  on  the  posterior  sur- 
face of  the  medulla  oblongata.  The  gray  matter  of  the  medulla 
oblongata  consists,  then,  of  two  portions  distinct  from  each  other; 
one  being  the  direct  continuation  of  the  gray  columns  of  the  spinal 
marrow,  and  the  other  being  made  up  of  certain  gray  nodules 
imbedded  here  and  there  among  the  white  strands.  These  latter, 
as  a  rule,  subserve  special  functions,  while  the  continuation  of 
the  gray  columns  of  the  spinal  cord,  which  are  spread  out  on  the 
floor  of  the  fourth  ventricle,  contains  the  nerve-cells  that  preside 
over  the  movements  which  are  most  important  for  the  every-day 
business  of  life. 

The  functions  of  the  medulla  may  be  conveniently  divided,  in 
the  same  manner  as  those  of  the  cord,  into  its  conducting  power 
and  its  use  as  a  central  nervous  organ. 

The  Medulla  Oblongata  as  a  Conductor. 

The  various  columns  of  the  spinal  cord  are  so  distributed  in 
the  medulla  that  the  anatomy  of  their  course  gives  some  indica- 
tion of  the  channels  by  which  impulses  are  carried  through  it. 
But  here,  as  in  the  spinal  cord,  we  should  remember  that  the 
white  fibres  must  be  regarded  as  the  direct  and  rapid  means  of 


MEDULLA   OBLONGATA."  629 

transit  of  impulses,  while  the  felt-work  of  fine  fibres  in  the  gray 
part  can  also  conduct  impulses  in  all  directions.  Though  the 
readiness  of  transit  is  much  less  along  the  thin-fibred  network  than 
via  the.  direct  medullated  routes,  the  complexity  of  paths  and  the 

Fig.  243. 


Diagram  of  Brain  and  Medulla  Oblongata.  (Cleland.) — a,  Spinal  cord  ; 
b,  b,  Cerebellum  divided,  and,  above  it,  the  valve  of  Vieussens  partially  di- 
vided ;  c,  Corpora  quadrigemina;  d,  d,  Optic  thalami;  e,  pineal  body;  /,/, 
Corpora  striata ;  g,  g,  Cerebral  hemispheres  in  section ;  h,  Corpus  callosum  ; 
i,  Fornix ;  /,  /,  Lateral  ventricles  ;  3,  Third  ventricle ;  4,  Fourth  ventricle ; 
5,  Fifth  ventricle,  bounded  on  each  side  by  septum  lucidum. 

variety  of  directions  in  which  they  lead,  is  much  greater  in  the 
gray  than  the  white  substance.  It  is  by  means  of  the  gray  sub- 
stance that  the  two  lateral  parts  of  the  medulla  are  so  nearly 


630  MANUAL   OF   PHYSIOLOGY. 

related  to  one  another,  and  the  local  centres  are  kept  in  communi- 
cation with  the  distant  parts. 

The  posterior  gray  columns  of  the  spinal  cord  are  partly  con- 
tinued on  by  fine  strands  into  the  cerebellum  by  its  peduncles, 
and  partly  are  carried  on  to  the  brain  through  the  cerebral 
peduncles.  The  antero-lateral  columns  are  also  distributed  in 
part  through  the  pyramids  and  peduncles  to  the  cerebrum,  and 
in  part  by  the  restiforra  bodies  and  peduncles  to  the  cerebellum. 
In  the  pyramids  the  decussation  of  the  anterior  columns  takes 
place,  and  it  is  believed  that  this  is  the  point  at  which  the  direct 
channels  carrying  voluntary  motor  impulses  to  the  skeletal  mus- 
cles pass  across  from  one  side  of  the  body  to  the  other. 

It  must  always  be  remembered  that  the  medulla  is  the  only 
route  between  the  brain  and  the  spinal  cord,  and  in  it  some 
medullated  channels  cross  and  separate  to  pass  to  their  cerebral 
connections,  and  the  gray  part  of  the  spinal  marrow  is  spread  out 
on  the  floor  of  the  fourth  ventricle,  and  amplified  by  the  addition 
of  several  separate  foci  of  gray  matter. 

The  Medulla  Oblongata  as  a  Central  Organ. 

A  number  of  groups  of  ganglion  cells  with  special  and  specific 
duties  are  located  in  the  medulla;  indeed,  those  acts  which  are 
obviously  most  important  for  the  due  execution  of  the  vegetative 
functions,  are,  for  the  most  part,  arranged  and  governed  by  the 
nerve-cells  of  the  medulla.  Some  of  these  centres  may  be  called 
automatic,  though  they  are  variously  affected  by  many  impulses 
arriving  from  distant  points,  and  others  are  purely  reflex  in  their 
action.  The  former  are  the  more  immediately  essential,  and  will 
therefore  be  considered  first. 

Respiratory  Centre. 

The  centre  which  regulates  the  motions  of  breathing  has  been 
known  to  be  situated  in  the  floor  of  the  fourth  ventricle  at  the 
upper  and  back  part  of  the  medulla  ever  since  Flourens  showed 
that  injury  of  this  spot — the  vital  point — was  followed  by  almost 
instant  cessation  of  respiratory  movements  and  death. 


CENTRES   OF   MEDULLA   OBLONGATA.  631 

This  centre  is  a  good  example  of  a  so-called  automatic  centre ; 
that  is  to  say,  the  blood  flowing  through  the  medulla  and  nour- 
ishing the  cells  suffices' to  supply  them  with  the  necessary  energy 
for  their  periodic  activity,  and  we  know  that  the  quality  of  the 
blood  reaching  this  part  modifies  the  activity  of  the  cells ;  for 
the  less  oxygen  and  the  more  carbonic  acid  contained  in  the 
blood,  the  more  powerfully  does  it  act  as  a  stimulant  to  the 
centre. 

Although  we  take  the  respiratory  centre  as  an  example  of  an 
automatic  centre,  its  daily  work  is  arranged  by  means  of  afferent 
impulses,  so  that  the  normal  rhythm  of  breathing  is  regulated 
and  maintained  by  reflex  action.  The  mechanical  states  of  the 
lungs — whether  distended  as  in  inspiration  or  contracted  as  in 
expiration — seem  to  excite  the  terminals  of  certain  fibres  of  the 
vagus,  which  carry  impulses  to  the  centre,  and  thus  excite  or  re- 
strain the  inspiratory  movements. 

But  this  automatic  centre  can  also  be  influenced  by  the  higher 
centres  of  the  brain,  for  by  our  will  we  can  obviously  regulate 
our  breathing  movements  or  stop  breathing  altogether  for  a  time. 
And  further,  the  action  of  the  respiratory  centre  can  be  much 
altered  by  impulses  arriving  from  the  surface,  as  may  be  seen  by 
the  gasping  inspirations  which  involuntarily  follow  the  sudden 
application  of  cold  to  the  surface. 

Again,  the  activity  of  the  centre  may  be  quite  altered  by  stimu- 
lations of  certain  parts  of  the  air- passages;  so  much  so,  that  con- 
vulsive actions  of  the  respiratory  muscles  are  brought  about, 
which  induced  some  to  speak  of  a  sneezing  centre  and  a  coughing 
centre  in  the  medulla.  But  sneezing  and  coughing  may  be  equally 
well  explained  as  a  peculiar  form  of  activity  of  the  respiratory 
centre,  or  a  reflex  alteration  in  the  respiratory  rhythm,  caused  by 
irritation  of  the  nasal  or  laryngeal  mucous  membranes,  as  by 
supposing  that  special  reflex  centres  exist  for  the  purpose  of  sneez- 
ing or  coughing. 

Though  the  action  of  the  respiratory  centre  can  be  modified 
by  (1)  the  will  and  by  (2)  various  peripheral  stimulations,  and 
is  habitually  regulated  from  the  periphery  through  the  (3)  vagi 
by  the  state  of  the  lungs,  the  condition  of  the  blood  supplied  to 


632  MANUAL   OF   PHYSIOLOGY. 

the  centre  may  be  such  that  these  remoter  influences  may  become 
quite  powerless.  This  uncontrollable  condition  of  the  centre  is 
established  when  the  blood  flowing  through  it  is  abnormally 
venous  and  the  cells  become  over-stimulated.  We  all  know  how 
short  a  time  we  can  hold  our  breath  by  voluntary  checking  of  the 
centre,  and  most  people  have  had  occasion  to  observe  the  inordi- 
nate and  painful  efforts  of  a  person  whose  respiration  is  interfered 
with  by  disease.  When  the  dyspnoea  becomes  intense,  nearly  all 
the  muscles  in  the  body  are  called  into  action.  Thus,  in  quiet 
breathing  comparatively  few  nerve  cells  in  the  medulla  carry  on 
the  work  of  respiration,  but  under  certain  emergencies  they  can 
call  to  their  aid  the  entire  motor  areas  of  the  gray  substance  of 
the  spinal  cord,  and  thus  give  rise  to  a  kind  of  general  effort. 
Hence  we  often  hear  of  a  convulsive  centre  in  the  medulla  being 
placed  in  close  relation  to  the  respiratory  centre.  In  cases, 
namely,  irritation  of  the  air-passages,  or  imperfect  oxidation  of 
the  blood,  the  convulsive  centre  comes  under  the  command  of  the 
cells  of  the  respiratory  centre,  which  can  then  excite  coughing, 
sneezing,  or  convulsive  inspiratory  effort. 

As  already  mentioned,  the  convulsion  of  asphyxia  may  also,  in 
part  at  least,  be  explained  by  the  impure  blood  acting  as  a  stim- 
ulus to  the  cells  of  the  cord  itself. 

The  Vaso-motor  Centre. 

It  has  already  been  stated  that  groups  of  cells  exist  in  the  gray 
part  of  the  spinal  cord,  which,  according  to  the  class  of  animal, 
have  more  or  less  direct  influence  upon  the  muscles  in  the  coats 
of  the  vessels.  Thus  in  a  frog,  whose  brain  and  medulla  have 
been  destroyed,  in  some  hours  the  vessels  of  the  web  regain  a 
considerable  degree  of  constriction,  which  is  again  lost  if  the  cord 
be  destroyed.  In  the  dog  the  vessels  of  the  hinder  limb  also 
recover  their  tone  more  or  less  perfectly  in  a  few  days  after  the 
spinal  marrow  has  been  cut  in  the  dorsal  region,  although  just 
after  the  section  they  are  widely  dilated  from  the  paralysis  of 
their  muscular  coats.  In  a  few  days,  then,  the  cells  of  the  cord 
can  learn  to  accomplish,  of  their  own  accord,  work  which  they 


VASO-MOTOR  CENTRE.  633 

had  been  in  the  habit  of  doing,  only  under  the  direction  of  the 
higher  centre.  From  this  we  conclude  that,  though  the  cord  con- 
tains local  vaso-raotor  centres  distributed  throughout  its  gray 
matter,  these  are  all  under  the  direction  and  control  of  the  vaso- 
motor centre  in  the  medulla,  and  this  centre  is  really  the  chief 
station  from  which  impulses  destined  to  affect  the  whole  organism 
must  emanate. 

This  arrangement  is  quite  comparable  with  that  by  which  the 
ordinary  muscles  are  made  to  contract.  When  the  will  causes  a 
muscular  contraction,  the  impulse  starting  from  the  cerebral  cor- 
tex does  not  travel  directly  to  the  muscle,  but  it  passes  from  the 
brain  to  certain  cells  in  the  cord  and  thence  to  the  muscles.  In 
fact,  to  these  spinal  agents  the  ultimate  arrangement  and  coordi- 
nation of  the  act  is  confided.  So,  also,  the  chief  vaso-motor  cen- 
tre in  the  medulla  executes  its  orders  through  the  medium  of 
numerous  under  centres  placed  at  various  stations  along  the  cord. 

The  vaso-motor  centres  —  like  nearly  all  other  controlling 
groups  of  ganglion  cells — must  be  considered  to  be  made  up  of 
two  parts  antagonistic  one  to  the  other,  viz.,  a  constricting  and  a 
dilating  centre,  the  impulses  from  which  commonly  travel  along 
separate  nerve  channels.  The  constricting  impulses  are  mainly 
distributed  by  the  sympathetic  nerve,  while  the  dilating  impulses 
generally  run  in  the  ordinary  peripheral  nerves,  which  are  em- 
ployed in  calling  forth  the  ordinary  function  of  the  part  in  ques- 
tion. This  is  chiefly  true  of  the  internal  organs,  but  in  the  limbs 
all  the  nerve  channels  are  commonly  collected  together  to  form  a 
single  nerve. 

From  what  has  been  said  as  to  the  wide  distribution  of  centres 
influencing  the  bloodvessels,  an  attempt  to  localize  exactly  the 
position  of  the  medullary  vaso-motor  cells  is  not  satisfactory.  In 
the  lower  animals — frogs — the  cells  are  evenly  diffused  through- 
out medulla  and  cord.  In  man  the  localization  is  difficult  to 
demonstrate,  though  we  have  reasons  for  thinking  it  much  more 
definitely  circumscribed  than  in  the  lower  animals.  In  the  rab- 
bit it  has  been  tolerably  accurately  localized  to  the  floor  of  the 
fourth  ventricle,  in  the  immediate  neighborhood  of  the  respiratory 

53 


634  MANUAL  OF   PHYSIOLOGY. 

and  cardiac  centres.     From  this  the  nerves  pass  into  the  cord  to 
the  spinal  roots,  by  which  they  reach  the  sympathetic. 

The  vaso-raotor  centre  exerts  a  tonic  or  continuing  action  on 
the  vessels,  holding  them  in  a  state  of  partial  constriction  or  tone. 
In  this  it  may  possibly  be  said  to  have  an  automatic  action. 
Though  tonic  state  of  activity  of  the  centre  may  be  called  auto- 
matic, it  is  really  under  the  control  of  many  complex  reflex  in- 
fluences, which  constantly  vary  the  general  tone,  or  effect  local 
changes  in  the  degree  of  constriction  of  this  or  that  vascular 
area.  Among  the  most  striking  afferent  regulating  impulses  are 
those  arriving  from  the  heart,  the  digestive  organs,  and  the  skin. 
In  some  animals,  a  special  nerve — the  depressor — has  been  dis- 
covered, which,  passing  from  the  heart  to  the  medulla,  keeps  the 
vaso-motor  centre  informed  as  to  the  degree  of  tension,  etc.,  of 
the  heart  cavities.  When  the  heart  becomes  overfull,  impulses 
pass  from  it  and  check  the  tonic  power  of  the  centre  so  as  to  re- 
duce the  arterial  pressure  against  which  the  ventricle  has  to  act. 
Electric  stimulation  of  this  nerve  causes  a  remarkable  fall  in  the 
general  blood  pressure.  The  vaso-motor  centres  regulate  the 
distribution  of  blood  to  the  viscera  and  skin,  according  to  the 
condition  of  activity  of  these  parts  as  described  in  another  chapter 
(XXXI.). 

The  Cardiac  Centre. 

Although  the  heart  beats  with  characteristic  periodicity  when 
cut  off"  from  the  nervous  centres,  its  normal  rhythm  is  under  the 
control  of  a  group  of  nerve  cells  in  the  medulla,  from  which 
some  of  the  fibres  of  the  vagus  conduct  special  regulating  im- 
pulses. The  action  of  this  centre  is  habitually  that  of  a  restrain- 
ing agent  lessening  the  rate  of  the  heart's  contractions,  and  is 
hence  called  a  tonic  inhibitory  centre  (see  page  274).  The  ac- 
tivity of  the  centre  is  influenced  by  the  condition  of  many  distant 
parts,  such  as  the  cortex  of  the  brain,  the  abdominal  viscera,  etc., 
■which  exert  a  kind  of  reflex  action  on  the  heart  through  this 
centre.  The  degree  of  inhibitory  power,  as  well  as  the  share 
taken  in  the  action  of  the  centre  by  automatism  and  reflection, 
diflTers  in  different  animals. 


THE  CARDIAC  CENTEE.  635 

In  the  medulla  there  also  exist  many  other  centres  connected 
with  the  organic  functions.  Among  these  the  centres  for  swallow- 
ing and  vomiting  may  be  mentioned.  For  further  details  on 
this  subject,  the  reader  may  consult  the  chapter  on  Digestion  (see 
page  112). 


CHAPTER  XXXVI. 

THE  BRAIN. 

As  we  pass  upwards  in  attempting  to  trace  the  destiny  of  the 
conducting  channels  of  the  medulla,  we  come  to  the  more  elab- 
orate system  of  nervous  textures  which,  together,  are  called  the 
brain.  This  is  anatomically  the  most  highly  developed,  and 
physiologically  the  most  intricate  part  of  the  central  nervous 
organs.  Besides  the  nerve  cells  and  various  kinds  of  conducting 
channels  with  which  we  have  already  become  familiar  in  the 
cord,  etc.,  there  are  in  the  brain  a  vast  number  of  smaller  elements 
which  do  not  possess  the  distinctive  characters  of  cells.  These 
granular  bodies  are  tightly  packed  together  in  many  parts  of  the 
centres,  and  must  have  some  important  function,  which  is,  how- 
ever, at  present  unknown. 

The  best  way  to  get  an  idea  of  the  general  plan  of  construc- 
tion of  the  brain,  is  to  follow  its  development  in  the  earlier  stages 
of  the  embryo,  from  the  time  when  it  forms  an  irregular  and 
thickened  part  of  the  tube  of  tissue  destined  to  become  the  spinal 
cord.  From  this  it  will  be  seen  that  the  brain  is  but  a  modified 
part  of  the  primitive  nervous  axis,  in  which  certain  swellings  may 
be  observed  at  an  early  period  of  embryonic  life.  These  swellings 
are  called  the  fore-brain,  the  mid-brain,  and  the  hind-brain,  and 
in  the  future  development  of  the  parts  give  rise  to  (1)  the  hemi- 
spheres and  basal  ganglia ;  (2)  the  corpora  quadrigemina,  pons, 
and  cerebellum  ;  and  (3)  the  medulla  oblongata.  The  great 
mass  of  the  brain — the  hemispheres — are  formed  by  an  excessive 
development  of  bud-like  processes  which  grow  out  from  the  sides 
of  the  fore-brain  at  an  early  period,  which  become  elaborately 
folded,  so  that  in  the  adult  it  is  difficult  to  trace  the  relationship 
to  the  original  form  (compare  p.  677,  etc.). 

The  cells  of  the  brain  are,  like  those  in  the  cord,  grouped  to- 
gether in  the   complex  gray  substance,  while  the  white  part  is 


THE   MESENCEPHALON   AND   CEREBELLUM.  637 

made  up  exclusively  of  conducting  fibres.  The  gray  substance 
may  be  said  to  be  distributed  in  four  more  or  less  distinct  regions. 
(1.)  Of  these  one  can  be  traced  along  the  floor  of  the  fourth  ven- 
tricle, from  the  gray  matter  of  the  cord  to  the  base  of  the  brain, 
as  far  forwards  as  the  tuber  cinereum,  so  that  it  may  be  said  to 
be  representative  of  the  gray  matter  forming  the  inner  lining  of 
the  primitive  nervous  tube.  (2.)  Then  come  the  ganglia  of  the 
brain,  which  are  the  more  or  less  isolated  masses  of  gray  sub- 
stance within  the  brain,  known  as  the  corpora  quadrigemina, 
optic  thalami,  corpora  striata,  etc.  (3.)  The  gray  substance  of 
the  cerebellum  and  of  the  corpora  quadrigemina  is  derived  from 
the  upper  part  of  the  mid-brain.  (4.)  The  cortex  of  the  hemi- 
spheres of  the  brain  forms  the  most  extensive  gray  district,  and 
must  be  regarded  as  quite  distinct  from  the  preceding. 

Connecting  the  various  parts  of  these  gray  regions  are  sets  of 
fibres,  which  may  be  classified  as  follows: 

1.  Those  which  act  as  channels  of  intercommunication  between 
the  different  parts  of  the  same  region.  These  may  be  divided 
into  unilateral,  which  connect  together  the  cells  of  a  single  hemi- 
sphere, and  bilateral,  or  commissural  fibres,  which  unite  the  cor- 
responding masses  of  gray  matter  on  the  two  sides  of  the  brain. 

2.  Those  which  connect  the  different  regions  one  with  another. 
Under  this  head  naturally  fall  (1)  those  fibres  which  pass  between 
the  cortex  and  the  basal  ganglia ;  (2)  those  running  from  the 
cortex  to  the  cerebellum ;  and  (3)  those  connecting  the  above 
with  the  axial  or  spinal  gray  matter. 

The  Mesencephalon  and  Cerebellum. 

In  examining  the  functions  of  the  brain,  it  will  be  advantageous 
to  consider  the  various  parts  in  the  order  they  are  found  in  pro- 
ceeding from  the  medulla  towards  the  cerebral  hemispheres. 
Between  the  medulla  oblongata  and  the  hemispheres,  we  thus 
come  to  a  group  of  parts,  including  the  pons,  the  corpora  quad- 
rigemina, pons  varolii,  and  cerebellum,  which  may  be  called  the 
mesencephalon,  being  developed  from  the  raid-brain.  The  duties 
of  this  part  of  the  nervous  centres  can  be  investigated  by  observ- 
ing the  actions  of  lower  animals  in  which  the  hemispheres  have 


638  MANUAL  OF   PHYSIOLOGY. 

been  removed,  or  the  parts  directly  stimulated,  and  by  noting  the 
symptoms  produced  in  man  by  lesions  of  this  part  of  the  brain. 
The  former  gives  the  most  definite  results,  and  therefore,  for  our 
purpose,  deserves  most  attention. 

When  the  cerebral  hemispheres  have  been  removed  from  a 
frog,  the  animal  retains  the  power  of  carrying  out  coordinated 
motions  of  much  greater  complexity  than  those  performed  by 
command  of  the  spinal  cord  alone.  But  this  power  is  not  exer- 
cised spontaneously.  That  is  to  say,  the  animal  can  balance 
itself  accurately,  jump,  swim,  swallow,  etc.,  but  it  only  attempts 
these  acts  when  forced  to  do  so  by  stimulations  arising  from  its 
outer  surroundings.  Thus  on  a  flat  surface  it  sits  upright,  but 
does  not  stir  from  the  spot  where  it  has  been  placed  ;  if  the  sur- 
face upon  which  it  sits  be  inclined,  so  that  its  head  is  too  low,  it 
turns  round  to  regain  its  equilibrium.  If  the  surface  be  further 
inclined,  it  at  first  crouches  so  as  not  to  slip  off,  and  then  crawls 
upwards  to  find  an  even  resting-place.  Plunged  into  deep  water, 
it  swims  perfectly,  but  on  arriving  in  a  shallow  part,  it  either 
rests  quietly  with  its  nose  out  of  the  water  and  its  toes  touching 
the  ground,  or  crawls  out  to  sit  on  the  water's  edge,  where  it  finds 
its  balance.  AYhen  touched  on  the  leg,  it  jumps  away  from  the 
stimulus,  and  in  so  doing  avoids  any  obvious  dark  obstacle.  It 
swallows  if  a  substance  be  put  in  its  mouth,  but  it  does  not 
attempt  to  eat  even  when  surrounded  with  food.  In  short,  all 
movements,  even  the  most  complex,  may  be  brought  about  by 
adequate  stimulation — spontaneity  only  is  wanting.  The  pupil 
responds  by  reflex  contraction,  when  the  retina  is  exposed  to 
light ;  the  eyes  are  closed  if  the  light  be  intense ;  and  the  head 
may  follow  the  motions  of  a  flame  moved  from  side  to  side.  A 
sudden  or  loud  noise  causes  the  animal  to  move.  From  the  fore- 
going facts,  and  the  power  such  a  frog  has  of  avoiding  a  dark 
object,  we  may  conclude  that  the  impulses  arising  from  the  spe- 
cial sense  organs  are  all  duly  received  and  excite  more  or  less 
elaborate  response,  but  the  consciousness  of  the  arrival  of  these 
impulses  no  longer  exists. 

The  removal  of  the  hemispheres  of  birds  and  rabbits  leaves  the 
animal  in  somewhat  the  same  condition ;  but  the  response  to  the 


THE   MESENCEPHALON    AND    CEREBELLUM.  639 

special  sense  impulses  is  not  so  definite  or  well  marked  since  the 
animal  flies  or  runs  against  even  the  most  obvious  obstacles. 

We  may  conclude,  then,  that  while  the  medulla  controls  the 
coordinated  movements  absolutely  necessary  for  the  vegetative 
functions,  the  mid-brain  (including  the  cerebellum  of  birds  and 
mammals)  controls  the  still  more  complex  associations  of  coordi- 
nated movements  necessary  for  the  perfect  performance  of  such 
acts  as  balancing  our  bodies,  and  enables  us  at  the  same  time  to 
carry  on  elaborate  coordinated  motions  with  the  upper  extremi- 
ties, or  vocal  and  respiratory  muscles. 

The  enormous  number  of  muscles  simultaneously  used  in  some 
of  our  commonest  daily  actions,  concerning  which  we  have  but 
little  thought,  and  take  no  voluntary  trouble,  shows  the  great 
importance  of  this  part  of  the  brain.  If  we  take  a  simple  ex- 
ample, that  of  standing  in  the  upright  position  (equilibration)  (see 
page  478),  we  find  that  a  great  number  of  muscles  have  to  act 
together  with  the  most  exact  nicety  to  accomplish  what,  even  in 
man,  is  a  quite  thoughtless,  if  not  quite  involuntary  action.  In 
the  frog,  as  has  been  seen,  equilibration  is  performed  by  reflex 
action  alone.  In  man,  the  nervous  mechanisms  are  probably 
more  complicated  by  his  erect  attitude  and  the  addition  of  the 
cerebellum,  etc.,  but  they  are  nevertheless  comparable  with  those 
of  the  frog.  It  may  therefore  be  instructive  to  examine  the 
details  of  the  mechanisms  in  a  frog  deprived  of  its  cerebral  hemi- 
spheres. 

The  optic  lobes  (which  correspond  to  the  corpora  quadrigemina, 
and  also  take  the  place  of  the  cerebellum  of  the  higher  animals) 
form,  in  the  frog,  the  great  centx*es  of  equilibration,  locomotion, 
etc.  If  these  lobes  be  destroyed,  the  animal  can  no  longer  sit 
upright,  jump,  or  swim.  The  first  point  to  determine  is,  whence 
do  the  impulses  arrive  which  bring  about  these  complex  coordi- 
nations. The  first  set  are  those  coming  from  the  tactile  sense  of 
the  skin  of  the  parts  touching  the  surface  ;  another  set  of  impulses 
arrives  from  the  acting  muscles  acquainting  the  centres  with  the 
amount  of  work  done.  A  third  set  comes  from  the  eyes,  by  which 
the  position  of  the  surrounding  objects  is  gauged  ;  and  finally, 
from  the  semicircular  canals  of  the  internal  ear  come  impulses 


640  MANUAL   OP   PHYSIOLOGY. 

which  inform  the  equilibrating  centres  as  to  the  position  of  the 
head. 

By  depriving  a  frog  of  these  several  portals  by  which  incoming 
stimuli  direct  the  balancing  centres,  it  can  be  rendered  incapable 
of  any  of  the  acts  requiring  equilibration,  even  when  the  regu- 
lating centres  are  intact.  In  our  own  bodies  we  can  convince 
ourselves  of  the  absolute  importance  of  these  afferent  regulating 
impulses  arriving  from  the  ear,  eye,  skin,  and  muscles.  If  having 
bent  one's  forehead  to  the  handle  of  a  walking-stick,  the  end  of 
which  is  fixed  on  the  ground,  we  run  three  or  four  times  around 
this  axis,  and  then  quickly  walk  towards  any  near  object,  we  find 
how  helpless  our  volition  becomes  if  deprived  of  the  normal  in- 
coming stimulus,  for  thus  an  unwonted  disturbance  of  the  nerve- 
terminals  in  the  semicircular  canals  has  dispatched  conflicting 
impulses  to  the  coordinating  centre  of  equilibration.  Further, 
we  know  that  we  stand  less  fixedly  when  our  eyes  are  shut,  and 
we  move  unsteadily  when  our  feet  are  benumbed,  etc. 

Crura  Cerebri. 

Passing  above  the  Pons  Varolii,  we  come  to  a  thin  isthmus,  com- 
posed of  two  thick  strands  of  nerve  substance  connecting  the 
mesencephalon  with  the  cerebral  hemispheres.  These  are  called 
the  crura  cerebri.  They  diverge  slightly  in  their  upward  course 
towards  the  hemispheres,  and  lie  just  below  the  corpora  quadri- 
gemina,  which  have  already  been  referred  to.  Minute  examina- 
tion of  these  crura  brings  to  light  an  anatomical  difference  which 
corresponds  with  a  distinct  physiological  separ9.tion  between  the 
paths  taken  by  the  sensory  and  motor  impulses  in  each  crus. 
The  lower  or  more  anterior  part,  which  can  be  seen  on  the  base 
of  the  brain,  is  called  the  base  or  crusta.  This  is  made  up  of 
motor  nerve  channels  only.  The  posterior  or  upper  part,  which 
lies  next  to  and  is  connected  with  the  corpora  quadrigemina,  is 
called  the  tegmentum,  and  is  composed  of  sensory  fibres.  Ana- 
tomically the  separation  between  the  two  is  indicated  by  some 
scattered  nerve-cells  (locus  niger).  The  base,  or  crusta,  which  is 
the  great  bond  of  union  between  the  spinal  cord  and  the  cerebral 


BASAL   GANGLIA.  641 

motor  centres,  passes  iuto  the  corpus  striatum ;  and  the  tegmen- 
tum, or  great  sensory  tract,  is  directly  connected  with  the  optic 
thalamus. 

Basal  Ganglia. 

The  great  masses  of  gray  and  white  matter  seen  on  the  floor  of 
the  lateral  ventricles  are  called  the  corpora  striata  and  optic  thalami, 
and  together  are  spoken  of  as  the  basal  ganglia.  The  exact  re- 
lationship borne  by  their  functions  to  those  of  the  mesencephalon 
and  cerebral  cortex  is  not  perfectly  understood,  though  it  is,  no 
doubt,  intimate.  The  following  are  some  of  the  more  important 
points  in  the  evidence  on  the  subject : 

Corpora  Striata. — The  motor  tracts,  coming  from  below, 
lie  in  the  lower  part  of  the  crus  cerebri,  and  thence  one  on  each 
side  passes  into  the  corresponding  corpus  striatum.  Anatomi- 
cally, then,  this  part  may  be  regarded  as  the  ganglion  of  the 
motor  tract. 

Destructive  lesion  of  one  corpus  striatum  is  followed  by  loss  of 
power  of  the  muscles  of  the  other  side  of  the  body.  This  is 
equally  true  of  lesions  artificially  produced  in  animals  and  those 
resulting  from  disease  in  man.  When  the  crura  on  both  sides  are 
destroyed  the  animal  remains  motionless  and  prostrate. 

Electrical  stimulation  of  one  of  the  corpora  striata  causes  uni- 
lateral motions  of  the  other  side  of  the  body.  This  fact,  however, 
does  not  teach  us  much  concerning  the  functions  of  the  particular 
cells  of  its  gray  matter,  since  the  stimulus  cannot  be  kept  from 
affecting  the  fibres  passing  through  the  corpus  striatum  to  go 
directly  to  the  motor  tract. 

In  dogs,  and  still  more  so  in  rabbits,  the  corpora  striata  seem 
to  be  able  to  carry  out  some  complex  motions  which  in  man  are 
believed  to  require  the  cooperation  of  the  higher  cerebral  centres. 
It  has  been  stated  that  a  dog  whose  cerebral  cortex  is  completely 
destroyed  can  perform  movements  that  in  man  can  only  be  evoked 
by  the  cortex  of  the  hemispheres. 

It  would  appear  then  that  the  gray  matter  of  the  corpus  striatum 
is  a  motor  ganglion,  nearly  related  in  function  to  the  cerebral 
cortex.  The  cells  of  this  ganglion  are  the  first  agents  working 
under  the  direction  of  the  cortical  centres,  and  carry  out  the 

54 


642 


MANUAL   OF   PHYSIOLOGY. 


organization  and  distribution  of  voluntary  motor  impulses.  In 
animals  whose  hemispheres  are  less  complexly  developed,  such  as 
the  dog  or  rabbit,  the  "  basal  agent"  seems  capable  of  carrying  on 

Fig.  244. 


Diagram  of  some  of  the  paths  taken  by  nerve  impulses  in  the  brain  and 
spinal  cord.— C  Gray  substance  of  cerebral  cortex.  c\  Gray  substance 
of  cerebellum.  Cr.  Cranial  nerves,  some  afferent  and  some  efferent.  M. 
Motor  (efferent)  spinal  nerves.     S.  Sensory  (afferent)  spinal  nerves. 

more  elaborate  work,  independent  of  the  guidance  of  the  higher 
motor  centres  in  the  gray  matter  of  the  brain. 


OPTIC  THALAMI. 


643 


Fig.  245. 


Optic  Thalami. — The  evidence  concerning  these  ganglia  is 
far  from  being  as  satisfactory  or  conclusive  as  that  relating  to  the 
corpus  striatum. 

Anatomically  the  matter  is  equally 
clear ;  they  are  the  ganglia  of  the  sensory 
tracts,  since  the  tegmentum  or  sensory 
parts  of  the  crura  pass  directly  into  them. 
They  form,  in  fact,  the  only  routes  by 
which  the  impulses  giving  rise  to  the  dif- 
ferent kinds  of  sensory  impressions  can 
arrive  at  the  cerebral  cortex.  But  the 
evidence  we  can  obtain  by  the  physio- 
logical examination  of  sensory  impres- 
sions is  very  indistinct  in  comparison 
with  the  obvious  results  we  find  when 
motor  tracts  are  excited  ;  indeed,  in  the 
complete  absence  of  all  motion,  it  is  dif- 
ficult to  know  whether  an  animal  feels 
or  not,  as  we  have  no  signs  to  show 
whether  the  stimulus  takes  efitct. 
Further,  it  is  diflficult,  as  has  been  already 
seen,  to  stimulate  any  sensory  tract  with- 
out the  impulse  being  reflected  to  its 
motor  neighbors,  so  a  muscular  move- 
ment often  results  from  stimulation  of 
a  group  of  cells  purely  sensory  in  func- 
tion. 

When  we  take  into  consideration  the 
foregoing  points,  and  the  fact  that  it  is 
difficult,  if  not  impossible,  to  destroy  a 
portion  of  brain  substance  without  irri- 
tating it  and  the  neighboring  structures, 

we  cannot  be  surprised  that  experimenters  have  arrived  at  very 
contradictory  results,  both  by  stimulating  and  destroying  the 
optic  thalami.  Some  find  that  electric  stimulation  causes  muscu- 
lar movements;  others  find  that  it  does  not.  Some  authorities 
state  that  destruction  of  the  optic  thalami  interrupts  only  the  in- 


Section  through  the 
gray  matter  of  the  brain 
of  man,  sliowing  several 
layers  of  cells  into  which 
fine  fibres  pass  from  below. 


644  MANUAL   OF    PHYSIOLOGY. 

coming  sensory  impressions ;  others  say  it  gives  rise  to  motor 
paralysis. 

Human  pathology  helps  us  but  little,  for  it  is  impossible  to  say 
whether  a  given  lesion  simply  abolishes  the  function  of  the  part 
or  acts  as  an  irritant  to  it,  or  in  some  degree  produces  both  these 
effects.  Local  lesions  of  the  optic  thalami  have  been  met  with, 
in  some  of  which  sensory,  and  in  others,  both  sensory  and  motor 
defects  have  been  observed  in  the  patients. 

We  must,  then,  remember  that  the  occurrence  of  motion  as  the 
result  of  stimulation,  or  the  absence  of  muscular  power  as  the 
result  of  destruction  of  the  optic  thalami  must  not  be  accepted  as 
conclusive  evidence  of  the  motor  function  of  the  active  elements 
— the  nerve-cells — of  this  part,  because  these  results  may  depend 
on  the  indirect  influence  of  the  sensory  impulses  coming  from 
these  cells. 

Cerebral  Hemispheres. 

It  is  now  universally  regarded  as  a  recognized  fact  that  in  man 
the  hemispheres  of  the  brain  are  the  seat  of  the  mental  faculties — 
perception,  memory,  thought,  and  volition.  The  cerebral  cortex 
is  the  part  of  the  nervous  system  in  which  the  subjective  percep- 
tion of  the  various  sensory  impulses  takes  place,  and  in  which 
impulses  are  converted  into  impressions  or  mental  operations.  It 
is  in  the  cortical  nerve-cells  the  so-called  voluntary  impulses, 
causing  movement  of  the  skeletal  muscles,  have  their  origin.  It 
is  thus  a  sensory  and  a  motor  organ.  But  it  has  a  far  wider  range 
of  function  than  is  expressed  by  saying  it  is  both  sensory  and 
motor ;  indeed,  in  this  it  would  be  no  better  than  the  other  nerve- 
centres  in  the  spinal  cord,  etc.  The  cells  of  the  cortex  of  the 
brain  seem  to  differ  from  those  of  the  lower  nerve-centres  (which 
can  also  receive,  and  at  once  send  out,  corresponding  impulses), 
in  this :  when  an  impulse  arrives  at  the  cerebral  cells,  it  there 
excites  a  change,  which,  besides  producing  an  immediate  effect, 
leaves  a  more  or  less  permanent  impression ;  the  impression  per- 
sists, and  if  the  cell  be  well  supplied  with  chemical  energy  in 
the  shape  of  nutriment,  the  impression  may  be  reproduced  at  a 
subsequent  period.     This  revival  of  impressions,  the  effects  of 


CEREBRAL    HEMISPHERES.  645 

past  stimulations,  or  "re-collection,"  is  exclusively  the  property 
of  the  cerebral  cortex,  and  to  it  the  hemispheres  owe  their  mental 
faculties.  During  our  lifetime  seusory  impulses  are  continually 
streaming  into  the  cells  of  the  cortex  of  the  brain  from  the 
peripheral  seusory  organs.  Thus  innumerable  impressions  are 
left  stored  up  in  the  nerve-cells.  The  effect  of  the  continuing 
presence  of  these  impressions  in  the  active  cells  is  memory,  and 
by  an  association,  arrangement,  or  separation  of  these  persisting 
impressions,  the  activity  of  the  cells  gives  rise  to  thought  or 
ideation. 

In  close  relation  and  connection  with  these  cells  of  the  cortex, 
in  which  permanent  impressions  are  stored  and  ideation  is  ac- 
complished, are  those  other  groups  of  cells  which  have  been 
mentioned  as  being  in  direct  communication  with  the  lower  motor 
centres,  and  can  by  the  medium  of  the  latter  execute  voluntary 
movements. 

It  is  a  very  remarkable  fact,  as  far  as  the  mental  faculties  are 
concerned,  that  one  side  of  the  brain  seems  to  be  suflBcient  for 
their  perfect  performance.  Memory,  consciousness,  and  thought 
can  all  be  operative  in  a  perfectly  normal  way,  when  one  side  of 
the  brain  is  rendered  incapable  of  performing  its  functions  by 
disease  or  injury.  But  this  is  not  true,  as  regards  the  reception 
of  sensory  impressions,  or  the  emission  of  voluntary  impulses. 
The  difference  between  the  mental  powers  and  mere  motor  and 
sensory  functions  of  the  brain  can  be  seen  in  those  cases  of  paraly- 
sis known  as  hemiplegia.  The  patient  is  frequently  fully  con- 
scious, and  possesses  unimpaired  power  of  thought  and  memory, 
yet  he  is  unable  to  perceive  the  sensory  impulses  coming  from  one 
side  of  his  body  or  send  voluntary  impulses  to  the  muscles  of  the 
paralyzed  side. 

The  cells  which  act  as  the  immediate  receivers  of  afferent  and 
dispensers  of  efferent  impulses  to  one  or  other  side  of  the  body  are 
then  localized  to  one  hemisphere,  and  that,  we  have  already  seen, 
is  that  of  the  opposite  side. 


646  MANUAL   OF   PHYSIOLOGY. 


Localization  op  the  Cerebral  Functions. 

Whether  the  surface  of  the  hemispheres  cau  be  mapped  out 
into  small  areas,  each  of  which  is  set  apart  for  a  definite  duty,  or 
whether  a  comparatively  restricted  portion  of  the  cortex  suffices 
for  the  performance  of  all  the  functions  of  the  hemispheres,  are 
questions  surrounded  with  difficulty,  and  which,  up  to  the  present, 
cannot  be  answered  with  any  degree  of  certainty.  The  experi- 
mental evidence  hitherto  brought  forward  on  the  subject  seems, 
in  many  points,  to  be  contradictory,  a  fact  which  may  be  ex- 
plained partly  by  the  difficulties  with  which  such  experiments 
are  beset,  and  partly  by  different  observers  being  anxious  to  up- 
hold with  too  great  fervor  either  the  localization  or  non-localiza- 
tion theory  in  their  entirety. 

The  leading  experimental  experiences  which  have  been  recorded 
are  the  following : 

1.  Extensive  tracts  of  the  cortex  of  the  hemispheres  may  be  re- 
moved, by  accident  or  experiment,  without  interfering  with  the 
cerebral  functions  in  any  marked  or  tangible  way.  Both  men 
and  animals  have  lived  for  years,  after  the  loss  of  a  considerable 
quantity  of  brain  substance,  without  showing  impairment  of  either 
mental  or  bodily  faculties. 

2.  Lesion  of  a  certain  part  of  the  frontal  lobe  of  the  left  hemi- 
sphere of  man  (posterior  part  of  the  third  frontal  convolution) 
has  been  so  frequently  followed  by  the  loss  of  the  faculty  of  speech 
— aphasia  —  that  pathologists  now  call  that  spot  the  centre  of 
speech. 

3.  Destruction  of  the  convolutions  around  and  in  the  neighbor- 
hood of  the  fissure  of  Rolando  gives  rise  to  temporary  loss  of 
power  in  the  limbs  of  the  other  side,  voluntary  motion  being  abol- 
ished when  an  extensive  area  is  destroyed.  This  loss  of  power  is 
more  obvious  in  animals  with  complex  brains  (man  and  monkey) 
than  in  those  less  highly  organized  (dog,  cat,  rabbit),  which  rap- 
idly recover. 

4.  Destruction  of  the  surface  of  the  posterior  lobes  interferes 
with  the  reception  of  visual  impressions,  and  if  an  area  including 


LOCALIZATION  OF   THE   CEREBRAL    FUNCTIONS.        647 

the  angular  gyri  and  all  the  posterior  lobes  be  destroyed,  the  ani- 
mal remains  blind. 

5.  Extensive  areas  of  the  brain  surface  may  be  stimulated  me- 
chanically, chemically,  or  electrically,  without  the  least  response 
being  shown  by  the  animal,  to  indicate  either  sensory  or  motor 
excitations. 

6.  Stimulation  of  the  convolutions  around  the  fissure  of  Rolan- 
do, however,  gives  rise  to  definite  coordinated  movements  of  mus- 
cles of  the  other  side  of  the  body.  Indeed,  local  groups  of  muscles 
respond  with  surprising  constancy  to  the  electric  stimulation  of 
certain  definite  parts  of  the  cortex.  These  convolutions  have  thus 
been  mapped  out  into  motor  centres  for  hind  limb,  fore  limb,  face, 
etc. 

From  this  we  are  tempted  to  conclude  (1)  that  the  cortex  of 
the  posterior  region  of  the  hemispheres  is  related  to  the  reception 
of  some  sensory  impressions  ;  (2)  that  the  superior  and  lateral 
part  in  the  neighborhood  of  the  fissure  of  Rolando  is  related  to 
the  discharge  of  voluntary  motor  impulses ;  and  (3)  that  the  an- 
terior lobes  are  not  immediately  subservient  to  either  the  sensory 
or  motor  functions  of  the  hemispheres,  though  the  centre  presiding 
over  the  faculty  of  speech  is  placed  in  this  part  on  the  left  side. 

As  an  objection  to  the  soundness  of  these  conclusions,  the  re- 
markable and  undoubted  fact  has  been  urged,  that  no  matter  how 
thorough  is  the  destruction  of  the  centres,  the  function  returns 
after  the  lapse  of  a  variable  interval.  In  some  instances  the  loss 
of  function  only  remains  for  a  few  hours  after  the  operation  ;  in 
other  cases  (those  in  which  the  injury  is  extensive  and  deep,  and 
the  animal  belongs  to  a  class  with  high  mental  organization)  the 
recovery  is  slow  and  may  extend  over  several  weeks  and  months. 
In  man  and  monkeys  the  function  may  be  lost  forever,  and  the 
nerve  channels,  which  formerly  carried  the  impulses  to  or  from 
the  injured  centre,  become  degenerated. 

From  some  of  the  foregoing  facts — viz.,  the  constant  and  regu- 
lar response  of  certain  groups  of  muscles  to  the  stimulation  of 
certain  local  spots  of  the  brain  surface,  and  the  temporary  destruc- 
tion of  the  functions  of  some  organ  when  a  certain  point  is  injured 
— it  seems  definitely  fixed  that  certain  local  parts  of  the  brain 


648  MANUAL   OF   PHYSIOLOGY. 

surface  are  in  more  immediate  connection  with  certain  peripheral 
organs  than  are  others,  and  that  these  local  areas  have  been  in 
the  habit  of  receiving  (in  the  case  of  the  visual  impulses  coming 
to  the  angular  gyri)  or  sending  out  (in  the  case  of  motor  impulses 
starting  from  the  motor  centres)  impulses  of  a  special  and  definite 
kind. 

From  the  other  facts  mentioned — viz.,  the  recovery  of  function 
after  injury,  or  the  complete  absence  of  functional  lesion — we 
must  conclude  that  these  local  areas  are  by  no  means  the  only 
agents  which  can  carry  on  the  business  of  receiving  for  the  mind 
impulses  from  the  periphery,  and  sending  out  voluntary  impulses 
to  the  muscles ;  but  that  rather  there  are  many  groups  of  nerve- 
cells,  in  relation  with  the  nearest  sub-agents — the  basal  ganglia — 
which  can  take  on  the  duty  of  the  injured  cells,  and  act  as  cortical 
centres,  receiving  sensory,  and  discharging  motor  impulses.  In 
respect  of  this  capability  of  one  part  of  the  cerebral  cortex  to  carry 
on  the  duties  ordinarily  allocated  to  another,  we  have  a  complete 
analogy  in  the  gray  part  of  the  spinal  cord.  Partial  section  of 
the  gray  part  of  the  spinal  cord  (even  if  it  be  cut  at  two  or  three 
different  levels)  does  not  destroy  the  sensation  of  any  local  area 
of  skin,  showing  that  the  delicate  felt-work  of  nerve  fibrils  in  the 
gray  substance  can  conduct  the  impulses  in  many  directions,  so 
that  even  when  a  considerable  number  of  the  ordinary  routes  are 
blocked  by  section  of  fibrils  and  destruction  of  the  cells  at  the 
part  cut,  the  neighboring  channels  can  carry  on  the  work,  so  that 
after  a  little  time  the  sensory  impulses  are  carried  from  all  parts 
of  the  skin  to  the  brain  without  delay. 

It  has  already  been  pointed  out  that  the  function  of  any  given 
nerve  fibre  depends  on  the  function  of  its  terminals.  The  fibre 
itself  is  merely  a  conducting  agent.  In  somewhat  the  same  way 
the  functions  of  any  given  nerve-cell  must  depend  on  the  number 
and  character  of  its  connections.  If  it  be  attached  to  a  raotorial 
end-plate  in  a  muscle,  it  can  only  be  an  exciter  of  impulses  that 
give  rise  to  motion  :  if  it  be  connected  only  with  a  sensory  termi- 
nal, it  can  only  be  a  receiver  of  sensory  impulses.  But,  in  the 
gray  matter  of  the  spinal  cord,  and  still  more  so  in  that  of  the 
cerebral  cortex,  we  may  assume  that  all  the  cells  are  in  more  or 


LOCALIZATION   OF  THE   CEREBRAL   FUNCTIONS.       649 

less  intimate  connection  with  innumerable  other  cells.  In  fact, 
we  must  imagine  that  the  whole  of  the  gray  matter  of  both  cord 
and  brain  is  interwoven  into  a  complex  felt-work  of  fibrils  and 
cells,  which  in  no  part  are  isolated  from  the  rest,  but  that  all  the 
elements  form  a  continuous  system. 


CHAPTER  XXXVII. 

REPRODUCTION. 

Origin  of  Male  and  Female  Generative  Elements. 

One  of  the  chief  characteristics  of  a  living  being  is  the  power 
it  possesses  of  reproducing  itself;  that  is  to  say,  organisms  can, 
under  favorable  conditions,  form  out  of  themselves  other  living 
bodies  with  similar  lives  and  habits. 

In  the  lowest  form  of  animal  life  this  propagation  of  species 
may  take  place  by  the  division  of  a  single  cell ;  thus  an  amoeba 
reproduces  its  like  by  the  cleavage  of  its  mass  of  protoplasm,  which 
separates  the  main  body  into  two  amrebse.  In  such  a  case  the 
method  of  reproduction  is  purely  asexual,  the  amoeba  containing 
within  itself  the  power  of  forming  other  amoebjB  without  help  from 
other  individuals. 

As  we  ascend  the  animal  scale,  we  find  that  just  as  other  func- 
tions are  allotted  to  certain  groups  of  cells,  specially  differentiated 
for  the  purpose,  so  the  function  of  reproduction  is  performed  by 
certain  specially  modified  collections  of  cells.  Further,  we  find 
the  fact  that  the  production  of  a  new  being  requires  the  coopera- 
tion of  two  kinds  of  generative  elements,  each  of  which  is  com- 
monly produced  by  a  different  reproductive  organ.  These  repro- 
ductive organs  in  the  higher  animals  are  placed  in  different  indi- 
viduals of  the  same  species.  This  divides  most  organisms  into 
male  and  female  sexes,  and  is  hence  termed  the  sexual  method  of 
reproduction. 

The  sexual  method  of  reproduction  is  met  with  in  all  the  more 
highly  developed  forms  of  animal  and  vegetable  life.  The  male 
organ  produces  active  elements — the  spermatozoa;  the  female 
organ  produces  the  ovum,  which,  when  fertilized  by  the  sperma- 
tozoa, develops  into  the  embryo. 

In  mammalia  the  uterus  is  the  most  important  of  the  subsidiary 


DEVELOPMENT  OF   THE  SPERMATOZOA. 


651 


organs,  as  it  is  modified  for  the  development  and  growth  of  the 
embryo  :  its  function,  however,  can  be  performed  by  other  organs, 
as  is  seen  in  cases  of  extra-uterine  foetation,  when  the  ovum  de- 
velops in  some  unusual  situation,  such  as  in  the  Fallopian  tube, 
or  in  the  abdominal  cavity. 

The  spermatozoa  are  formed  by  the  cells  lining  the  tubuli  semi- 
niferi  of  the  testicle.  These  cells  are  cubical  masses  of  proto- 
plasm, which  undergo  rapid  proliferation.     The  nuclei  divide, 


Fig.  246. 


Section  of  the  tubuli  seminiferi  of  a  rat.  (Schafer.)— a,  Tubuli  in  which 
the  spermatozoa  are  not  fully  developed,  b,  Spermatozoa  more  developed, 
c.  Spermatozoa  fully  developed. 

and  from  each  part  resulting  from  this  division  arises  the  head  of 
a  spermatozoon,  and  the  body  is  developed  from  the  protoplasm 
of  the  cell.  The  spermatic  elements  escape  into  the  tubes,  and 
pass  down  the  vasa  deferentia  into  the  vesiculse  seminales,  where 
they  either  undergo  a  retrograde  change  or  are  cast  out  of  the 
body. 

The  ovum  arises  from  the  differentiation  of  one  of  the  cells  of 
the  germ-epif helium  covering  the  surface  of  the  ovary.  A  group 
of  these  cells,  entering  the  periphery  of  the  ovary,  becomes  there 


652 


MANUAL    OF   PHYSIOLOGY. 


imbedded  in  a  kiud  of  capsule  derived  from  the  surrounding 
areolar  tissue  of  the  stroma,  and  forms  an  immature  Graafian 
follicle.     One  of  the  cells  grows  rapidly  to  become  the  ovum,  the 


C/iOlAT.,, 


Section  of  the  ovary  of  a  cat,  showing  the  origin  and  the  development  of 
Graafian  follicles.  (Cadiat.) — a.  Germ-epithelium,  b.  Graafian  follicle 
partly  developed,  c.  Earliest  form  of  Graafian  follicle,  d.  Well-developed 
Graafian  follicle,  e.  Ovum.  /.  Vitelline  membrane,  g.  Veins,  h,  i.  Small 
vessels  cut  across. 


rest  increase  in  number  to  form  the  small  cells  of  the  granular 
tunic.     As  the  follicle  develops,  it  works  its  way  towards  the  cen- 


MENSTRUATION   AND   OVULATION.  653 

tre  of  the  ovary,  and  then  approaches  the  periphery  of  the  organ 
as  a  fully-developed  Graafian  follicle. 

Microscopically,  it  is  seen  to  be  surrounded  externally  by  a 
capsule — the  tunica  fibrosa,  which  is  ill-defined  from  the  stroma 
of  the  ovary  in  which  it  lies.  Beneath  this  is  a  layer  of  capil- 
lary bloodvessels,  the  tunica  vasculosa,  and  to  these  two  coats  col- 
lectively the  term  tunica  propria  is  applied. 

Inside  the  tunica  propria  are  granular  cells  of  small  size, 
which  occupy  a  considerable  space  in  the  follicle;  they  are  heaped 
up  at  one  spot  to  receive  the  ovum,  which  lies  imbedded  in  their 
midst.  These  cells  receive  the  name  of  the  tunica  granulosa,  and 
their  projecting  portion,  which  encircles  the  ovum,  is  called  the 
discus  proligerus.  Filling  up  the  remainder  of  the  follicle  is  a 
fluid — the  liquor  folliculi.  The  surface  of  the  ovary  is  covered  by 
columnar  cells,  which  are  continuous  with  the  endothelial  cells 
of  the  peritoneum.  When  the  follicle  is  fully  matured,  it  lies  at 
the  periphery  of  the  ovary  beneath  this  layer  of  cells,  which 
separates  it  from  the  abdominal  cavity. 

Menstruation  and  Ovulation. 

After  puberty,  at  intervals  averaging  about  four  weeks,  the 
genital  organs  of  the  female  become  congested,  and  at  the  same 
time  a  Graafian  follicle  is  ruptured  and  its  contained  ovum  set 
free.  Coincidently  with  the  rupture  of  the  follicle,  the  fimbriated 
extremity  of  the  Fallopian  tube  becomes  closely  approximated 
to  the  spot  where  the  follicle  lies,  so  that  the  ovum,  instead  of 
falling  into  the  abdominal  cavity,  passes  into  the  canal  of  the 
Fallopian  tube,  down  which  it  is  conveyed  to  the  uterus. 

The  usual  place  for  the  ovum  to  meet  the  spermatozoa,  and  to 
be  impregnated,  is  the  Fallopian  tube. 

When  the  ovum  reaches  the  uterus,  if  it  be  unimpregnated,  it 
is  cast  out  with  the  surface  cells  of  the  mucous  membrane  of  the 
uterus,  which  are  destroyed,  and  escape  along  with  a  sanious  fluid. 
The  whole  of  the  phenomena  constitute  a  menstrual  act. 

If,  however,  the  ovum  become  impregnated,  it  remains  in  the 
Fallopian  tubes  some  days,  during  which  time  the  mucous  raem- 


654 


MANUAL   OF   PHYSIOLOGY. 


brane  of  the  uterus  becomes  so  hypertrophied  and  developed  as 
to  retain  the  ovum  when  it  reaches  that  organ. 

The  human  ovum  is  a  cell  consisting  of  a  mass  of  protoplasm 
inclosing  a  nucleus  and  a  nucleolus,  and  surrounded  by  a  cell- 
wall.  On  its  outer  surface  is  an  irregular  layer  of  cells,  the  re- 
mains of  that  part  of  the  tunica  granulosa  which  encircled  the 
ovum  in  the  Graafian  follicle.  The  cell-wall  of  the  ovum  is  called 
the  vitelline  membrane  or  zona  pellucida,  and  the  mass  of  granular 
protoplasm  itencircles,  the?u7e^/i(sor  yolk,  and  in  this  is  a  nucleus 
— the  germinal  vesicle,  which  contains  a  nucleolus — the  germinal 
spot. 

Beneath  the  outer  covering  of  calcareous  material  of  the  hen's 
egg  there  is  a  white  membrane,  which  incloses  a  transparent  albu- 
minous substance  known  as  the  white  of  egg.     Inside  this  is  a 

Fig.  *J48. 


Ovum.  (Robin.) — a.  Zona  pellucida  or  vitelline  membrane.  6.  Yolk, 
c.  Germinal  vesicle  or  nucleus,  d.  Germinal  spot  or  nucleolus,  e.  Interval 
left  bv  the  retraction  of  the  vitellus  from  the  zona  pellucida. 


yellow  fluid  mass,  the  yolk,  which  is  surrounded  by  a  delicate 
membrane,  the  vitelline  membrane.  The  yolk  is  made  up  of  two 
varieties  of  material  of  different  shades  of  color,  the  white  and 
the  yellow  yolk.  Of  these  the  yellow  forms  the  greater  part,  the 
white  being  arranged  in  thin  layers,  which  separate  the  yellow 
yolk  into  strata.  In  the  centre  of  the  yolk  it  forms  a  flask- 
shaped  mass,  with  its  neck  turned  to  the  upper  surface,  upon 


THE   OVUM. 


655 


which  a  portion  of  the  yolk  called  the  cicatricula  rests.  This 
cicatricula,  which  lies  between  the  vitelline  membrane  and  the 
white  yolk,  is  the  active  growing  part  of  the  egg,  and  out  of  it  are 
developed  the  chick  and  the  embryonic  membranes. 

Extending  through  the  albumin  from  the  vitelline  membrane 
to  the  ends  of  the  egg  are  two  twisted  membranous  cords — the 


ff.^J. 


Diagram  of  a  section  of  an  unimpregnated  fowl's  egg.  (From  Foster  and 
Balfour,  after  Allen  Thomson.) — bl.  Blastoderm  or  cicatricula.  w.y.  White 
yolk.  y.y.  Yellow  yolk.  ch.l.  Chalaza.  i.s.m.  Inner  layer  of  shell  mem- 
brane, s.m.  Outer  layer  of  shell  membrane,  s.  Shell,  a.ch.  Air  space, 
to.  The  white  of  the  egg.  vt.  Vitelline  membrane,  x.  The  denser  albu- 
minous layer  which  lies  next  to  the  vitelline  membrane. 


chalazoe,  which  fix  and  protect  the  delicate  yolk  from  shocks,  but 
allow  it  to  rotate,  so  that  the  cicatricula  is  always  the  uppermost 
part  of  the  yolk  when  the  egg  is  on  its  side. 

The  main  structural  differences  between  the  human  ovum  and 
that  of  a  fowl  are  apparent  from  the  above  description  ;  the  essen- 
tial peculiarity  of  the  development  of  the  hen's  egg  is  that  only 
a  portion  of  the  yolk  is  engaged  in  the  formation  of  the  first  signs 


056  MANUAIi   OF    PHYSIOLOGY. 

of  the  chick  and  its  membranes,  by  far  the  greater  part  of  the 
egg,  both  yolk  and  albumin,  being  utilized  in  supplying  the  nour- 
ishment during  the  subsequent  stages  of  development. 

After  the  egg  has  been  laid,  it  obtains  no  help  from  the  out- 
side world,  except  the  oxygen  of  the  air  and  the  heat  of  the 
mother's  body  ;  it  is,  as  it  were,  fenced  in  with  a  protecting  mem- 
brane, garrisoned  with  the  quantity  of  provisions  required,  and 
by  the  warmth  of  the  hen's  body  stimulated  to  growth  and  ac- 
tivity. 

The  whole  of  the  human  ovum,  on  the  other  hand,  undergoes 
segmentation  and  differentiation  in  the  primary  formation  of  the 
embryo,  which  subsequently  is  supplied  with  the  necessary  nour- 
ishment from  the  maternal  circulation.  The  life  and  growth  of 
the  human  embryo,  in  fact,  depends  entirely  upon  supplies  from 
the  mother,  the  ovum  not  having  wnthin  itself  any  store  of  nutri- 
ent material. 

Changes  in  the  Ovum  Subsequent  to  Impregnation. 

The  first  changes  in  the  ovum  independent  of  impregnation 
consist  in  the  shrinking  of  the  yolk  from  the  vitelline  membrane, 
and  the  extrusion  from  it  of  certain  granular  bodies  which  lie 
between  it  and  the  vitelline  membrane,  and  are  called  the  polar 
globules.  The  germinal  spot  and  germinal  vesicle  also  disappear, 
and  are  thought,  by  some  observers,  to  form  these  polar  globules. 
After  the  union  of  the  male  and  female  elements,  a  new  nucleus 
appears  in  the  vitellus  which  forms  what  is  called  the  segmenta- 
tion sphere.  This  divides  at  first  into  two  segments,  then  into 
four,  eight,  sixteen,  and  so  on,  until  a  large  mass  of  cells  occupies 
the  yolk.  To  this  condition  the  name  of  morula  is  given,  from 
its  supposed  likeness  to  a  mulberry.  Fluid  now  collects  among 
the  cells,  and  separates  some  of  them  from  the  others,  and  the 
cells  arrange  themselves  into  an  outer  layer  and  an  inner  layer, 
consisting  of  different  kinds  of  cells.  The  inner  cells  finally  be- 
come aggregated  at  one  part  of  the  ovum  in  contact  with  the 
outer  cells.  The  ovum  now  receives  the  name  of  the  blastodermic 
vesicle. 


FORMATION  OF   THE   LAYERS   OF   THE   BLASTODERM.      657 

In  the  hen's  egg  the  cleavage  is  confined  to  the  cicatricula  or 
blastoderm,  and  does  not  include  the  rest  of  the  yolk.  Such  an 
ovum,  from  the  fact  that  the  cleavage  of  the  yolk  is  only  partial, 

Fig.  250. 


eJlL 


^■P- 


Sections  of  the  ovum  of  a  rabbit,  showing  the  formation  of  the  blasto- 
dermic vesicle.  (E.  Van  Beneden.) — a,  b,  c,  d,  are  ova  in  successive  stages 
of  development,  z.p,  Zona  pellucida.  eet,  Ectomeres,  or  outer  cells,  ent, 
Entomeres,  or  inner  cells. 


receives  the  name  of  meroblastic.     The  human  ovum,  which  under- 
goes complete  segmentation,  is  called  a  holobladic  ovum. 

The  cells  in  the  blastodermic  vesicle  become  arranged   into 

55 


668  MANUAL   OP   PHYSIOLOGY. 

three  definite  layers,  which  are  called  respectively,  from  their 
position  in  the  blastoderm,  the  epiblaat,  the  mesoblast,  and  the 
hypoblast. 

From  these  layers  are  developed  the  embryo  and  the  membranes 
surrounding  it,  each  layer  being  developed  into  certain  tissues 
and  forming  these  only. 

Thus  from  the  epiblast,  or  outer  layer,  arise  the  epidermis  ot 
the  skin,  the  brain  and  spinal  cord,  and  certain  parts  of  the 
organs  of  special  sense  ;  whilst  it  also  aids  in  the  formation  of  the 
chorion  and  the  amnion.  From  the  mesoblast  are  developed  the 
skeleton,  the  connective  tissues,  the  muscles,  the  nerves,  in  ad- 
dition to  the  vascular  system  and  the  supporting  tissue  of  the 


Fig.  251. 


Transverse  section  of  the  medullary  groove,  and  half  the  blastoderm  of  a 
chick  of  eighteen  hours.  (Foster  and  Balfour.) — a.  Epiblast.  b.  Meso- 
blast. c.  Hypoblast,  mf.  Medullary  fold.  mc.  Medullary  groove,  ch. 
Notochord. 

glands ;  one  kind  of  tessellated  cells  arise  from  this  layer,  viz., 
the  endothelium,  forming  the  surface  of  all  serous  membranes. 
From  the  hypoblast  springs  the  epithelial  lining  of  the  alimen- 
tary canal,  of  the  glands  which  are  diverticula  from  it,  and  of 
the  lungs ;  it  also  forms  the  lining  membrane  of  the  allantois  and 
yolk  sac. 

The  blastoderm  of  the  hen's  ovum,  which  is  comparatively 
easily  studied,  consists  of  a  small  clear  central  portion,  called  the 
area  pellucida,  from  which  the  body  of  the  chick  arises.  Sur- 
rounding the  area  pellucida  is  a  much  larger  zone,  which  appears 
less  transparent ;  this,  the  area  opaca,  is  devoted  to  the  formation 
of  the  membranes. 

The  embryo  is  developed  from  the  rest  of  the  blastoderm  in 


FOEMATION    OF   THE.  LAYERS   OF   THE    BLASTODERM.       659 

the  following  manner.  At  the  front  of  the  area  pellucida  a  fold, 
or  dipping  in  of  the  blastoderm  takes  place;  this  consists  of  a 
projecting  part  or  fold  above  and  a  groove  below,  and  consti- 
tutes what  is  known  as  the  cephalic  or  head  fold.  The  upper 
projecting  portion  of  the  fold  tends  to  grow  forwards,  whilst  the 
groove  grows  gradually  backwards.  Later  on,  another  fold 
appears  at  the  posterior  part  of  the  area  pellucida;  this  is  the 
tail  fold.  At  the  sides  of  the  area  pellucida  folds  appear,  which 
tend  to  grow  downwards  and  inwards  so  as  to  reach  the  under 
surface  of  the  blastoderm  and  unite  with  the  head  and  tail  folds. 

Fig.  252. 


r.so. 


Diagrammatic  longitudinal  section  through  the  axis  of  an  embryo  chick. 
(Foster  and  Balfour.) — N.C.  Neural  canal.  Ch.  Notochord.  B.  Foregut. 
F.So.  Somatopleure.  F.Sp.  Splanchnopleure.  Sp.  Splanchnopleure  form- 
ing the  lower  wall  of  the  foregut.  Ht.  Heart,  pp.  Pleuroperitoneal  cavity. 
Am.  Amniotic  fold.     A.  Epiblast.     B.  Mesoblast.     C.  Hypoblast. 

By  the  approximation  of  all  these  folds  a  canal  is  formed — the 
embryonal  sac — which  is  closed  above  by  the  main  portion  of  the 
area  pellucida,  in  front  by  the  head  fold,  behind  by  the  tail  fold, 
at  the  sides  by  the  lateral  folds,  whilst  below  it  is  open  to  the 
vitellus.  This  canal  ultimately  becomes  subdivided  into  an  inner 
tube,  the  alimentary  tract,  and  an  outer  one,  which  forms  the 
body  walls,  the  final  place  of  union  of  the  folds  being  marked  by 
the  umbilicus.  It  must  be  clearly  understood  that  these  primary 
folds  which  form  the  embryo  include  in  their  layers  the  epiblast, 
the  whole  thickness  of  the  mesoblast,  and  the  hypoblast,  whereas 
the  folds  giving  rise  to  the  membranes  do  not  comprehend  all 
these  layers. 


(J60 


MANUAL   OF   PHYSIOLOGY. 


Formation  of  the  Membranes. 

(1)   The  Amnion. — The  mesoblast  around  the  embryo  becomes 
thickened,  and  is  split  into  two  distinct  layers ;  this  cleavage  is  at 


A  / 


Fig. 

253 

A. 

e 

yt 

of 

/' 

iiVai -- 

IP 

^'% 

.^^^ 

vl 


vt 


I  ^i^. 


«/ 


ul- 


•iSf 


Figs.  253  A,  253  B,  and  254  are  diagrammatic  views  of  sections  through 
the  developing  ovum,  showing  tlie  formation  of  the  membranes  of  the  chick. 
(Foster  and  Balfour.) — A,  B,  C,  D,  E,  and  F  are  vertical  sections  in  the  long 
axis  of  the  embryo  at  different  periods,  showing  the  stages  of  development 
of  the  amnion  and  of  the  yolk-sac.  I.,  II.,  III.,  and  IV.  are  transverse  sec- 
tions at  about  the  same  stages  of  development,  i.,  ii.,  and  iii.  give  only  the 
posterior  part  of  the  longitudinal  section,  to  show  three  stages  in  the  forma- 
tion of  the  allantois.  e.  Embyro.  y.  Yolk.  pp.  Pleuroperitoneal  fissure. 
vt.  Vitelline  membrane.     «/.  Amniotic  food.     al.  allantois. 

first  confined  to  the  neighborhood  of  the  embryo,  but  gradually 
spreads  over  the  whole  blastoderm. 

The  upper  of  these  two  layers  of  the  blastoderm  receives  the 
name  of  the  somatopleure,  and  is  engaged  in  the  formation  of  the 
body  walls  of  the  embryo  and  the  amnion.     The  lower  one  is 


DEVELOPMENT   OF   THE    AMNION. 


661 


called  the  splanchnopleure,  and  forms  the  walls  of  the  alimentary 
canal,  the  allantois,  and  the  yolk-sac.  The  space  intervening 
between  these  layers  is  called  the  pleuroperitoneal  cavity.  At  a 
point  in  front  of  the  cephalic  fold,  an  upward  projection  of  soma- 


FiG.  253  B. 


D 


e.  Embryo,    a.  Amnion,     a''.  Alimentary  canal .    t'<.  Vitelline  membrane. 
a^.  Amniotic  fold .    ac.  Amniotic  cavity,     y.  Yolk.     al.  Allantois. 

topleure  takes  place,  conveying  with  it  the  overlying  epiblast. 
Along  the  sides  of  the  embryo  and  behind  the  caudal  fold,  pro- 
jections of  the  somatopleural  mesoblast  and  epiblast  also  occur. 


662 


MANUAL   OF   PHYSIOLOGY. 


Thus  folds  are  developed,  consisting  of  somatopleural  mesoblast 
and  of  epiblast,  which  tend  to  grow  upwards  and  meet  over  the 
back  of  the  embryo.  These  folds  are  the  amniotic  folds,  and  each 
of  them  presents  two  surfaces,  one  looking  towards  the  embryo 
and  the  other  towards  the  vitelline  membrane.  As  they  meet 
over  the  back  of  the  embryo  the  folds  become  fused,  the  mem- 
branes looking  towards  the  embryo  joining  to  form  the  amnion 

Fig.  254. 


Diagrammatic  sections  of  an  embryo,  showing  the  destiny  of  the  yolk- 
sac,  ys.  vt.  Vitelline  membrane,  pp.  Pleuroperitoneal  cavity,  ac.  Cavity 
of  the  amnion,     o.  Amnion.     a\  Alimentary  canal,     ys.  Yolk-sac. 


proper,  whilst  those  next  the  vitelline  membrane  unite  to  form 
the  false  amnion,  which,  separating  from  the  amnion  proper,  re- 
tires towards  the  vitelline  membrane,  with  which  it  unites  to  form 
the  primitive  chorion. 

The  true  amnion  then  is  a  sac  formed  of  an  outer  layer  derived 
from  the  mesoblast  and  an  inner  layer  derived  from  the  epiblast. 
The  false  amnion  likewise  consists  of  mesoblast  and  epiblast,  but 


DEVELOPMENT   OF   THE    AMNION.  663 

here  the  epiblast  is  external.  The  true  amnion  is  continuous  with 
the  skin  of  the  embryo,  and  when  the  foetus  is  mature,  the  connec- 
tion may  be  traced  by  the  umbilical  cord,  around  which  it  forms 
a  sheath  to  be  continuous  with  the  skin  at  the  umbilicus.  This 
membranous  sac  enlarges,  and  in  mammalia  eventually  becomes 
the  large  bag  of  liquid  which  contains  the  foetus.     The  amniotic 


Diagrammatic  longitudinal  section  of  a  chick  on  the  fourth  day.  (Allen 
Thomson.) — ep,  Epiblast.  hy,  Hypoblast,  sm,  Somatopleure.  v.m,  Splanch- 
nopleure.  af.pf,  Folds  of  the  amnion,  pp,  Pleuroperitoneal  cavity,  am, 
Cavity  of  amnion,  a/,  Allantois.  a,  Position  of  the  future  anus.  A,  Heart. 
i,  Intestine,  vi,  Vitelline  duct,  ys,  Yolk,  s,  Foregut.  m,  Position  of  the 
mouth,    me,  The  mesentery. 

liquid  is  of  low  specific  gravity,  consisting  mainly  of  water  con- 
taining traces  of  nitrogenous  matter,  and  also  phosphates  and 
chlorides. 

It  contains  albumin  and  some  other  nitrogenous  constituents, 
and  a  minute  quantity  of  urea,  which  is  thought  to  be  derived 
from  the  foetal  kidneys. 

This  fluid  preserves  the  child  from  the  effects  of  any  jolts  or 
jars  caused  by  the  movements  of  the  mother,  and  similarly  pro- 
tects the  uterus  of  the  mother  by  acting  as  a  buffer  between  the 
foetus  and  the  uterine  wall.  Before  delivery  it  helps  to  dilate  the 
OS  uteri,  so  that  when  the  amnion  is  ruptured  the  head  of  the 
foetus  occupies  the  opening  which  has  been  gradually  made  by 
the  fluid  wedge.     The  outer  part  of  the  amniotic  membrane,  de- 


664  MANUAL   OF    PHYSIOLOGY. 

rived  from  the  mesoblast,  is  of  a  tougher  character  than  the  inner 
epithelial  layer,  and  it  is  said  to  possess  muscular  fibre  and  to  be 
capable  of  rhythmical  contractions. 

(2)  The  Yolk  Sac  is  that  part  of  the  blastoderm  which  grows 
and  envelops  the  yolk,  which  previously  was  only  surrounded  by 
the  vitelline  membrane.  After  the  mesoblast  has  split  into  two 
layers,  the  splanchnopleure  becomes  bent  inwards  at  a  point  some 
distance  from  its  origin,  carrying  with  it  the  hypoblast.  By  this 
curve  an  upper  constricted  canal  is  differentiated  from  the  large 
lower  cavity.  This  upper  canal  becomes  eventually  the  alimen- 
tary tract,  the  lower  cavity  the  yolk-sac,  whilst  the  constricted 
portion  leading  from  the  one  to  the  other  is  the  canal  leading 
from  the  intestine  to  the  yolk,  called  the  ductus  vitello-intedinalis. 

At  first  the  splanchnopleure  incloses  only  the  upper  part  of  the 
yolk,  but  as  development  proceeds  it  grows  around,  and  at  last 
completely  encircles  it.  The  yolk-sac  is  thus  derived  from  the 
splanchnopleural  layer  of  the  mesoblast,  and  its  lining  hypoblast. 

The  yolk  is  continually  used  up  for  the  nutrition  of  the  embryo, 
and  its  covering  shrinks  in  size,  becoming  smaller  with  the  growth 
of  the  foitus,  until  eventually  it  forms  but  a  shrivelled  protrusion 
from  the  intestine,  lying  in  the  umbilical  cord. 

The  importance  of  the  yolk-sac  differs  largely  in  mammalia  and 
birds.  In  man  it  is  not  highly  developed,  as  its  place  is  early 
supplied  by  the  placenta.  In  birds,  however,  it  develops  to  a 
much  higher  degree,  being  the  seat  of  a  special  circulation,  which 
carries  nourishment  from  the  yolk  to  the  chick.  The  vessels  are 
developed  in  the  mesoblastic  portion  of  the  membrane,  and  are 
called  the  omphalo-mesenteric  vessels,  which  convey  blood  to  and 
from  the  primitive  heart. 

(3)  The  Allantois,  or  urinary  vesicle,  in  the  chick  is  of  import- 
ance, as  the  vessels  developed  in  it  are  used  for  respiratory  pur- 
poses, being  spread  out  beneath  the  porous  shell.  In  the  mamma- 
lian embryo  it  is  still  more  important,  as  it  is  the  seat  of  the 
circulation,  which  performs  the  chief  function  of  the  foetal  pla- 
centa. The  allantois  arises  at  the  tail  of  the  embryo,  as  a  budding 
outwards  of  a  portion  of  the  splanchnopleure  forming  the  wall  of 
the  primitive  intestine.     It  is  lined  by  hypoblast,  and  projects 


DEVELOPMENT   AND    FUNCTIONS   OF   THE    ALLANTOIS.       665 

into  the  pleuro-peritoneal  cavity.  As  it  grows  away  from  the 
embryo  it  extends  between  the  layers  of  the  true  and  false  amnion 
and  approaches  towards  the  vitelline  membrane,  but  remains  con- 
nected to  the  intestine  by  a  narrow  tube.  When  it  reaches  the 
periphery  of  the  ovum,  it  spreads  over  the  chorion  as  a  complete 
lining,  and  sends  processes  into  the  villi  of  that  organ.  It  becomes 
chiefly  developed,  however,  at  that  part  of  the  chorion  which  is 
opposite  the  decidua  serotina  of  the  mother.     In  the  mesoblastic 


Diagram  of  an  embryo,  showing  the  relationship  of  the  vascular  allantois 
to  the  villi  of  the  chorion.  (Cadiat.) — a,  Lies  in  cavity  of  the  amnion  under 
the  embryo,  b,  Yolk-sac.  c,  Allantois.  d,  Vessels  of  the  allantois  dipping 
into  the  villi  of  the  chorion,     e,  Chorion. 


layer  of  the  allantois  bloodvessels  arise  which  are  connected  with 
large  trunks,  proceeding  from  the  primitive  aorta,  called  the  um- 
bilical arteries ;  these  will,  however,  be  further  described  when 
treating  of  the  foetal  placenta. 

As  the  foetus  becomes  developed,  the  part  of  the  allantois  in 
connection  with  the  body  becomes  gradually  obliterated.  A  part 
of  it  remains  as  the  urinary  bladder,  and  the  rest  forms  a  fibrous 

56 


6Q6  MANUAL  OF    PHYSIOLOGY. 

cord,  which  runs  from  the  apex  of  the  bladder  to  the  umbilicus, 
and  is  known  as  the  urachus. 

(4)  r/ie  CAorj'oH  is  the  external  covering  of  the  ovum.  At  first 
it  consists  simply  of  the  zona  pellucida  or  vitelline  membrane, 
and  then  it  is  called  the  primitive  chorion.  Later,  however,  it  is 
supplemented  by  the  part  of  the  soraatopleure  removed  from  the 
embryo  in  the  process  of  forming  the  amnion.  This  blends  with 
the  primitive  chorion  and  strengthens  it,  and  while  lying  beneath 
the  zona  pellucida,  receives  the  name  of  the  subzonal  membrane. 
The  chorion  at  first  is  a  smooth  membrane,  but  villous  processes 
early  grow  out  from  it.  These  villi  are  chiefly  developed  at  its 
upper  part,  where  they  aid  in  the  formation  of  the  foetal  placenta. 

The  allantois,  when  it  has  spread  over  the  chorion,  becomes 
blended  with  this  membrane,  and  fills  the  villous  processes  with 
the  bloodvessels  it  contains. 

The  Placenta. 

The  placenta  is  a  most  important  organ  to  the  mammalian  em- 
bryo. It  conveys  not  only  nourishment,  but  also  oxygen  from 
the  maternal  blood  to  that  of  the  foetus.  It  is,  of  course,  neces- 
sary that  the  animals  whose  ova  do  not  contain  large  stores  of 
food,  should  in  some  way  provide  the  substances  necessary  for  the 
life  of  their  embryo,  and  it  is  by  means  of  the  placenta  that  this 
is  brought  about.  The  embryo  of  oviparous  animals  does  not 
require  a  placenta  for  its  nutrition,  since  inside  the  egg  is  a  large 
store  of  highly  nutritious  albuminous  and  fatty  materials  ;  the 
shell  is  pervious  to  air,  and  the  chick's  blood  can  in  the  allantois 
be  oxidized  by  the  air  directly.  A  bird's  egg  contains  in  itself 
all  the  necessaries  which  the  placenta  supplies,  and  when  impreg- 
nated only  requires  the  heat  of  the  mother's  body  to  develop  a 
chick. 

While  an  ovum  is  descending  the  Fallopian  tube,  the  mucous 
membrane  of  the  uterus  becomes  turgid,  and,  as  before  mentioned, 
if  the  ovum  be  unimpregnated  it  is  cast  out  of  the  body,  part  of 
the  substance  of  the  lining  membrane  of  the  uterus  is  desqua- 
mated and  discharged  with  a  fluid  largely  composed  of  blood. 
This  takes  place  approximately  every  four  weeks,  and  hence  is 


THE   PLACENTA. 
Fig.  257. 


667 


Series  of  diagrams  representing  the  relationship  of  the  decidua  to  the 
ovum  at  different  periods.  The  decidua  are  colored  black,  and  the  ovum  is 
shaded  transversely.  In  4  and  5  the  vascular  processes  of  the  chorion  are 
figured  (copied  from  Dalton). — 1.  Ovum  entering  the  congested  mucous 
membrane  of  the  fundus — decidua  serotina.  2.  Decidua  reflexa  growing 
round  the  ovum.  3.  Completion  of  the  decidua  around  the  ovum.  4.  Gen- 
eral growth  of  villi  of  the  chorion.  5.  Special  growth  of  villi  at  placental 
attachment,  and  atrophy  of  the  rest. 


668  MANUAL  OF   PHYSIOLOGY. 

called  menstruation.  If,  however,  the  ovum  be  impregnated,  the 
mucous  membrane  of  the  uterus  not  only  becomes  turgid,  but  its 
cells  proliferate,  and  considerable  thickening  of  the  tissue  takes 
place.  The  mucous  membrane  is  then  called  the  decidua.  When 
the  ovum  reaches  the  uterus,  it  ordinarily  becomes  imbedded  in 
that  part  of  the  decidua  which  occupies  the  fundus  of  the  uterus. 
The  decidua  here  grows  excessively,  and  becomes  much  thickened, 
and  on  either  side  of  the  ovum  a  projection  is  sent  from  the  de- 
cidua which  meets  below  the  ovum,  and  completely  encircles  it. 

To  the  membrane  lining  the  general  cavity  of  the  uterus  the 
name  decidua  vera  is  given,  whilst  that  part  lining  the  fundus,  to 
which  the  ovum  is  attached,  is  called  the  decidua  serotina,  its 
processes  surrounding  the  ovum  receiving  the  name  of  the  decidua 
reflexa. 

The  placenta  is  developed  from  two  sources,  one  arising  from 
the  membranes  of  the  foetus,  and  the  other  belonging  to  the 
mother. 

Relation  of  the  Fatal  to  Maternal  Placenta. — The  maternal  part 
is  formed  from  the  decidua  serotina,  which  becomes  much  thick- 
ened and  very  vascular  where  the  placenta  is  attached.  The 
foetal  placenta  is  derived  from  the  chorion,  which  sends  out  a 
number  of  finger-like  processes,  which  subdivide,  and  into  which 
the  allantois,  as  it  spreads  over  the  chorion,  sends  prolongations. 
The  mesoblastic  layer  of  the  allantois  gives  rise  to  the  capillaries 
which  are  in  these  processes.  The  capillaries  spring  from  the 
branches  of  the  umbilical  arteries  which  pass  along  the  umbilical 
cord  to  reach  the  chorion.  The  vessels  of  the  decidua  serotina 
or  maternal  placenta  end  in  large  sinuses,  lined  by  endothelial 
cells.  The  blood  is  carried  to  these  sinuses  by  the  uterine  arte- 
ries, and  from  them  by  the  uterine  veins.  The  walls  of  the  si- 
nuses are  provided  with  unstriped  muscular  tissue,  which  can  close 
the  inlets  from  the  arteries,  and  thus  shut  out  the  blood.  The 
villi  of  the  foetal  placenta,  dipping  into  these  uterine  sinuses,  are 
covered  with  a  single  layer  of  thin  scaly  cells,  so  that  the  foetal 
blood  is  only  separated  from  the  maternal  by  the  walls  of  the 
capillar4es  and  these  thin  cells,  and  thus  the  interchange  of  nu- 
trient materials  and  gases  readily  goes  on  between  them  ;  it  is  very 


THE   PLACENTA.  669 

similar  to  the  coaditions  of  the  lung  alveoli,  where  the  blood  is 
separated  from  the  air  with  which  it  interchanges  gases  by  the 
cells  of  the  capillary  wall  and  of  the  lung  alveolus. 

Though  the  capillaries  of  the  foetus  are  in  such  close  relation 
to  the  blood  of  the  mother,  it  must  be  distinctly  understood  that 

Fig.  258. 


Antero-posterior  section  through  a  gravid  uterus  and  ovum  of  five  weeks 
(semi-diagrammatic).  (Allen  Thomson.) — a.  Anterior  wall  of  uterus,  p.  Pos- 
terior wall  of  uterus,  m.  Muscle  substance,  g.  Glandular  layer,  ss.  De- 
cidua  serotina.  r.  Decidua  reflexa.  v.  Decidua  vera.  ch.  Chorion,  u.u. 
Uterine  cavity,     c.  Cavity  of  the  cervix. 

there  is  no  direct  communication  between  the  vessels  of  the  foetus 
and  those  of  the  mother,  and  therefore  it  is  not  possible  to  inject 
the  vessels  of  the  mother  through  those  of  the  foetus,  or  vice  versa  . 


670  MANUAL   OF    PHYSIOLOGY. 

The  nutrient  materials  frona  the  maternal  blood  together  with 
oxygen  diffuse  through  the  walls  of  the  foetal  capillaries,  the  effete 
matter,  ou  the  other  hand,  passing  from  the  capillaries  to  the 
blood  in  the  veins  which  surrounds  and  bathes  these  vessels.  The 
placenta  increases  with  the  growth  of  the  foetus  till  shortly  before 
birth,  when  it  is  said  to  undergo  a  certain  amount  of  degenera- 
tion. It  is  cast  out  of  the  uterus  after  the  expulsion  of  the  foetus 
with  the  membranes  attached  to  it.  It  is,  however,  only  the 
superficial  layer  of  the  maternal  placenta  (which  is  intimately 
connected  with  the  foetal  placenta)  that  is  cast  off,  the  deeper 
layer  remaining  in  the  uterus,  and  undergoing  various  changes 
during  the  reduction  of  this  organ  to  its  normal  size. 

After  ligature  of  the  umbilical  cord,  the  intimate  relationships 
of  the  maternal  and  foetal  circulations  cease,  and  it  is  thought 
that  this  causes  the  inlets  of  the  uterine  sinuses  to  contract,  so 
that  when  the  placenta  separates  from  the  uterine  walls,  the  arte- 
rioles leading  to  the  sinuses  are  contracted  and  possibly  occluded 
with  clots.  The  uterine  blood  current  is  thus  prevented  from 
escaping  into  the  uterine  cavity  after  parturition,  and  causing 
profuse  hsemorrhage. 

The  uses  of  the  placenta  may  be  briefly  summed  up  as: 

(1)  Alimentary,  as  it  supplies  the  place  of  the  alimentary  canal. 

(2)  Respiratory,  as  it  performs  the  function  of  the  lungs. 

(3)  Excretory,  as  it  does  duty  for  the  kidneys  and  some  other 
excretory  organs. 


CHAPTER  XXXVIII. 


DEVELOPMENT  OF  THE  SPECIAL  SYSTEMS. 


Development  of  the  Vertebral  Axis. 

The  earliest  evidence  of  the  differentiation  of  the  blastoderm 
consists  in  the  appearance  of  the  primitive  streak  which  forms  the 
first  sign  of  the  embryo.    This 

is  a  line  which  appears  near  ^^^-  2^^- 

what  is  to  be  the  tail  end  of 
the  embryo,  and  runs  for- 
wards. This  primitive  line 
or  streak  is  due  to  the  thick- 
ening of  the  mesoblast,  and 
it  becomes  converted  into  a 
groove  by  a  depression  ap- 
pearing in  its  centre,  forming 
the  primitive  groove.  This 
extends  in  a  forward  direc- 
tion, but  never  reaches  the 
head  fold  of  the  embryo, 
which,  in  the  chick,  appears 
a  few  hours  after  the  forma- 
tion of  the  primitive  groove. 

In  front  of  the  primitive 
groove,  and  stretching  back- 
wards to  overlap  it  at  the 
sides,  arise  two  folds  of  the 
epiblast,  called  the  laminse 
dorsales,  or  the  medullary 
folds. 

These  are  the  elevations  of 
the  epiblast,  beneath  which  the  mesoblast  is  thickened.     They 
arise  in  front,  where  they  are  joined  immediately  behind  the  head 


View  of  the  urea  pellucida  of  a  chick 
of  eighteen  honrs  seen  from  above. 
(Foster  and  Balfour.) — A.  Medullary 
folds,  mc.  Medullary  groove,  pr. 
Primitive  streak  and  groove. 


672 


MANUAL   OF   PHYSIOLOGY. 


fold,  whilst  posteriorly  they  diverge,  and,  passing  on  either  side 
of  the  primitive  groove,  gradually  become  lost.    Between  the  two 


Fig.  2G0. 


Transverse  section  of  the  embryo  of  a  chick  at  the  end  of  the  firet  day. 
(Kolliker.) — sp.  Mesoblast.  Pv.  Medullary  groove.  Rf.  Medullary  fold. 
d.d.  Hypoblast,  m.  Medullary  plate,  h.  Epiblast.  ch.  Chorda  dorsalis. 
uwp.  Protovertebral  plate.     uv:h.  Division  of  mesoblast. 

folds  is  a  furrow  lined  by  epiblast,  which  is  called  the  medullary 
groove. 

The  medullary  folds  growing  upwards  turn  in  towards  one 
another,  and  eventually  coalesce  at  their  line  of  meeting,  con- 

FiG.  261. 


Transverse  section  of  an  embryo  of  a  chick  at  the  latter  end  of  the  second 
day.  (Kolliker.) — rw.  Medullary  fold.  //.  Medullary  groove.  A.  Epi- 
blast. ao.  Aorta,  del.  Hypoblast,  p.  Pleuro-peritoneal  cavity,  sp.  Ex- 
ternal plate  of  mesoblast  dividing,     uwp.  Protovertebral  plate. 

verting  the  medullary  groove  into  a  channel — the  medullary 
canal;  this  union  of  the  folds  takes  place  from  before  backwards. 
The  medullary  canal  thus  formed  lies  in  the  axis  of  the  em- 
bryo on  the  uncleft  mesoblast;  it  is  covered  in  superficially  by 
several  layers  of  epiblastic  cells,  which  also  line  its  walls.     The 


DEVELOPMENT    OF   THE    SPINAL    COLUMN.  673 

canal  is  the  earliest  representative  of  the  nervous  centres,  and 
eventually  becomes  the  brain  and  spinal  cord.  The  front  part 
of  the  canal,  when  completely  closed  in,  becomes  dilated  into  a 
bulb,  thus  forming  the  earliest  indication  of  the  brain.  The 
hind  part  of  the  medullary  groove  remains  unclosed  consider- 
ably later  than  the  forepart.  It,  however,  gradually  becomes 
converted  into  a  canal  at  the  tail  end,  and  as  it  extends  back- 
wards it  obliterates  the  primitive  streak  and  groove,  which  are 
lost,  and  take  no  permanent  part  in  the  formation  of  the  embryo. 


Transverse  section  through  the  embryo  of  a  chick  on  the  second  day  where 
the  medullary  canal  is  closed.  (KoUiker.) — mr.  Medullary  canal,  h.  Epi- 
blast.  uwh.  Cavity  of  protovertebra  uw.  ung.  WolfiBan  duct.  mp.  Meso- 
blast  dividing  into  hjpl.  Somatopleure.  dj.  Splanchnopleure.  sp.  Pleuro- 
peritoneal  cavity,     dd.  Hypoblast,    c^.  Notochord. 

Beneath  the  medullary  canal  the  cells  of  the  mesoblast  are 
altered  to  form  a  rod-shaped  cellular  body,  which  following  the 
line  of  the  canal  lies  in  the  axis  of  the  embryo  ;  this  is  the  chorda 
dorsalis  or  notochord. 

Supporting  the  medullary  canal  on  either  side  of  the  chorda 
dorsalis  are  masses  of  mesoblast,  somewhat  quadrangular  in  sec- 
tion, which  are  termed  the  vertebral  plates;  continuous  with  these 
externally  are  other  thinner  masses  of  mesoblast  called  the  lateral 
plates. 

The  lateral  plates  become  divided  into  an  upper  part  or  somato- 
pleure, which  is  in  close  relationship  to.  the  epiblast,  and  a  lower 
part,  the  splanchnopleure,  which  is  next  to  the  hypoblast ;  the 
space  between  these  being  the  pleuro-peritoneal  cavity.  The  ver- 
tebral plates  become  separated  from  the  lateral  plates  by  a  longi- 


674 


MANUAL   OF    PHYSIOLOGY. 


tudinal  partition,  so  that  on  either  side  of  the  neural  canal  is  a 
mass  of  undivided  mesoblast  extending  laterally  towards  the 
divided  mesoblast. 

In  each  vertebral  plate  there  appear  transverse  vertical  inter- 
ruptions at  definite  intervals  which  split  the  plate  up  into  a  num- 

FiG.  264. 


Fig.  263. — iLinbryo  chick  at  the  end  of  the  second  day,  seen  from  below. 
(Kolliker.) — FA.  Forebrain.  ^6.  Optic  vesicles.  CA.  Notochord.  if.  Heart. 
ovi.  Omphalo-mesenteric  veins.      Vd.  Lower  opening  of  foregut. 

Fig.  264. — Division  of  the  vertebral  column  of  a  chick.  (Kolliker  after 
Remak.) — 1.  Notochord.  2.  Points  of  separation  of  the  original  protover- 
tebrae.  3.  Points  of  division  of  the  permanent  vertebrae.  4.  Arches  of  the 
vertebrae.  5.  Spinal  ganglia,  c.  Body  of  first  cervical  vertebra,  d.  One 
of  the  lower  vertebrae. 


ber  of  quadrangular  blocks  of  mesoblast,  known  as  the  protover- 
tebrce ;  the  number  of  these  corresponds  to  the  number  of  vertebrae 
of  the  animal. 

These  protovertebrse  become  subdivided  by  transverse  fissures 


DEVELOPMENT   OF   THE   SPINAL    COLUMN. 

Fig.  265i 


675 


Transverse  section  through  the  dorsal  region  of  an  embryo  chick  of  forty- 
five  hours.  (Foster  and  Balfour.) — A.  Epiblast.  M.e.  Medullary  canal. 
P.v.  Protovertebrse.  W.d.  Wolffian  duct.  p.p.  Pleuro-peritoneal  cavity. 
S.o.  Somatopleure.  S.p.  Splanchnopleure.  v.v.  Vessels,  a.o.  Aorta.  B. 
Mesobla«t.  C.  Hypoblast,  o.p.  Line  of  union  of  opaque  and  pellucid 
areas,     w.  Spheres  of  the  white  yolk. 


676  MANUAL   OF   PHYSIOLOGY. 

into  external  parts,  the  muscle  plates,  which  form  eventually  the 
dorsal  and  other  muscles,  and  internal  parts  which  become  the 
permanent  vertebrse. 

From  these  inner  portions  processes  of  mesoblast  grow  upwards 
over  the  medullary  canal  to  meet  with  processes  from  the  pro- 
tovertebrse  of  the  opposite  side.  Mesoblastic  tissue  also  grows 
inwards  between  the  medullary  canal  and  the  notochord,  and 
between  the  notochord  and  the  subjacent  hypoblast. 

These  projections  beneath  the  notochord  meet  with  projections 
from  a  mass  of  the  mesoblast,  which  lies  between  the  protover- 
tebrse  and  the  cleft  mesoblast,  which  is  known  as  the  intermediate 
cell  mass. 

The  portions  of  the  protovertebrse  above  the  medullary  canal 
form  the  arches  of  the  vertebrae;  from  those  surrounding  the 
notochord  the  bodies  of  the  vertebrae  are  developed. 

The  outer  part  of  each  protovertebra  divides  into  an  anterior 
or  pre-axial  part,  from  which  arises  the  ganglion  of  a  spinal  nerve, 
and  into  a  posterior  or  post-axial  part. 

After  this  the  original  lines  of  separations  between  the  proto- 
vertebrse  disappear,  and  the  spinal  column  is  fused  into  a  carti- 
laginous mass.  New  segmentation  now  appears  in  the  centre  of 
each  original  protovertebra,  midway  between  the  primary  divi- 
sions. Thus  the  vertebral  column  is  divided  into  a  number  of 
component  parts,  each  of  which  is  destined  to  become  a  permanent 
vertebra. 

The  vertebrae  do  not  then  correspond  to  the  original  protover- 
tebree,  but  rather  to  the  posterior  half  of  that  which  lay  in  front 
of  the  primary  division  joined  to  the  anterior  half  of  the  one  be- 
hind. The  ganglia  of  the  spinal  nerves,  therefore,  by  this  arrange- 
ment, instead  of  belonging  to  the  front  of  the  vertebra  become 
joined  to  the  posterior  part  of  the  vertebra,  to  which  they  belong. 

The  notochord  atrophies  with  ossification  of  the  vertebrae,  and 
finally  is  represented  only  by  a  mass  of  soft  cells  in  the  centre  of 
an  intervertebral  disk. 

In  connection  with  the  vertebrae  in  the  dorsal  region,  processes 
grow  horizontally,  these  are  the  rudiments  of  the  ribs. 


DEVELOPMENT    OF   THE   SPINAL   CORD. 


677 


Development  of  the  Central  Nervous  System. 

Spinal  Cord. 

Soon  after  the  closure  of  the  medullary  or  neural  canal  at  its 
anterior  or  cranial  end,  it  is  dilated  in  this  region  into  three  vesi- 
cles known  as  the  first,  second,  and  third  cerebral  vesicles,  from 
which  the  brain  is  developed.     The  spinal  cord  is  formed  from 


Fig.  266. 


Transverse  section  of  the  spinal  column  of  the  human  embryo  of  from 
nine  to  ten  weeks.  (Kolliker.) — dm.  Dura  mater,  p'.  Columns  of  GoU. 
p.  Posterior  column,  pr.  Posterior  root.  na.  Arch  of  vertebra,  g.  Gan- 
glion of  a  spinal  nerve,  a.  Anterior  column,  ar.  Anterior  root.  cA.  Noto- 
chord.  b.  Body  of  the  vertebra,  n.  Spinal  nerve,  c.  Central  canal,  e. 
Epithelium  of  canal. 


the  part  of  the  medullary  canal  which  lies  over  the  chorda  dor- 
salis.  The  medullary  canal  is  lined  by  columnar  cells  derived 
from  theepiblast,  which,  shortly  after  they  are  shut  off  from  the 
general  epiblast,  develop  at  the  sides  of  the  canal,  so  as  to  narrow 
the  lumen  of  the  tube  by  the  increase  in  thickness  of  its  sides. 


678 


MANUAL   OF   PHYSIOLOGY. 


The  upper  and  lower  parts  of  the  caual  do  not,  however,  become 
thickened.  The  lateral  walls  approximate  to  the  centre,  decreas- 
ing laterally  the  lumen  of  the  canal,  which  becomes  narrow  in 
the  middle  with  a  dilatation  above  and  below.     The  lateral  walls 

Fig.  267. 
pt  ^ 


Transverse  section  of  the  spinal  cord  of  a  chick  of  seven  days.  (Foster 
and  Balfour.) — ep.  Epithelium  lining  the  medullary  canal,  ^f.  Part  of  the 
cavity  of  the  medullary  canal  which  becomes  the  posterior  fissure,  s^c. 
Permanent  medullary  tube  or  central  canal  of  the  spinal  cord.  age.  Ante- 
rior gray  commissure,  a/.  Anterior  fissure,  not  yet  well  formed,  c.  Tissue 
filling  in  the  upper  part  of  the  posterior  Ussure.  ipc.  Cells  forming  the  pos- 
terior gray  matter  pew.  Posterior  white  column,  ct.  Mesoblast  surround- 
ing the  spinal  cord.  /cic.  Lateral  white  column,  ojchk.  Anterior  white  col- 
umn.    «c.  Cells  forming  the  anterior  gray  matter. 


of  the  canal,  thus  approximated,  unite  in  their  centre,  and  con- 
vert the  medullary  canal  into  two  separate  tubes,  a  dorsal  and  a 
ventral. 

The  lower  or  ventral  tube  of  the  divided  canal  becomes  the 


DEVELOPMENT   OF   THE    BRAIN.  679 

central  canal  of  the  spinal  cord,  and  the  columnar  cells  of  the 
epiblast  form  a  lining  of  ciliated  columnar  epithelium. 

The  epiblast  at  the  lower  part  of  the  canal  becomes  converted 
into  the  anterior  gray  columns,  in  connection  with  which  arise 
the  anterior  roots  of  the  spinal  nerves ;  whilst  at  the  upper  part 
the  posterior  gray  columns  are  formed  in  connection  with  the  pos- 
terior roots  of  the  spinal  nerves  and  their  ganglia. 

The  white  columns  are  thought  by  some  authors  to  be  derived 
from  the  mesoblast  surrounding  the  canal,  but  by  others  they  are 
assigned  to  the  epiblast. 

The  upper  or  dorsal  canal  becomes  converted  into  a  fissure  by 
the  absorption  of  its  root,  and  is  thus  changed  into  the  posterior 
fissure  of  the  spinal  cord. 

The  anterior  fissure  is  formed  by  the  downgrowth  of  the  ante- 
rior columns,  which  diverge,  leaving  between  them  an  interval 
which  becomes  occupied  by  the  pia  mater. 

The  commissures  are  not  formed  between  the  lateral  halves  of 
the  cord  until  later.     The  gray  commissure  appears  first. 

The  Brain. 

Anterior  cerebral  vesicle. — As  already  mentioned,  the  brain  is 
formed  from  the  primitive  neural  canal,  the  anterior  part  of 
which  becomes  dilated  into  three  little  swellings  called  the  ante- 
rior, middle,  and  posterior  cerebral  vesicles.  From  the  anterior, 
or  first  cerebral  vesicle,  at  an  early  period  spring  two  processes, 
which  become  the  optic  vesicles.  These  ultimately  become  de- 
veloped into  the  retina  and  other  nervous  parts  of  the  eye,  with 
the  history  of  which  the  changes  occurring  in  them  will  be  de- 
scribed. 

The  optic  vesicles  are  displaced  downwards  by  two  processes 
growing  forwards  from  the  anterior  cerebral  vesicle,  which 
becomes  divided  into  two  parts,  the  anterior  of  which  is  subse- 
quently developed  into  the  cerebral  hemispheres  and  the  olfac- 
tory lobes,  while  the  hinder  part  receives  the  name  of  thalamen- 
cephalon. 

The  cavity  of  the  thalamencephalon  opens  behind  into  the 


680 


MANUAL   OF   PHYSIOLOGY. 


cavity  of  the  middle  cerebral  hemisphere,  and  in  front  it  commu- 

FiG.  268. 


Diagram  of  the  cerebral  vesicles  of  the  Ijrain  of  a  chick  at  the  second  day. 
(Cadiat.) — 1,  2,  3,  Cerebral  vesicles.     0.  Optic  vesicles. 

nicates  with  the  hollow  rudiments  of  the  cerebral  hemispheres, 


Diagram  of  a  vertical  longitudinal  section  of  the"  developing  brain  of  a 
vertebrate  animal,  showing  the  relation  of  the  three  cerebral  vesicles  to  the 
different  parts  of  the  adult  brain.  (Huxley.) — 0//'.  Olfactory  loTjes.  F.M. 
Foramen  of  Monro.  C.^S".  Corpus  striatum.  TA.  Optic  thalamus.  Pn..  Pineal 
gland.  3/.6.  Mid-brain.  C6.  Cerebellum.  J/. 0.  Medulla  oblongata.  Hmp. 
Central  hemispheres.  TAJ5^.  Thalamencephalon.  Py.  Pituitary  body.  C.Q. 
Corpora  quadrigemina.  C'.C.  Crura  cerebri.  P.  F.  Pons  Varolii.  /. — XII. 
Regions  from  which  spring  the  cranial  nerves.  1.  Olfactory  ventricle.  2. 
Lateral  ventricle.     3.  Hind  ventricle.     4.  Fourth  ventricle. 

and  eventually  it  becomes  the  cavity  of  the  third  ventricle.     The 


DEVELOPMENT   OF   THE   BRAIN, 


681 


floor  of  the  thalamencepbalon  is  ultimately  developed  into  the 
optic  chiasma  and  part  of  the  optic  nerves,  as  well  as  the  infun- 
dibulum.  The  latter  comes  in  contact  with  a  process  from  the 
mouth,  which  is  ultimately  changed  into  the  pituitary  body.  The 
anterior  part  of  the  roof  of  the  thalamencepbalon  becomes  very 
thin,  and  its  place  is  finally  occupied  by  a  vascular  plexus,  which 


-jcs 


M6 


Diagram  of  a  horizontal  section  of  a  vertebrate  brain.  (Hnxley.) — Olf. 
Olfactory  lobes.  L.t.  Lamina  terminalis.  C.S.  Corpus  striatum.  Th.  Optic 
thalamus.  Pn.  Pineal  gland.  3f.b.  Mid-brain.  Cb.  Cerebellum.  3f.O. 
Medulla  oblongata.  1.  Olfactory  ventricle.  2.  Lateral  ventricle.  3.  Hind 
ventricle.     4.  Fourth  ventricle.     +  Iter  a  tertio  ad  quartum  ventriculum. 

persists  in  the  roof  of  the  third  ventricle  (choroid  plexus).  The 
pineal  gland — a  peculiar  outgrowth,  of  unknown  function —  is  de- 
veloped from  the  posterior  part  of  the  roof  of  the  thalamence- 
pbalon, and  from  its  sides,  which  become  extremely  thickened, 
are  developed  the  optic  ihalami. 

57 


682 


MANUAL   OF   PHYSIOLOGY. 


The  primitive  cerebral  hemispheres  first  appear  as  two  lobes 
growing  out  from  the  front  of  the  anterior  part  of  the  first  cerebral 


Fig.  271. 


Chick  on  the  third  day,  seen  from  beneath  as  a  transparent  object,  the 
head  being  turned  to  one  side.  (Foster  and  Balfour.) — a''.  False  amnion, 
a.  Amnion.  CH.  Cerebral  hemisphere.  F.B.,  M.B.,  H.B.  Anterior,  Mid- 
dle, and  Posterior  cerebral  vesicles,  op.  Optic  vesicle,  ot.  Auditory  vesi- 
cle, ofv.  Omphalo-mesenteric  veins.  Ht.  Heart.  Ao.  Bulbus  arteriosus. 
Ch.  Notochord.  Of.a.  Omphalo-mesenteric  arteries.  Pv.  Protovertebrae. 
X.  Point  of  divergence  of  the  splanchnopleural  folds,  y.  Termination  of  the 
fore-gut,  V. 

vesicle.    The  floor  of  these  lobes  thickens  to  give  rise  to  the  cor- 
pora striata,  and  the  roof  develops  into  the  hemispheres  proper. 


THE   INTESTINAL   CANAL.  683 

The  cavities  of  these  lobes  become  the  lateral  ventricles,  and  are 
connected  by  means  of  the  foramen  of  Monro,  which  at  the  earlier 
periods  is  very  wide,  but  subsequently  becomes  narrowed  to  a 
mere  slit.  The  cerebral  hemispheres  are  separated  by  the  in- 
growth of  a  septum,  which  is  ultimately  formed  into  the  falx  cerebri. 
The  hemispheres  are  then  greatly  enlarged  in  the  backward  di- 
rection, so  that  they  quite  overlap  the  thalameucephalon  and  the 
parts  developed  from  the  middle  cerebral  vesicle.  The  corpus 
callosum  is  subsequently  formed  by  the  fusion  of  the  juxtaposed 
parts  of  the  hemispheres. 

From  the  anterior  part  of  the  cerebral  hemispheres  arise  two 
prolongations,  which  develop  into  the  olfactory  bulbs,  these  grow 
forwards,  and  soon  lose  their  cavities  which  at  first  communicated 
with  those  of  the  ventricles. 

Middle  cerebral  vesicle. — By  the  cranial  flexure  the  brain  is  bent 
at  the  junction  of  the  first  and  second  cerebral  vesicles,  the  first 
cerebral  vesicle  is  thus  turned  downwards,  leaving  the  second 
vesicle  as  the  most  anterior  part  of  the  brain. 

The  upper  walls  of  the  second  cerebral  vesicle  are  developed 
into  the  corpora  quadrigemina. 

The  cavity  of  this  vesicle  persists  as  a  narrow  channel,  and 
forms  a  communication  between  the  third  ventricle  in  front  and 
the  fourth  ventricle  behind,  and  receives  the  name  in  the  adult 
brain  of  the  iter  a  terlio  ad  quartum  ventriculum.  The  crura  cere- 
bri arise  from  the  lower  wall  of  this  middle  vesicle. 

The  third  cerebral  vesicle  is  divided  into  an  anterior  and  a  pos- 
terior part.  From  the  upper  part  of  the  anterior  division  arises 
the  cerebellum,  and  from  its  lower  part  the  pons  Varolii. 

The  posterior  division  gives  rise  to  the  medulla  oblongata. 

The  cavity  of  this  vesicle  is  called  the  fourth  ventricle.  It  is 
continuous  with  the  central  canal  of  the  spinal  cord.  Its  upper 
wall  is  thinned  and  forms  the  valve  Vieussens.  It  communicates 
with  the  subarachnoid  space  through  the  foramen  of  Majendie. 

The  Alimentary  Canal  and  its  Appendages. 

When  the  blastoderm  is  bent  at  its  anterior  extremity  to  form 
the  cephalic  fold,  it  closes  in  and  forms  the  anterior  boundary  of 


684 


MANUAL   OF   PHYSIOLOGY. 


a  short  canal,  the  upper  wall  of  which  is  formed  by  the  general 
blastoderm,  and  its  lower  wall  by  that  part  of  the  splanchnopleure 
which  runs  backwards,  leaving  the  somatopleure  to  form  the 
pleuro-peritoneal  space.  It  then  turns  forwards  to  meet  with 
the  uncleft  mesoblast,  forming  the  wall  of  the  yolk-sac,  which 
communicates  freely  with  this  rudimentary  part  of  the  alimentary 
tract. 

This  canal  becomes  closed  in  for  a  considerable  extent,  and  is 
then  called  the  fore-gut.     It  is  the  precursor  of  the  pharynx,  the 


Fig.  272. 


Alimentary  canal  of  an  embryo  wliilst  the  rudimentary  mid-gut  is  still  in 
continuity  with  the  yolk-sac.  (Kolliker  after  Bischoff.) — A.  Vieivfrom  below  : 
a.  Pharyngeal  plates,  b.  The  pharynx,  c.e.  Diverticula  forming  the  lungs. 
d.  The  stomach.  /.  Diverticula  of  the  liver,  g.  Membrane  torn  from  the 
yolk-sac.  h.  Hind-gut.  B.  Longitudinal  section:  a.  Diverticulum  of  a 
lung.     b.  Stomach,     c.  Liver,     d.  Yolk-sac. 

lungs,  the  oesophagus,  the  stomach,  and  the  duodenum.  The 
mouth,  which  at  this  period  is  unformed,  is  developed  later  by  an 
involution  of  the  epiblast  and  the  removal  of  the  tissue  between 
the  fore-gut  and  the  buccal  cavity. 

The  tail  fold,  in  a  somewhat  similar  manner,  shuts  off  a  canal 
called  the  hind-gxd,  which  becomes  developed  into  the  posterior 


THE   INTESTINAL,   CANAL. 


685 


part  of  the  alimentary  canal.  This  hind-gut  until  the  further  de- 
velopment of  the  bladder,  etc.,  is  in  connection  with  the  allantois 
which  arises  as  a  bud  from  the  lower  part  of  the  rudimentary 
hind-gut. 

Between  these  two  canals  an  intermediate  one  is  formed  by  the 
splanchnopleure  of  the  mesoblast,  which,  at  a  distance  from  its 
origin,  becomes  constricted,  and  shuts  off  an  upper  canal,  the  mid- 


Fig.  273. 


Position  of  the  various  parts  of  the  alimentary  canal  at  different  stages. 
A.  Embryo  of  five  weeks ;  B.  Of  eight  weeks ;  C.  Of  ten  weeks.  (Allen 
Thomson.) — /.  Pharynx,  s.  Stomach,  i.  Small  intestine.  i\  Large  in- 
testine. (/.  Genital  duct.  u.  Bladder,  cl.  Cloaca,  c.  Caecum,  vi.  Ductus 
vitello  intestinalis.     si.  Urogenital  sinus,     v.  Yolk-sac. 

gut,  from  a  lower  larger  organ,  the  yolk-sac,  the  connection  be- 
tween the  two  forming  the  ductus  vitello-intestinalis. 

Thus  the  primitive  alimentary  canal  consists  of  an  anterior  and 
a  posterior  canal,  which  are  closed  below,  and  a  canal  interme- 
diate between  these,  which  opens  at  its  lower  surface  into  the  yolk- 
sac. 

As  the  placental  circulation  becomes  more  and  more  developed, 
80  the  yolk-sac  .shrinks  and  atrophies,  until  at  last  it  is  represented 
by  a  fold  of  tissue  connected  with  the  primitive  intestine.     The 


686 


MANUAL   OF   PHYSIOLOGY. 


ductus  vitello-intestinalis  accordingly  becomes  obliterated,  and 
thus  the  mid-gut  is  closed  at  its  lower  aspect. 

The  primitive  intestine  placed  at  the  inferior  aspect  of  the  em- 
bryo, just  below  the  protovertebrai,  is  lined  internally  by  hypo- 
blast, and  covered  externally  by  raesoblast.  The  cephalic  or 
anterior  extremity  of  the  canal  is  formed  by  uncleft  mesoblast ; 
the  rest  of  the  canal  is  formed  by  the  splanchnopleural  layer  of 
the  mesoblast. 

A  dilatation  of  a  part  of  the  fore-gut  gives  origin  to  the  primi- 
tive stomach  ;  this  is  quite  straight  at  first,  lying  below  the  ver- 

Fio.  274. 


Longitudinal  section  of  a  foetal  sheep. 
Commencement  of  Diaphragm,  c.  Heart, 
ynx.    /.  Origin  of  lung.     g.  Liver. 


(Cadiat.) — o.  Pericardium,     h. 
d.  Branchial  arches,     e.  Phar- 


tebral  column,  with  which  it  is  connected  by  mesoblast.  After 
a  time  the  stomach  becomes  turned  to  the  right  side,  so  that  the 
lefl  surface  of  the  organ  comes  to  lie  anteriorly  and  the  right  sur- 
face posteriorly,  the  mesoblast  connecting  it  with  the  vertebral 
column,  being  developed  into  the  peritoneal  processes  of  the  organ. 


THE  INTESTINAL   CANAL. 


687 


:  The  lower  part  of  the  fore-gut  is  of  much  smaller  calibre  than 
the  dilated  portion  forming  the  stomach ;  it  becomes  the  duode- 
num, in  connection  with  which  arise  two  important  viscera,  the 
liver  and  the  pancreas. 

The  mid-gut  and  hiud-gut  form  the  small  and  large  intestines, 
these  being  at  first  one  straight  tube,  of  which  the  small  intestine 
has  the  larger  calibre.     The  small  intestine,  as  it  grows,  falls  into 


Fig.  275. 


Diagram  of  the  alimentary  canal  of  a  chick  at  the  fourth  day.  (Foster 
and  Balfour,  after  Gotte.) — Ig.  Diverticulum  of  one  lung.  St.  Stomach.  I. 
liver,    p.  Pancreas. 

folds,  and  the  mesoblast  connecting  it  to  the  vertebral  column 
forms  the  mesentery. 

The  large  intestine  is  at  first  a  straight  tube  lying  to  the  left  of 
the  embryo ;  it  becomes  bent,  and  part  of  the  tube  is  directed 
towards  the  right  side ;  this  develops  another  flexure,  the  portion 
of  intestine  below  which  grows  downwards.  Thus  that  part  re- 
maining on  the  left  side  forms  the  rectum,  the  sigmoid  flexure, 
and  the  descending  colon  ;  whilst  that  part  between  the  flexures 
becomes  the  transverse  colon,  and  that  on  the  right  side  the  as- 
cending colon. 

The  caecum  is  developed  from  the  ascending  colon,  the  ileo-csecal 
valve  arising  and  shutting  off"  the  one  part  of  the  intestinal  canal 
from  the  other.     The  vermiform  appendix  originates  from  the 


688  MANUAL  OF   PHYSIOLOGY. 

inferior  extremity  of  the  csecum,  which,  owing  to  its  feeble  growth, 
is  of  much  smaller  calibre  than  the  upper  part. 

The  epithelial  lining  of  the  intestines  is  derived  from  the  hypo- 
blast, and  the  muscular,  vascular,  connective  tissue,  and  serous 
coverings  are  mesoblastic  in  their  origin. 

The  liver  is  developed  from  two  diverticula  of  the  duodenum, 
in  connection  with  which  arise  cylinders  of  cells.  The  hypoblast 
develops  into  the  liver  cells  and  the  cells  lining  the  ducts,  the 
mesoblast  furnishing  the  vascular  and  connective  tissue  parts  of 
the  organ.  The  two  diverticula  are  connected  by  a  transverse 
piece,  and  form  the  right  and  left  lobes  of  the  liver. 

The  process  connecting  the  liver  to  the  duodenum  forms  the 
common  bile  duct,  and  from  this  the  gall  bladder  is  developed  as 
an  outgrowth. 

The  vessels  of  the  embryo  which  are  in  relation  to  the  liver 
will  be  described  under  the  vascular  system. 

The  pancreas  arises  as  an  outgrowth  from  the  duodenum,  its 
constituent  parts  originating  in  a  manner  similar  to  those  of  the 
liver. 

The  spleen  is  derived  from  the  mesoblast,  and  is  developed  in 
one  of  the  peritoneal  processes  of  the  stomach. 

The  lungs  are  developed  in  connection  with  the  oesophagus,  of 
which  they  are  early  outgrowths. 

The  canal  of  the  fore-gut  at  a  certain  point  becomes  laterally 
constricted,  its  transverse  section  presenting  an  hour-glass  shape, 
consisting  of  an  upper,  and  a  lower  dilated  portion,  united  by  a 
central  constricted  neck.  The  lower  of  these  cavities  becomes 
subdivided  by  the  outgrowth  of  the  lateral  portions  and  the  up- 
growth of  a  part  of  the  lower  wall  which  forms  a  central  septum, 
so  that  the  fore-gut  comes  to  be  composed  of  an  upper  undivided 
tube,  giving  ofi'two  appendages. 

These  appendages  consist  of  hypoblastic  tissue,  and  as  they 
grow  into  the  surrounding  mesoblast  they  divide  and  subdivide, 
until  at  last  they  come  to  consist  of  very  minute  tubules,  which 
terminate  in  dilated  extremities.  The  undivided  canal  forms  the 
permanent  trachea,  the  appendages  the  main  bronchi,  whilst 


THE    WOLFFIAN   DUCT.  689 

their  minute  subdivisions  are  the  bronchioles,  which  terminate 
in  the  dilated  alveoli. 

The  hypoblast  forms  the  delicate  lining  membrane  of  the  air- 
passages,  and  the  mesoblast  gives  rise  to  the  supporting  tissue 
holding  them  together,  as  well  as  to  the  bloodvessels,  the  muscu- 
lar, cartilaginous,  and  connective  tissue  of  the  bronchial  tubes. 

The  pleurse  surrounding  the  lungs  are  like  the  other  serous 
membranes,  also  mesoblastic  in  their  origin. 

Genito-urinaey  Apparatus. 

In  the  interval  between  the  protovertebrae  and  the  cleavage  of 
the  mesoblast  into  its  somatopleural  and  splanchnopleural  layers, 
there  appears  a  mass  of  cells,  which  arrange  themselves  into  the 

Fig.  276. 


Transverse  section  through  the  embryo  of  a  chick  on  the  second  day  where 
the  medullary  canal  is  closed.  (Kolliker.) — mr.  Medullary  canal.  A.  Epi- 
blast.  nwh.  Cavity  of  protovertebra  nw.  ung.  Wolffian  duct.  mp.  Meso- 
blast dividing  into  hpl.  Somatopleure.  df.  Splanchnopleure.  sp.  Pleuro- 
peritoneal  cavity,     dd.  Hypoblast,     ch.  Notochord. 

form  ox  a  ridge.  This  ridge,  which  lies  beneath  the  epiblast, 
becomes  hollow,  and  thus  a  tube  is  produced,  which  is  called  the 
Wolffian  duct. 

From  this  tube  diverticula  arise,  which  extend  into  the  sur- 
rounding mesoblast ;  they  are  tubular,  and  communicate  with  the 
central  duct,  whence  they  arise.  The  processes  become  twisted, 
and  at  their  extremities  the  neighboring  mesoblast  undergoes  dif- 
ferentiation, and  forms  vascular  capsules  corresponding  in  struc- 
ture to  the  Malpighian  corpuscles.     This  part  of  the  WolflBan 

58 


690 


MANUAL   OP   PHYSIOLOGY. 


duct,  which  has  acquired  a  glandular  structure,  is  the  Wolffian 
body  or  primitive  kidney  of  the  embryo,  whilst  the  Wolffian  duct 
corresponds  to  the  primitive  ureter. 

The  epithelium  lining  the  interval  between  the  somatopleure 
and  splanehnopleure  (pleuro-peritoneal  cavity),  close  to  their 
origin  from  the  uncleft  mesoblast,  becomes  columnar  in  character. 

Fig.  277. 


Section  of  the  inner  part  of  the  pleuro-peritoneal  cavity  through  the 
origin  of  the  genito-urinary  organs.  (Waldeyer.) — L.  Somatopleure.  m. 
Splanehnopleure  a.  Germinal  epithelium.  C,  o.  Primitive  ova.  E  Meso- 
blast forming  the  ovary.  WK.  WolfiBan  body.  y.  Wolffian  duct.  a^.  Epi- 
thelium giving  rise  to  the  duct  of  Miiller  z. 


It  receives  the  name  of  the  germinal  epithelium.  An  involution 
of  this  takes  place  into  the  mesoblast,  just  below  the  somato- 
pleure, and  becomes  shut  off,  and  forms  a  hollow  cylinder. 

By  this  means  a  second  duct  is  formed  in  close  relation  to  the 


THE    MULLERIAN    DUCT. 


691 


first;  this  is  the  Mullerian  dxict.     This  duct  is  developed  from 
before  backwards. 

According  as  the  embryo  is  a  male  or  a  female,  so  one  or  other 
of  these  ducts  develops.     In  the  male  the  Wolffian  duct  remains 


Fig.  278. 


Transverse  section  through  the  lumbar  region  of  an  embryo  cliick  at  the 
end  of  the  fourth  day.  (Foster  and  Balfour.)  — TF.72.  Wolffian  ridge,  (j.e. 
Germinal  epithelium.  ^1.0.  Dorsal  aorta.  J/.  Mesentery.  /S'P.  Splanch- 
nopleure.  d.  Alimentary  canal.  V.  Vessels,  m.p.  Commencing  Miil- 
lerian  duct.  So.  Somatopleure.  W.b.  Wolffian  body.  W.d.  Wolffian 
duct.  V.cM.  Posterior  cardinal  vein.  c./t.  Notochord.  A.  W.C  Anterior 
white  column  of  spinal  cord.  a.r.  Anterior  root.  A.G.C.  Anterior  gray 
column,    p.r.  Posterior  root.     m.p.  Muscle  plate,     nc.  Canal  of  spinal  cord. 

as  the  vas  deferens,  and  the  Mullerian  duct  becomes  atrophied. 
In  the  female,  on  the  other  hand,  the  Mullerian  duct  forms  the 


692 


MANUAL   OF    PHYSIOLOGY. 


organs  for  the  conveyance  of  the  ova  out  of  the  body,  and  the 
Wolffian  duct  is  represented  by  a  rudimentary  structure  near  the 
ovary. 

Part,  however,  of  the  Wolffian  duct  in  both   sexes  develops 
similarly  ;  this,  the  metanephros,  corresponds  to  that  part  of  the 

Fig.  279. 


Diagram  of  the  genital  organs  of  an  embryo  previous  to  ?exiial  distinc- 
tion. (Allen  Thomson.)— PF.  WolflSan  body.  3.  Ureter.  4.  Bladder.  5. 
Urachus.  jrc.  Genital  cord.  cp.  Clitoris,  or  penis,  m.  Miillerianduct.  w. 
Wolffian  duct.  i.  Intestine,  ug.  Urogenital  sinus,  cl.  Cloaca.  Is.  Part 
from  which  the  scrotum  or  the  labia  majora  are  developed,  ot.  Origin  of 
the  ovary  or  testicle  respectively,  x.  Part  of  WolflSan  body  subsequently 
developed  into  the  coni  vasculosi. 


duct  nearest  to  the  tail  end  of  the  embryo.  It  forms  part  of  the 
urinary  organs,  and  develops  into  the  permanent  ureter  and  the 
kidney. 

From  the  metanephros  a  projection  arises,  which  grows  quickly, 
and  opens  into  the  cloaca  ;  this  remains  as  the  ureter.     From  the 


THE   TESTICLE.  693 

upper  part  of  the  ureter  arise  small  csecal  evolutions,  which  be- 
come convoluted  at  certain  points  and  surrounded  by  mesoblast ; 
these  canals  are  the  urinary  tubules,  and  at  the  extremity  of  each 
is  developed  a  tuft  of  vessels,  which  thus  forms  a  Malpighiau 
corpuscle. 

The  straight  tubes  group  themselves  together  at  the  inner  part 
of  the  gland,  whilst  the  convoluted  tubules,  with  the  Malpighian 
corpuscles,  are  aggregated  at  the  periphery  of  the  gland. 

At  the  junction  of  the  ureter  to  the  glandular  mass,  changes 
take  place  by  which  this  tube  is  split  up  into  several  subdivisions, 
which  are  the  primary  calices  of  the  kidney,  the  dilated  part  of 
the  ureter  forming  the  pelvis. 

The  testicle  arises  partly  from  the  germinal  epithelium  lining 
the  inner  extremity  of  the  pleuro-peritoneal  cavity,  lying  close  to 
the  splanchnopleure,  and  partly  from  the  mesoblast  surrounding 
the  Wolffian  body. 

The  germinal  epithelium,  the  cells  of  which  are  not  so  well 
devoloped  as  in  the  female,  sends  processes  into  the  mesoblast, 
and  these  are  said  to  form  the  spermatic  cells,  the  mesoblast  be- 
coming differentiated  around  them  to  form  the  walls  of  the  tubuli 
seminiferi. 

The  Wolffian  duct,  which  persists  as  the  vas  deferens,  aids  in 
forming  the  testicle,  the  epididymis  being  merely  a  convoluted 
part  of  it,  and  the  vas  aberrans  one  of  the  csecal  tubes  in  con- 
nection with  the  duct.  The  coni  vasculosi  are  thought  to  be 
formed  from  some  of  the  tubules  of  the  Wolffian  body  ;  they  are 
connected  to  the  testicle  by  means  of  a  tube  which  is  itself  split 
up  into  a  number  of  divisions  which  form  the  vasa  efferentia. 

The  Wolffian  duct  forms,  besides  the  vas  deferens,  the  vesicula 
serainalis  (which  is  merely  a  blind  diverticulum  from  its  extrem- 
ity), and  terminates  in  the  ejaculatory  duct. 

The  two  Miillerian  ducts,  in  the  male,  join  and  form  a  single 
tube;  this  is  not  further  developed,  but  atrophies,  leaving  as  its 
representative  the  sinus  pocularis,  which  is  situated  in  the  floor 
of  the  prostate.  The  upper  extremities  of  the  Miillerian  ducts 
form  the  hydatids  of  Morgagni. 

The  ovary,  like  the  testicle,  is  formed  from  the  germinal  epithe- 


694 


JfANUAL   OF   PHYSIOLOGY. 


lium,  which  multiplies  and  forms  a  projection  close  to  the  Wolf- 
fian body.  The  cells  of  the  epithelium  become  involuted  and 
surrounded  by  the  uncleft  mesoblast,  to  form  ova  and  Graafian 
follicles.     The  glandular  part  of  the  ovary  thus  arises  from  the 

Fig.  280. 


Diagram  of  the  sexual  organs  of  the  male  embryo.  (Allen  Thomson.) — 
3.  Ureter.  4.  Bladder.  5.  Urachns.  t.  Testicle,  m.  Atrophied  duct  of 
Miiller  (hydatid  of  Morgagni).  e.  Epididymis,  g.  Gubernaculum  testis. 
vs.  Vesicula  seminalis.  i.  Intestine,  pr.  Prostate.  W.  Organ  of  Giraldds. 
vh.  Vas  aberrans.  vd.  Vas  deferens.  C.  Cowper's  gland,  cp.  Penis,  sp. 
Spongy  part  of  the  urethra,  t^.  Position  tlie  testicle  ultimately  assumes, 
s.  .Scrotum. 


germinal  epithelium,  and  its  stroma  springs  from  the  mesoblast 
in  the  neighborhood  of  the  Wolffian  body. 

The  ducts  of  Miiller  are  the  precursors  of  the  female  genital 
passages.  They  approach  one  another  and  unite  along  a  certain 
distance  at  their  lower  extremities.  Of  this  united  part,  the  upper 
end  forms  the  uterus,  and  the  lower  the  vagina,  whilst  the  un- 
united parts  of  the  Miillerian  ducts  form  the  Fallopian  tubes, 


FEMALE    EMBRYO. 


695 


which  become  counected  to  the  ovaries,  whilst  their  cavities  re- 
main continuous  with  the  pleuro-peritoneal  space. 

In  the  female  the  Wolffian  duct  and  body  atrophy,  the  paro- 
varium being  in  the  adult  the  representative  of  the  Wolffian 
body. 

The  bladder  is  merely  a  dilated  portion  of  that  part  of  the 
allantois  which  is  in  immediate  connection  with  the  alimentary 

Fig.  281. 


Diagram  of  the  sexual  organs  of  a  female  embryo.  (Allen  Thomson.) — 
/.  Fimbriated  extremity  of  the  left  Fallopian  tube.  W.  Kemains  of  the 
WolflBan  tubes,  g.  Kound  ligaments,  o.  Ovary,  po.  Parovarium,  u. 
Uterus.  dG.  Kemains  of  Wolffian  duct,  or  duct  of  Gaertner.  m.  Right 
Fallopian  tube  cut  short,  iv.  Right  obliterated  Wolffian  duct.  va.  Vagina. 
3.  Ureter.  4.  Bladder.  5.  Urachus.  h.  Inferior  opening  of  vagina.  C. 
Gland  of  Bartholin,  v.  Vulva,  sc.  Vascular  bulb.  cc.  Clitoris,  n.  Nympha. 
I.  Labium,     i.  Rectum. 


canal,  and  the  urachus  is  the  narrowed  part  of  the  allantois  con- 
necting the  bladder  to  the  remainder  of  the  allantois  which  is 
without  the  body  walls  of  the  foetus. 

While  the  alimentary  canal  is  in  connection  with  the  allantois, 
the  intestinal  and  genito-urinary  pa.ssages  open  into  a  common 
cavity  at  their  termination ;  this  is  the  cloaca,  and  it  is  in  the 
further  development  of  the  embryo  that  a  septum  arises,  dividing 


696  MANUAL   OF    PHYSIOLOGY. 

this  iutoan  alimentary  or  aual  portion,  and  an  anterior  or  urinary 
portion.  The  septum,  dividing  the  urogenitary  from  the  alimen- 
tary portion  of  the  cloaca,  forms,  externally,  the  perinieura. 

At  the  aperture  of  the  cloaca  an  eminence  arises  which  develops 
into  the  penis  in  the  male,  the  clitoris  in  the  female.  Around 
this  eminence  is  a  fold  of  integuments,  which  forms  the  labia  in 
the  female,  the  scrotum  in  the  male. 

In  the  female  this  integumentary  covering  enlarges  much  more 
than  the  clitoris,  and  covers  it  in,  the  urethral  orifice  opening 
just  below  the  clitoris. 

In  the  male  the  urethral  orifice  at  first  opens  at  the  base  of  the 
penis,  but  eventually  a  groove  is  formed  on  the  under  surface  of 
this  organ,  which  becomes  converted  into  a  canal,  and  forms  the 
urethra. 

Blood- Vascular  System. 

In  the  mammalian  embryo  this  may  be  appropriately  divided 
into  two  systems  of  different  dates ;  the  first,  or  early  circulation, 
which  is  confined  to  the  yolk  sac ;  and  the  second,  or  later  circu- 
lation, which  passes  through  the  placenta. 

The  Primitive  Heart  arises  from  the  splanchnopleural  layer  of 
the  raesoblast,  just  at  the  point  where  this  forms  the  under  wall 
of  the  forepart  of  the  alimentary  canal.  When  the  formation  of 
the  folds  of  the  embryo  was  described,  it  was  stated  that  the  groove 
of  the  cephalic  fold  tended  to  grow  backwards  towards  the  tail 
end  of  the  embryo.  This  groove  is  limited  behind  by  the  somato- 
pleural layer  of  the  mesoblast,  and  posteriorly  to  this  is  a  cavity 
formed  by  the  cleavage  of  the  mesoblast,  called  the  pleuroperi- 
toneal  cavity.  In  the  early  stages  of  development,  the  posterior 
wall  of  this  small  cavity  is  formed  by  the  splanchnopleural  layer 
of  the  mesoblast.  The  heart  arises  at  the  point  at  which  the 
splanchnopleure  tends  to  travel  forwards  to  meet  the  uncleft  me- 
soblast, and  thus  completes  the  pleuro-peritoneal  cavity. 

The  heart  consists  at  first  of  a  single  cylinder,  which,  in  the 
human  embryo,  probably  is  formed  by  the  coalescence  of  two  pri- 
mary tubes.  At  first  it  has  no  distinct  cavity,  but  soon  the  cells 
of  the  mesoblast  within  the  mass  forming  the  heart  become  trans- 


BLOOD-VASCULAR   SYSTEM, 


697 


formed  into  blood  corpuscles,  and  thus  it  is  hollowed  out.  A  layer 
of  endothelial  cells  lines  the  cavity,  and  becomes  the  endocardium. 

The  primitive  heart  is  connected  at  its  upper  end  with  the  two 
aortse,  and  at  its  lower  end  with  the  omphalo-mesenteric  veins. 

After  a  time  the  tube  shows  signs  of  division  into  three  parts  ; 
the  upper  part  becomes  the  aortic  bulb,  next  to  which  is  formed 
the  cavity  of  the  ventricle,  continuous  with  which  is  the  auricular 


Fig.  282. 


ent 


Transverse  section  tlirough  the  region  of  the  heart  of  a  rabbit's  embryo 
of  nine  days  old.  (Kolliker.j — -jf;.  Jugular  veins,  ao.  Aorta,  ph.  Fore-gut. 
bl.  Blastoderm,  hp.  Body  wall  reflected  in  ect.  ent.  Hypoblast,  e^.  Pro- 
longation of  hypoblast  between  the  two  halves  of  the  heart,  ah.  Outer  wall 
of  the  heart,  p.  Cavity  of  the  pericardium,  ih.  Inner  lining  of  the  heart. 
ect.  Epiblast.     df.  Visceral  mesoblast. 

space.  The  tube  also,  which  at  first  lies  in  a  straight  line,  now 
becomes  twisted  on  itself,  the  auricular  part  becoming  posterior 
and  superior,  whilst  the  ventricle,  with  the  aortic  bulb,  remains 
anterior  and  somewhat  below. 

Each  primitive  cavity  of  the  heart  is  divided  into  two  by  the 
gradual  growth  of  partitions,  and  thus  the  four  permanent  heart 
cavities  are  developed. 

Externally  a  notch  shows  the  division  of  the  ventricle  into 
right  and  left  cavities,  whilst  from  the  inside  of  the  right  wall 
there  grows  a  projection  which  subdivides  the  ventricle  internally. 


698 


MANUAL    OF    PHYSIOLOGY. 


This  septum  is,  however,  not  at  once  complete  at  its  upper  part,  a 
communication  between  the  right  and  left  sides  of  the  heart  re- 
maining for  some  time  above  this  partition.  With  the  growth  of 
the  inter-ventricular  septum,  the  external  notch  becomes  less 
prominent,  but  it  is  less  permanently  recognizable  as  the  inter- 
ventricular groove. 

In  the  auricles  a  fold  develops  from  the  anterior  wall,  which 
ultimately  unites  with  a  process  of  later  development  from  the 

Fig.  283. 


Diagrammatic  views  of  the  under  surface  of  an  embryo  rabbit  of  nine 
days  and  three  hours  old,  showing  the  development  of  the  heart.  (Allen 
Thomson.) — A.  View  of  entire  embryo.  B.  An  enlarged  outline  of  the 
heart  of  A.  C.  A  later  stage  of  the  development  of  B.  h  h.  Ununited 
heart,     a  a.  Aortie.      V  V.  Vitelline  veins. 


posterior  wall.  This  septum  is  not  complete  during  foetal  life, 
but  is  interrupted  by  an  opening  leading  from  one  auricle  to  the 
other,  called  the  foramen  ovale. 


DEVELOPMENT  OF  THE  HEART. 


699 


Simultaueously  with  the  appearance  of  the  posterior  process  of 
the  septum,  another  fold  arises,  which  is  placed  at  the  mouth  of 
the  inferior  vena  cava,  and  forms  the  Eustachian  valve. 

The  aortic  bulb  likewise,  by  a  projection  from  the  inner  wall 
of  the  cavity,  becomes  divided  into  two  canals,  the  anterior  of 


Fig.  284. 


Fig.  285. 


Fig.  284. — Development  of  the  heart  in  the  human  embryo,  from  the 
fourth  to  the  sixth  week. — A.  Embryo  of  four  weeks.  (Kolliker  after  Coste.) 
B.  Anterior,  and  G.  posterior  views  of  the  heart  of  an  embryo  of  six  weeks. 
(Kolliker  after  Ecker.)    a.  Upper  limit  of  buccal  cavity,    b.  Buccal  cavity. 

c.  Lies  between  the  ventral  ends  of  the  second  and  third  branchial  arches. 

d.  Buds  of  upper  limbs,  e.  Liver.  /.  Intestine.  L  Superior  vena  cava. 
1'.  Left  superior  vena  cava  or  connection  between  the  left  brachio-cephalic 
vein  and  the  coronary  vein.  1^^.  Opening  of  inferior  vena  cava.  2.  2''. 
Right  and  left  auricles.     3.  3''.  Right  and  left  ventricles.     4.  Aortic  bulb. 

Fig.  285. — Human  embryo  of  about  three  weeks.  (Allen  Thomson.) — 
uv.  Yolk  sac.  al.  Allantois.  am.  Amnion,  ae.  Anterior  extremity,  pe. 
Posterior  extremity. 


which  remains  in  continuity  with  the  right  ventricle,  and  the  pos- 
terior canal  is  continuous  with  the  left  ventricle.  The  anterior 
thus  becomes  the  pulmonary  artery,  and  the  posterior  the  perma- 
nent aorta. 

The  primitive  circulations  of  a  human  embryo  may  be  divided 


700 


MANUAL   OF   PHYSIOLOGY. 


into  two,  which  differ  in  their  time  of  appearance  and  in  the  ac- 
cessory organs  to  which  they  are  distributed.  Though  they  may, 
for  the  sake  of  clearness,  be  described  as  two  independent  circu- 


JW 


Diagram  of  the  circulation  of  a  chick  at  the  end  of  the  third  day.  (Foster 
and  Balfour  ) — H.  Heart.  AA.  Aortic  arches  (2d,  3d,  and  4th).  Ao.  Dor- 
sal aorta.  L.qfA.,  R.ofA.  Eight  and  left  omphalo-mesenteric  arteries. 
S.T.  Sinus  terminalis.  R.of,  and  L.of.  Right  and  left  omphalo-mesenteric 
veins.  S.V.  Sinus  venosns.  D.C.  Duct  of  Cuvier.  S.Ca.and  V.Ca.  Supe- 
rior and  inferior  cardinal  veins. 


lations,  they  are  not  strictly  so,  as  they  exist  for  a  short  time 
coincidently,  and  arise  in  connection  with  one  another  from  the 
same  heart. 

(a)  The  earlier  or  vitelline  circulatio7i  is  that  which  is  directed 


VITELLINE    CIRCULATION.  701 

to  the  yolk  sac,  the  embryo  obtaining  nourishment  from  the 
vitellus  or  yolk;  this  is,  however,  an  organ  of  quite  secondary 
importance  in  the  mammalian  embryo,  and  hence  this  circulation 
may  be  better  studied  in  some  such  animal  as  the  chick,  which 
depends,  throughout  its  embryonic  life,  on  the  vitellus  for  nourish- 
ment. In  the  human  embryo  the  vitelline  circulation  is  chiefly 
of  importance  for  the  few  days  immediately  preceding  the  devel- 
opment of  the  placental  circulation. 

The  aortic  bulb  is  continuous  with  two  vessels  which  run  on 
either  side  of  the  primitive  pharynx  ;  these  are  the  aortse,  and 
from  each  of  them  a  large  branch  is  given  off.  These  omphalo- 
mesenteric arteries  pass  to  the  yolk  sac,  and  there  become  split 
up  into  a  number  of  small  vessels,  the  blood  from  them  being  re- 
turned partly  by  corresponding  omphalo-mesenteric  veins,  partly 
by  a  large  vein  running  round  the  periphery  of  the  vascular  area 
known  as  the  sinvs  terminalis.  The  sinus  terminalis  opens  partly 
into  the  right  and  partly  into  the  left  omphalo-mesenteric  veins, 
the  omphalo-mesenteric  veins  themselves  subsequently  uniting  into 
a  common  venous  trunk,  called  the  sinus  venosus,  which  is  con- 
tinuous with  the  primitive  auricle. 

This  vitelline  circulation  in  the  human  embryo  persists  but  a 
short  time.  After  the  fifth  or  sixth  week  of  foetal  life  it  becomes 
obliterated,  the  yolk  then  being  atrophied,  and  the  placental  cir- 
culation well  developed. 

(6)  The  later,  or  placental  circulation,  is  developed  in  the  meso- 
blastic  layer  of  the  allantois,  especially  in  that  part  which  is  in 
relation  with  the  decidua  serotina.  The  allantois,  when  fully  de- 
veloped, extends  to  the  chorion,  over  which  it  spreads,  sending  in 
processes  to  occupy  the  villi.  These  chorionic  villi  are  imbedded 
in  the  decidua  of  the  uterus,  and  are  especially  developed  at  the 
upper  part,  which  is  in  connection  with  the  decidua  serotina  or 
maternal  placenta. 

The  primitive  aortse,  which  were  at  first  two  separate  tubes,  be- 
come united  in  the  dorsal  region  of  the  embryo,  so  that  the  two 
aortic  arches  end  in  a  single  vessel,  which  extends  to  the  middle 
of  the  embryo,  and  there  divides  into  two  branches,  each  of  which 
gives  off  a  vessel  called  the  vitelline  or  omphalo-mesenteric  artery. 


702 


jrANUAL   OF   PHYSIOLOGY. 


From  the  branches  of  the  aorta^  arise  two  large  vessels,  which, 
running  along  the  allantois,  spread  out  over  the  chorion,  being 
especially  directed  to  the  upper  part  of  this  membrane  ;  these  are 


Fio.  287. 


Diagram  of  the  vascular  system  of  a  human  fcetus.  ( Huxley.) — H.  Heart. 
T.A.  Aortiotrunk.  c.  Common  carotid  artery,  c'.  External  carotid  artery, 
c'''.  Internal  carotid  artery,  s.  Subclavian  artery,  v.  Vertebral  artery.  1, 
2,3,4,5.  Aortic  arches.  A^.  Dorsal  aorta.  1.  Omphalo-mesenteric  artery. 
dv.  Vitelline  duct.  o'.  Omphalo-mesenteric  vein.  v'.  Umbilical  vesicle. 
vp.  Portal  vein.  L.  Liver,  uu.  Umbilical  arteries,  u"  u" .  Their  endings 
in  the  placenta,  u' .  Umbilical  vein.  Dv.  Ductus  venosus.  vh.  Hepatic 
vein.  CO.  Vena  cava  inferior,  vil.  Iliac  veins,  az.  Venaazygos.  fc.  Pos- 
terior cardinal  vein.     DC.  Duct  of  Cuvier.     P.  Lungs. 

the  umbilical  or  hypogastric  arteries,  which  carry  the  blood  from 
the  aortse  to  the  foetal  placenta. 

Veins  arise  from  the  terminal  networks  of  these  arteries,  and 
combine  to  form  the  two  umbilical  veins.  The  umbilical  veins 
take  a  similar  course  to  the  arteries,  and  convey  the  blood  to  the 
venous  trunk  formed  by  the  junction  of  the  omphalo-mesenteric 
veins. 


PORTAL   SYSTEM. 


703 


After  a  time  the  right  umbilical  and  right  omphalo-raesenteric 
veins  disappear,  whilst  from  the  trunk  formed  by  the  junction  of 
the  left  umbilical  and  left  oraphalo-mesenteric  veins,  branches 
are  given  off  to  the  liver  (the  vence  advehentes),  and  at  a  point 
nearer  the  heart,  vessels  are  received  from  the  liver  fthe  veiice 
revehentes). 

To  the  part  of  the  vessel  intervening  between  the  origin  of  the 
vense  advehentes  and  the  entrance  of  the  vense  revehentes  is  given 
the  name  of  the  ductus  venosus. 

Thus  it  may  be  seen  that  in  the  placental  circulation,  the  blood 
is  conveyed  from  the  aorta,  by  the  umbilical  arteries,  to  the  foetal 


Fig.  288. 


Y  '  *      «- 

Diagram  of  the  heart  and  principal  arteries  of  the  cliick.  (Allen  Thom- 
son.) B.  and  care  later  than  A. — 1,1.  Oraphalo-mesenteric  veins.  2.  Au- 
ricle. 3.  Ventricle.  4.  Aortic  bulb.  5,  5.  Primitive  aortse.  6,  6.  Om- 
phalo-mesenteric  arteries.     A.  United  aorta. 

placenta,  and  here  it  undergoes  changes,  owing  to  its  close  rela- 
tionship to  the  maternal  blood.  From  the  placenta  it  is  returned 
by  the  umbilical  vein,  which  sends  a  part  through  the  liver  and 
a  part  direct  to  the  heart.  The  more  minute  details  of  foetal  cir- 
culation will  be  described  later  on. 


704 


MANUAL   OF   PHYSIOLOGY. 


The  Arterial  iSystem. — Around  the  pharynx  are  developed  five 
pairs  of  aortic  arches.  These  coraraeuce  anteriorly  from  the  two 
primitive  aortce,  and,  passing  along  the  side  of  the  pharynx,  end 
in  the  aortse  as  they  descend  to  become  united  in  the  dorsal  region 


Fig.  289. 


Diagram  of  the  aortic  arches  ;  the  permanent  vessels  arising  from  them 
are  shaded  darkly.  (Allen  Thomson  after  Rathke.) — 1,  2,  3,  4,  5.  Primitive 
aortic  arches  of  right  side.  i.  ii.  iii.  iv.  Pharyngeal  clefts  of  the  left  side, 
showing  the  relationship  of  the  clefts  to  the  aortic  arches.  A.  Aorta.  P. 
Pulmonary  artery,  d.  Ductus  arteriosus.  a\  Left  aortic  root.  a.  Right 
aortic  root.  ^^.  Descending  aorta.  jDn.pn.^  Right  and  left  vagi.  s.s'.  Right 
and  left  subclavian  arteries.  v.v\  Right  and  left  vertebral  arteries,  c. 
Common  carotid  arteries,  ce.  External  carotid.  cL  ci'.  Right  and  left  in- 
ternal carotid. 


of  the  embryo.  The  points  of  origin  of  the  arches  are  termed 
their  anterior  roots,  and  the  points  of  termination  their  posterior 
roots. 

Though  all  these  arches  do  not  exist  at  the  same  time,  still,  in 
describing  the  vessels  which  arise  from  them,  they  may  be  con- 
veniently considered  together. 

On  the  right  side  the  fifth  arch  disappears  completely.     On  the 


AOETIC   AECHES.  705 

left  side  the  anterior  root  and  neighboring  part  of  the  fifth  arch 
are  transformed  into  the  pulmonary  artery ;  the  remaining  part 
of  this  arch  continues  as  the  ductus  arteriosus,  which  connects  the 
pulmonary  artery  with  the  permanent  aorta. 

The  fourth  left  arch,  in  mammalia,  becomes  the  permanent 
aorta.  At  the  junction  of  the  fourth  and  fifth  left  posterior 
roots  the  left  subclavian  artery  is  given  off.  In  birds  the  right 
fourth  arch  is  transformed  into  the  permanent  aorta ;  and  in  ex- 
amining the  development  of  the  aortic  arch  of  the  chick,  it  must 
be  borne  in  mind  that  it  is  on  the  opposite  side  to  that  it  occupies  in 
man. 

On  the  right  side  the  anterior  root  of  the  fourth  arch,  and  the 
part  of  the  aortic  trunk  leading  to  it,  persists  as  the  innominate 
artery,  the  fourth  arch  being  represented  by  the  right  subclavian 
artery. 

The  part  of  the  primitive  aortic  trunk  joining  the  fourth  and 
third  right  anterior  roots  becomes  the  common  carotid  artery  of 
the  same  side,  whilst  arising  from  this  is  the  internal  carotid, 
which,  taking  the  position  of  the  third  arch,  passes  to  the  poste- 
rior roots,  and  occupies  the  trunk  of  the  primitive  aorta  from  the 
third  to  the  first  arches. 

The  external  carotid,  arising  from  the  common  carotid  at  the 
third  anterior  root,  occupies  the  position  of  the  vessel  joining  this 
root  to  those  of  the  second  and  first  arch. 

On  the  left  side  the  common  carotid  and  its  branches  are  de- 
veloped similarly  to  those  on  the  right,  the  only  difference  being 
that  the  common  carotid  arises  from  the  aorta  and  not  from  the 
innominate. 

The  iliac  arteries  are  developed  from  the  hypogastric.  At 
first  they  appear  as  branches,  but  with  the  growth  of  the  limbs 
they  become  so  much  larger  that  after  birth  they  appear  to  be 
the  main  branches  from  the  point  of  division  of  the  aorta,  the 
hypogastric  arteries  now  being  merely  small  branches  of  the  iliac 
vessels. 

With  the  development  of  the  organs  and  limbs,  vessels  in  con- 
nection with  those  above  described  arise  in  the  mesoblast.     It  is, 

59 


706 


MANUAL   OF   PHYSIOLOGY. 


FlO.  290. 


A.  Plan  of  principal  veins  of  the  foetus  of  about  four  weeks  old.  B.  Veins 
of  the  liver  at  an  earlier  period.  C.  Veins  after  the  establishment  of  the 
placental  circulation.  D.  Veins  of  the  liver  at  the  same  period. — j.  Primi- 
tive jugular  veins,  dc.  Ducts  of  Cuvier.  ca.  Cardinal  veins,  ci.  Inferior 
vena  cava.  I.  Ductus  venosus.  u.  Umbilical  vein.  p.  Portal  vein.  o. 
Vitelline  vein.  cr.  External  iliac  veins,  o^.  Right  vitelline  vein.  «'. 
Eight  umbilical  vein.  l^.  Hepatic  veins  (venae  revehentes).  p^p\  Vense 
advehentes.  m.  Mesenteric  veins,  az.  Azygos  vein.  ca^.  Remains  of  left 
cardinal  vein.  s.  Subclavian  vein.  It.  Cross  branch  from  left  jugular  which 
becomes  the  left  branchio-cephalic  vein.  ri.  Right  innominate  vein.  8.s. 
Subclavian  veins,  h.  Hypogastric  veins,  il.  Division  of  inferior  vena  cava 
into  the  common  iliac  veins. 


VENOUS   SYSTEM.  .       707 

however,  beyond  the  scope  of  this  work  to  describe  in  detail  the 
origin  of  the  lesser  vessels. 

Venotis  System. — The  blood  is  returned  from  the  head  by  the 
two  primitive  jugulars,  which  unite  with  the  cardinal  veins  con- 
veying the  blood  from  the  trunk  and  lower  extremities  to  form  a 
vessel  on  each  side,  called  the  duct  of  Cuvier. 

From  the  lower  extremity  of  the  embryo  the  inferior  vena  cava 
commences  by  the  union  of  the  external  iliac  veins  ;  this  passes  up 
and  opens  into  the  venous  trunk  common  to  the  left  vitelline  and 
left  umbilical  veins. 

The  left  vitelline  becomes  continuous  with  the  vessels  from  the 
common  trunk  going  to  the  right  side  of  the  liver  (the  right  vena 
advehens),  and  forms  the  main  trunk  of  the  portal  vein  (v.  Fig. 
290,  B.  &  D.). 

j^  t  this  stage  of  the  formation  of  the  veins  there  are  three  trunks 
opening  into  the  auricle,  the  right  and  left  ducts  of  Cuvier  and 
the  inferior  vena  cava. 

As  development  proceeds,  the  lower  parts  of  the  cardinal  veins 
join  the  external  iliac  veins,  forming  the  common  iliacs,  and  so 
return  their  blood  into  the  inferior  vena  cava. 

The  upper  parts  of  the  cardinal  veins  become  continuous  with 
the  posterior  vertebral  veins  which  convey  the  blood  from  the 
parietes  of  the  embryo.  Between  the  latter  a  communicating 
branch  is  established,  which  helps  in  the  formation  of  the  azygos 
vein. 

The  ducts  of  Cuvier,  which  at  first  were  placed  almost  at  right 
angles  to  the  auricle,  become  more  oblique  in  their  direction  as 
the  heart  descends. 

Between  the  primitive  jugular  veins  a  cross  branch  is  developed, 
which  conveys  the  blood  from  the  left  side  of  the  head  and  upper 
extremity  to  the  duct  of  Cuvier  of  the  opposite  side. 

The  left  duct  of  Cuvier,  below  the  communicating  branch, 
atrophies  and  forms  part  of  the  coronary  veins  of  the  heart  ; 
the  connection  between  this  and  the  vein  above  the  cross  branch 
being,  in  the  adult,  represented  by  a  small  vein,  or  a  band  of 
fibrous  tissue,  called  the  vestigial  fold  of  the  pericardium. 

The  cross  branch  from  the  left  to  the  right  jugular  becomes  the 


708  MANUAL   OF   PHYSIOLOGY. 

left  innomiuate  vein.  The  right  duct  of  Cuvier  and  the  right 
jugular  below  the  entrance  of  this  cross  branch,  forms  the  supe- 
rior vena  cava  ;  whilst  the  part  of  the  right  primitive  jugular 
immediately  above  the  entry  of  the  left  innomiuate  vein,  forms 
the  right  innominate  vein. 

The  posterior  vertebral  vein  of  the  right  side  forms  the  vena 
azygos  major;  the  corresponding  branch  of  the  opposite  side, 
together  with  the  part  of  the  left  primitive  jugular  below  the 
cross  branch,  forms  the  left  superior  intercostal  vein  and  the  supe- 
rior vena  azygos  minor.  The  lower  part  of  the  left  posterior 
vertebral  vein,  together  with  the  connecting  branch  to  the  right 
vein,  remain  as  the  inferior  vena  azygos  minor. 

Foetal  Circulation. — The  course  taken  by  the  blood  through 
the  heart  and  vessels  of  the  embryo  differs  essentially  from  that 
which  persists  in  adult  life. 

Tracing  the  blood  from  the  placenta,  it  passes  along  the  um- 
bilical vein  towards  the  liver,  here  it  may  take  either  of  two  courses 
to  reach  the  vena  cava,  one  which  follows  the  ductus  venosus  and 
avoids  the  liver,  the  other  which  passes  by  the  venae  advehentes 
(portal  veins)  to  the  liver,  and  proceeds  by  the  venre  reveheutes 
(hepaticveins)to  the  inferior  vena  cava,  which  receives  all  the  blood 
passing  by  both  of  these  channels.  From  this  the  blood  is  emptied 
into  the  right  auricle,  and  hence  is  guided  by  the  Eustachian 
valve  through  the  septum  by  the  patent  foramen  ovale  to  the  left 
auricle.  From  the  left  auricle  it  passes  to  the  left  ventricle, 
which  contracts  and  sends  the  blood  into  the  aortic  arch,  where  it 
is  split  up  into  two  streams,  one  of  which  passes  into  the  vessels 
of  the  head  and  neck,  the  other  by  the  descending  aorta  to  the 
trunk  and  lower  extremities. 

The  blood  from  the  head  and  neck  is  returned  to  the  right 
auricle  by  the  superior  vena  cava.  The  blood  from  this  vein 
passes  through  the  auricle  to  the  right  ventricle,  which  sends  it 
through  the  pulmonary  artery  towards  the  lungs. 

The  pulmonary  artery,  however,  in  the  embryo,  has  one  very 
large  branch,  called  the  ductus  arteriosus,  which  joins  the  aorta 
at  a  point  just  below  the  origin  of  the  vessels  of  the  head  and 
neck ;  hence  the  main  part  of  the  blood  passing  from  the  right 


FCETAL  CIRCULATION. 


709 


ventricle  reaches  the  aorta  by  the  ductus  arteriosus,  and  only  a 
very  small  part  goes  to  the  lungs,  to  be  returned  from  them  by 
the  pulmonary  veins  to  the  left  auricle. 


Fig.  291. 


Diagram  illustrating  the  circulation  through  the  heart  and  the  principal 
vessels  of  a  foetus.  (Cleland.) — a.  Umbilical  vein.  6.  Ductus  venosus.  /. 
Portal  vein.  e.  Vessels  to  the  viscera,  d.  Hypogastric  arteries,  c.  Ductus 
arteriosus. 


The  blood  from  the  ductus  arteriosus  blends,  therefore,  with 
that  in  the  aorta  which  is  passing  to  the  viscera  and  lower  ex- 
tremities.    The  main   part  of  this  blood  travels  by  two  large 


710  MANUAL  OF   PHYSIOLOGY. 

branches  of  the  aorta  (the  hypogastric  arteries)  to  the  placenta, 
where  it  is  aerated  and  purified,  etc. 

It  is  evident,  then,  that,  as  the  placenta  is  the  great  renovating 
organ  of  the  blood  of  the  foetus,  the  blood  in  the  umbilical 
vein  is  the  most  arterial  in  the  foetal  circulation.  The  blood  in 
the  ascending  vena  cava  and  first  part  of  the  aorta  is  likewise 
fairly  arterial,  but  the  blood  in  the  descending  aorta  is  of  a  mixed 
character,  as  it  contains  blood  which  has  nourished  the  head  and 
neck,  besides  blood  which  has  come  from  the  placenta  by  the 
inferior  vena  cava  through  the  right  auricle,  foramen  ovale,  left 
auricle,  and  left  ventricle. 

As  the  foetal  lungs  are  not  called  into  play  until  after  birth, 
but  little  blood  passes  to  them  in  the  foetus ;  this  state  of  things 
is,  however,  completely  altered  at  birth,  when  the  lungs  of  the 
child  expand,  the  pulmonary  arteries  increase  in  size,  and  the 
ductus  arteriosus  dwindles  in  a  corresponding  degree. 

The  liver,  which  in  the  foetus  is  of  relatively  greater  size  than 
in  the  adult,  receives  much  blood  coming  from  the  placenta  to 
the  heart,  and  is  thought  to  contribute  to  it  several  essential  con- 
stituents. 

The  head  and  brain,  which  are  largely  developed  in  the  foetus, 
receive  well-aerated  blood,  namely,  the  placental  blood,  which 
has  passed  through  the  liver,  and,  in  the  inferior  vena  cava,  is 
mixed  with  blood  coming  from  the  lower  limbs.  The  rest  of  the 
foetus  receives  blood  that  is  less  well  aerated,  as  it  is  mixed  with 
that  which  is  returned  from  the  head  and  neck  to  the  right  side 
of  the  heart,  and  which  is  sent  through  the  ductus  arteriosus  to 
join  the  general  blood  current  in  the  aorta  going  to  the  viscera 
and  lower  extremities. 


Development  of  the  Eye. 

The  optic  vesicles  arise  from  the  anterior  cerebral  vesicle  at  a 
very  early  period,  and  their  cavities  are  continuous  with  that  of 
the  fore-brain.  With  the  development  of  the  rudimentary  cerebral 
hemispheres  the  optic  vesicles  become  displaced  downwards,  and 
their  cavities  open  into  the  junction  of  the  cavities  of  the  cerebral 


THE   EYE.  711 

hemispheres  aud  that  of  the  thalamencephalon,  which  becomes 
the  third  ventricle.  Later,  the  optic  vesicles  open  directly  into 
the  third  ventricle,  and  finally  are  displaced  backwards,  and 
come  into  connection  with  the  mid -brain. 

The  optic  vesicles  are  at  first  hollow  prolongations,  which  con- 
sist of  an  anterior  dilated  portion,  forming  the  primary  optic 
vesicle,  and  a  posterior  tubular  portion  or  stalk  joining  the  vesicle 
to  the  fore-brain.     This  stalk  forms  the  optic  nerve. 

As  each  vesicle  grows  forwards  towards  the  epiblast  covering 
the  head  of  the  embryo,  the  epiblastic  cells  at  the  spot  overlying 
the  vesicle  become  thickened,  and  an  involution  of  the  epiblast 
takes  place  towards  the  optic  vesicle,  and  indents  the  latter, 
approximating  its  anterior  to  its  posterior  wall. 

By  this  means  the  anterior  and  posterior  walls  of  the  primary 
optic  vesicle  come  into  close  contact,  and  the  cavity  of  the  vesicle 
is  obliterated.  The  two  layers  of  the  vesicle  are  now  cup-shaped, 
and  receive  the  name  of  the  secondary  optic  vesicle  or  the  optic 
cup.  This  ultimately  becomes  the  retina,  and  the  optic  stalk 
losing  its  cavity  is  transformed  into  the  optic  nerve. 

Meanwhile,  the  local  involution  of  the  epiblast  over  the  optic 
cup,  which  is  the  rudiment  of  the  crystalline  lens,  becomes  grad- 
ually separated  from  the  general  epiblast  giving  origin  to  it,  and 
is  finally  detached  from  its  point  of  origin.  It  now  lies  as  a 
somewhat  spherical  body  in  the  cavity  of  the  optic  cup  within 
the  superficial  mesoblast,  which  has  closed  over  it. 

The  secondary  optic  vesicle  grows  (except  at  its  lower  part, 
just  at  the  junction  of  the  optic  stalk),  so  as  to  deepen  the  optic 
cup,  which  contains  the  rudimentary  lens.  At  the  lower  part 
an  interval  is  left,  which  receives  the  name  of  the  choroidal  fissure. 
Through  this  gap  in  the  secondary  optic  vesicle  the  mesoblast 
enters  and  separates  the  lens  from  the  optic  cup,  and  forms  the 
vitreous  humor. 

The  mesoblast  surrounding  the  optic  cup  develops  two  cover- 
ings of  the  eye,  an  outer  fibrous  capsule  called  the  sclerotic  coat, 
and  a  vascular  coat,  the  choroid,  which  lies  in  contact  with  the 
outer  layer  of  the  optic  cup. 

In  front  of  the  lens,  beneath  the  epiblast,  the  mesoblast  forms 


712  MANUAL   OF   PHYSIOLOGY. 

the  corneal  tissue  proper.  The  epiblast  forms  the  epithelial  or 
conjunctival  covering  of  the  eyeball. 

The  involution  of  mesoblast  through  the  choroidal  fissure, 
which  forms  the  vitreous  humor,  indents  the  optic  stalk,  and 
forms  the  central  artery  of  the  retina.  The  choroidal  fissure  is 
gradually  obliterated,  and  its  position  may  sometimes  be  marked 
by  a  permanent  fissure  in  the  iris  (colohoma  iridis). 

The  rudimentary  lens  is  a  spherical  body,  hollow  in  the  centre, 
made  up  of  an  anterior  and  posterior  wall,  each  of  which  is  formed 

Fig.  292. 


V-'IGiii'tt 


Section  through  the  head  of  a  chick  at  the  third  day,  showing  the  origin 
of  the  lens. — a.  Epiblast  thickened  at  c,  which  is  the  point  of  origin  of  the 
lens.  0.  Optic  vesicle.  Vz-  Anterior  cerebral  vesicle.  Fj-  Posterior 
cerebral  vesicle. 

of  columnar  cells.  The  posterior  wall  of  the  lens  increases 
greatly  in  thickness,  and  approaching  the  anterior  wall  obliter- 
ates the  original  cavity  of  the  lens. 

The  cells  forming  this  wall  become  very  much  elongated,  and 
form  long  fibre-like  columnar  cells.  The  cells  of  the  anterior 
wall,  from  being  a  columnar  epithelium,  are  modified  to  a  flat- 
tened epithelium,  and  finally  become  the  layer  of  epithelium 
lining  the  anterior  surface  of  the  capsule  of  the  lens.     The  cap- 


THE    EETINA. 


713 


sule  of  the  lens  has  been  variously  considered  as  arising  from  the 
cells  of  the  lens  substance,  or  as  originating  from  a  thin  layer  of 


Diagrammatic  section  of  the  primitive  eye,  showing  the  choroidal  fissure. 
(Foster  and  Balfour.)— D.  Horizontal  section.  E.  Vertical  transverse  sec- 
tion just  striking  the  posterior  part  of  the  lens.  F.  Vertical  longitudinal 
section  through  the  optic  stalk,  and  the  fissure  through  which  the  mesoblast 
passes  to  form  the  vitreous  humor,  h.  Superficial  epiblast.  x.  Point  of 
origin  of  the  lens.  v.h.  Vitreous  humor,  r.  Anterior  layer  of  the  optic 
vesicle,  u.  Posterior  layer  of  the  optic  vesicle,  c.  Cavity  of  the  optic 
vesicle.  /.  Choroidal  fissure,  s.  Optic  stalk,  s^.  Cavity  of  the  optic  stalk. 
I.  Lens.     V.  Cavity  of  the  lens. 

mesoblast,  which  forms  not  only  the  lens  capsule,  but  also  the 
hyaloid  membrane,  which  is  continuous  with  it. 


Fig.  294. 


Later  stages  in  the  development  of  the  lens.     (Cadiat.) — a.  Epiblast. 
c.  Rudimentary  lens.     o.  Optic  vesicle. 

The  optic  cup  gives  origin  to  the  retina.     The  inner  or  an- 
terior layer  of  the  cup  becomes  thickened,  and  from  it  are  differ- 

00 


714 


MANUAL  OF   PHYSIOLOGY. 


entiated  the  various  layers  of  the  retina,  except  the  layer  of  pig- 
ment cells  which  lies  next  to  the  choroid.  The  posterior  layer 
develops  this  layer  of  pigment  cells,  which,  from  their  intimate 
connection  to  the  choroid,  were  formerly  considered  as  part  of 
that  membrane. 

Fio.  295. 


A  further  stage  of  the  development  of  the  lens.  (Cadiat.) — a.  Elongating 
epithelial  cells  forming  lens ;  b,  Capsule  ;  c.  Cutaneous  tissue  becoming  con- 
junctiva; d,  e,  Two  layers  of  optic  cup  forming  retina;  /,  Cell  of  mucous 
tissue  of  the  vitreous  humor  ;  g,  Intercellular  substance ;  h,  Developing 
optic  nerve. 


The  thickening  of  the  inner  or  anterior  layer  of  the  optic  cup 
ceases  at  the  ora  serrata.  The  outer  layer  with  its  contiguous 
choroid  is  thrown  into  a  number  of  folds — the  ciliary  processes — 
and,  passing  in  front  of  the  lens,  helps  to  form  the  iris. 

In  front  of  the  ora  serrata  the  anterior  layer  of  the  cup  is  no 


THE   EAR.  715 

longer  differentiated  iuto  the  special  retinal  elements,  but  joins 
with  the  posterior  to  form  a  layer  of  columnar  cells, — the  pars 
ciliaris  retince.  In  front  of  this  the  anterior  rim  of  the  optic  cup 
passes  forwards  and  lines  the  posterior  surface  of  the  iris,  forming 
the  uvea  of  that  organ,  and  terminates  at  the  margin  of  the  pupil. 

The  rest  of  the  substance  of  the  iris  is  developed  from  the 
mesoblast,  from  which  also  arise  the  choroid,  the  cornea,  and  the 
sclerotic. 

The  development  of  the  eye  may  be  thus  briefly  described. 
An  offshoot  of  nervous  matter  from  the  fore-brain  forms  the 
retina  and  the  uvea,  and  its  stalk,  or  connection  with  the  brain, 
develops  iuto  the  optic  nerve. 

An  involution  of  epiblast  which  grows  into  the  nervous  cup 
forms  the  lens,  whilst  from  the  adjacent  mesoblast  the  surround- 
ing parts  of  the  eye  arise.  The  vitreous  is  produced  by  the  meso- 
blast growing  through  a  fissure  in  the  lower  part  of  the  optic  cup 
to  fill  its  cavity. 

Development  of  the  Ear. 

The  ear  is  developed  chiefly  from  the  epiblast,  a  special  and 
independent  involution  of  which  forms  both  its  essential  nervous 
structures,  and  the  general  epithelium  lining  the  membranous 
labyrinth.  The  mesoblast  supplies  the  surrounding  firmer  struc- 
tures, such  as  the  fibrous  substance  of  the  inner  ear,  and  the  bony 
parts  in  which  the  organ  lies.  The  auditory  nerve  grows  as  a 
bud  from  the  neural  tissue  forming  the  hind-brain,  and  connects 
it  to  the  delicate  specialized  auditory  cells. 

The  process  begins  by  the  appearance  of  a  depression  of  the 
general  epiblast  covering  the  head,  which  soon  forms  a  tubular 
diverticulum,  lying  in  the  mesoblast  at  the  side  of  the  hind-brain. 

This  diverticulum  becomes  separated  from  the  epiblast  by  the 
obliteration  of  its  outer  extremity,  which  united  it  to  the  super- 
ficial epiblast,  and  is  then  converted  iuto  a  cavity  and  receives 
the  name  of  the  otic  vesicle.  It  soon  becomes  somewhat  triangular 
in  shape,  the  base  of  the  triangle  lying  upwards. 

From  the  lower  angle  arises  a  projection,  which  is  the  rudi- 


716 


MANUAIi   OF   PHYSIOLOGY. 


mentary  canal  of  the  cochlea.  The  angle  lying  next  to  the  neural 
epiblast  sirailarly  gives  off  a  tubular  process,  which  forms  the 
recessus  vestibuli. 

Elevations  in  the  primitive  vesicle  indicate  the  origin  of  the 
semicircular  canals,  which  become  tubular,  opening  at  their  ends 

Fig.  296. 


Transverse  section  through  the  head  of  a  foetal  sheep  in  the  region  of  the 
hind-brain.  (Boettcher.) — hb.  Hind-brain,  cc.  Canal  of  the  cochlea.  RC. 
Recessus  vestibuli.  vb.  Vertical  semicircular  canal,  go.  Auditory  gan- 
glion. g\  Auditory  nerve.  N.  Connection  of  auditory  nerve  to  the  hind- 
brain. 


into  the  general  cavity  of  the  vesicle.  The  two  superior  canals 
are  the  first  to  appear,  the  horizontal  canal  rising  somewhat 
later. 

The  part  of  the  otic  vesicle  in  connection  with  the  canal  of  the 
cochlea  becomes  separated  from  the  latter  by  a  narrow  constric- 
tion, which  forms  the  canalis  reuniens,  the  part  of  the  vesicle  be- 
yond this  developing  into  the  saccule. 


THE    EAR. 


717 


The  utricle  arises  from  that  part  of  the  vesicle  which  is  in  con- 
nection with  the  semicircular  canals.  It  is  at  first  in  direct  con- 
nection with  the  saccule,  but  after  a  time  it  is  only  found  to  com- 
municate by  means  of  a  narrow  canal  with  a  similar  one  from 
the  saccule ;  these  two  canals  are  connected  with  a  third,  which 
lies  in  the  aqueductus  vestibuli. 

Fig.  297. 


Section  through  tlie  liead  of  a  fcetal  sheep.  (Boettcher.) — r.v.  Eecessus 
vestibuli.  v.B.  Vertical  semicircular  canal,  h.b.  Horizontal  semicircular 
canal.     G.  Auditory  ganglion,     c.c.  Canal  of  the  cochlea. 


The  canal  of  the  cochlea  is  at  first  a  straight  tube,  but  as  it 
develops  it  becomes  coiled  upon  itself. 

The  walls  of  the  primitive  otic  vesicle,  formed  from  the  epiblast, 
become  developed  into  the  epithelium  lining  the  internal  ear. 
The  mesoblast  immediately  surrounding  the  vesicle  forms  a  sup- 


718  MANTJAI-   OF    PHYSIOLOGY. 

porting  capsule  of  fibrous  tissue,  which  completes  the  membranous 
parts  of  the  internal  ear. 

Part  of  the  mesoblast  around  the  otic  vesicle  becomes  liquefied, 
and  gives  origin  to  the  canals  and  spaces  in  which  the  membran- 
ous labyrinth  lies;  the  neighboring  mesoblast  is  changed  into  car- 
tilage, which  ossifies  and  forms  the  bony  parts  of  the  ear. 

The  auditory  nerve  is  developed  from  the  hind-brain,  and  grows 
through  the  mesoblast  towards  the  otic  vesicle.  It  is  recognizable 
from  its  having  some  ganglion  cells  in  its  growing  extremity  from 
a  very  early  period  of  its  development. 

The  Eustachian  tube  and  the  tympanum,  or  cavity  of  the 
middle  ear,  are  formed  in  connection  with  the  inner  part  of  the 
first  visceral  cleft,  and  the  ossicles  are  developed  from  the  corre- 
sponding visceral  arch,  namely,  the  hyo-mandibular. 

The  merabrana  tyrapani  is  formed  at  the  surface  of  the  embryo ; 
the  adjacent  parts  grow  outwards  and  give  rise  to  the  external 
auditory  meatus. 

Development  of  the  Skull  and  Face. 

The  bones  of  the  roof  of  the  skull  and  of  the  face  are  chiefly 
derived  from  membrane,  those  of  the  base  of  the  skull  being  laid 
down  in  cartilage. 

At  the  cephalic  extremity  of  the  notochord  is  a  mass  of  uncleft 
mesoblast,  called  the  investing  mass,  corresponding  to  that  from 
which  the  vertebrae  are  developed. 

From  this  arise  two  prolongations,  which  diverge  and  then 
unite  again,  leaving  an  interval ;  and  the  united  portion  becomes 
once  more  divided  into  two  processes,  the  traheculoe.  cranii. 

The  mesoblast  behind  the  interval  receives  the  name  of  the 
occipito-sphenoid  portion  ;  the  interval  is  the  rudiment  of  the 
sella  turcica,  which  is  occupied  by  the  pituitary  body.  The  part 
of  the  mesoblast  in  front  of  this,  is  called  the  spheno-ethmoidal 
portion. 

From  the  occipito-sphenoidal  portion  are  developed  the  basi- 
occipital  together  with  the  posterior  part  of  the  sphenoid.  At  the 
sides  of  the  medulla  oblongata  processes  are  sent  up,  which  unite 


THE   SKULL. 


719 


round  it  and  form  the  foramen  magnum.  Laterally  the  mesoblast 
envelops  the  auditory  vesicles  and  forms  the  side  portions  of  the 
occipital  bone. 

In  the  cartilaginous  antecedent  of  the  temporal  bone  there  are 
three  centres  of  ossification — the  epiotic,  which  develops  the  mas- 
toid process ;  the  prootic,  which  is  in  the  region  of  the  superior 
semicircular  canal ;  and  the  opisthotic,  which  is  at  the  cochlea. 


Fio.  298. — Basis  cranii  of  a  chick,  sixth  day.  (Huxley.) — 3.  Trabeculae. 
4.  Pituitary  space.     1.  Notochord.     5.  Internal  ear. 

Fig.  299. — Longitudinal  section  through  the  head  of  an  embryo  of  four 
weeks  (Kolliker). — v.  Cavity  of  cerebral  hemisphere,  a.no.  Optic  vesicle, 
z.  Cavity  of  third  ventricle,  m.  Cavity  of  mid-brain,  h.  Cerebellum,  n. 
Medulla,  o.  Auditory  depression,  t.  Basis  cranii.  t^.  Tentorium,  p. 
Pituitary  body. 


The  spheno-ethmoidal  portion  develops  the  anterior  part  of  the 
sphenoid  together  with  the  ethmoid  bones  and  the  cartilage  of  the 
septum  of  the  nose,  the  first  arising  from  the  back  part  is  developed 
from  membrane.  The  trabeculae  are  carried  forwards,  and  bending 
down  at  the  nasal  depression  form  the  lateral  nasal  cartilages  and 
the  anterior  part  of  the  septal  cartilage. 

The  face  is  developed  in  connection  with  ridges  known  as  the 
visceral  folds  or  arches,  between  .which  are  a  number  of  clefts,  the 
visceral  clefts. 

The  eyes  and  the  openings  of  the  nose  are  in  the  face ;  whilst 


720 


MANUAL  OF   PHYSIOLOGY. 


the  ear  arises  at  the  side  of  the  face,  in  connection  with  one  of  the 
visceral  clefts. 

The  nasal  depressions  or  pits  appear  in  the  wall  of  the  head, 
covering  the  anterior  part  of  the  brain. 

Just  above  the  first  visceral  arch  or  fold  is  the  depression  which 
ultimately  becomes  the  buccal  cavity,  and  unites  with  the  alimen- 
tary tract  to  form  the  mouth. 

The  first  fold  is  called  the  mandibular ;  this  gives  off  at  either 
end  a  process  which  grows  upwards  and  inwards,  forming  the  ru- 
diment of  the  superior  maxillary  bone  and  side  of  the  face. 

Between  these  is  a  median  process,  the  frontonasal,  which  gives 
off,  on  the  inner  sides  of  the  nasal  grooves,  projections  which  form 


Fig.  300. 


Different  stages  of  the  development  of  the  liead  and  f;ioe  of  a  human  em- 
bryo.— A.  Embryo  of  four  weeks.  (Allen  Thomson.)  B,  Embryo  of  six 
weeks.  (Ecker.)  C.  Embryo  of  nine  weeks.  (Allen  Thomson.)  a.  Au- 
ditory vesicle.     1.  Lower  jaw.     V.  First  pharyngeal  cleft. 

the  inner  nasal  processes  ;  these  unite  with  the  superior  maxillary 
processes  to  close  in  the  nostril  and  form  the  lip. 

The  outer  nasal  process  is  a  thickening  on  the  outer  side  of  the 
nasal  depression,  which,  running  down  towards  the  superior  max- 
illary process,  forms  eventually  the  lachrymal  duct. 

The  mandibular  arch  forms  the  lower  jaw,  and  between  this  and 
the  superior  maxillary  process  the  buccal  cavity  is  developed 
chiefly  by  the  outgrowth  of  the  surrounding  tissue ;  the  epiblast 


DEVELOPMENT   OF   THE    NOSE. 


721 


lining  this  becomes  thinned  away,  and  the  subjacent  mesoblast 
and  hypoblast  disappear ;  and  thus  the  buccal  cavity  is  made  con- 
tinuous with  that  of  the  alimentary  canal. 

The  cavities  of  the  nasal  depressions  at  first  open  into  the  buccal 
cavity  by  means  of  the  nasal  grooves  ;  after  a  time,  however,  pro- 
cesses arise  from  the  superior  maxillae  which  grow  inwards,  and 
finally  meet  one  another  in  the  middle  line,  forming  a  broad  plate 

Fig.  301. 


c%  ch 

Vertical  section  of  the  head  of  an  embryo  of  a  rabbit.  (Mihalkovics.) — 
In  A.  there  is  no  connection  between  the  buccal  cavity  and  the  fore-gut.  In 
B.  the  connection  is  established. — m.  Epiblast  of  neural  canal,  h.  Heart, 
c.  Cavity  of  fore-brain,  mc.  Cavity  of  mid-brain,  mo.  Cavity  of  medulla. 
sp.o.  Spheno-occipital  parts  of  the  basis  cranii.  sp.e.  Spheno-ethmoidal  part 
of  the  basis  cranii.  be.  Part  of  basis  cranii  which  receives  the  pituitary  body. 
am.  Amnion,  py.  Part  of  heart  cavity  going  to  form  the  pituitary  body. 
i.f.  Fore-gut.     ch.  Notochord.     if.  Infundibulum. 

of  tissue  intervening  between  the  nasal  cavity  above  and  the  buc- 
cal cavity  below.  The  palate  when  complete  in  front  gradually 
closes  towards  the  back  of  the  buccal  cavity,  and  here  the  com- 
munication between  the  nose  and  the  pharynx  is  left. 

Imperfect  development  of  these  parts  gives  rise  to  the  common 
congenital  deformities,  cleft-palate  and  hare-lip. 

The  first  cleft  is  the  hyo-mandibular ;  it  forms  the  tympano- 
Eustachian  cavity,  which  becomes  separated  from  the  surface  by 
the  closure  of  its  outer  end  by  the  growth  of  the  membrana  tym- 


722  MANUAL    OF    PHYSIOLOGY. 

pani,  the  external  auditory  meatus  and  ear  being  formed  by  an 
outgrowth  of  the  tissue  surrounding  the  tympanic  membrane. 

The  mandibular  arch  contains,  close  to  its  connection  with  the 
superior  maxillary  process,  a  rod  of  cartilage,  called  Meckel's  car- 
tilage. This  becomes  partly  converted  into  the  malleus,  partly 
into  the  internal  lateral  ligaments  of  the  teraporo-maxillary  ar- 
ticulation. 

The  second,  or  hyoid  arch,  gives  origin  to  the  incus,  the  stylo- 
hyoid process  and  ligament,  together  with  the  lesser  wings  of  the 
hyoid  bone. 

From  the  third  arch  arise  the  body  and  greater  wings  of  the 
hyoid  bone  together  with  the  thyroid  cartilage. 


GLOSSARY. 


Abscissa.  The  line  forming  the  basis  of  measurement  of  graphic 
records,  along  which  the  time  measurement  is  commonly  made. 

Accommodation.     The  focussing  of  the  eye  for  different  distances 
it  depends  upon  changes  in  the  lens,  which  becomes  more  convex 
for  near  objects. 

Acinous  glands.  Secreting  organs  composed  of  small  saccules  filled 
with  glandular  epithelium  connected  with  the  twigs  of  a  branched 
duct  ;  like  the  berries  on  a  bunch  of  grapes. 

Adenoid  tissue.  The  follicular  part  of  lymphatic  glands  composed 
of  recticular  tissue  containing  the  lymph  corpuscles. 

Adequate  stimulus.  The  particular  form  of  stimulus  which  excites 
the  nerve-endings  of  a  special  sense  organ. 

Afferent  nerves.  Nerves  bearing  impulses  to  the  great  nervous 
centres,  from  the  various  parts  of  the  body,  so  as  to  give  infor- 
mation to  the  sensorium,  or  to  excite  reflex  actions. 

Agminate  glands.  A  name  applied  to  those  lymph  follicles  that 
occur  in  groups  in  the  lower  part  of  the  small  intestine. 

Albumin,  A  term,  derived  from  the  Latin  for  the  white  of  egg 
(albumen),  used  in  physiology  to  denote  a  complex  chemical 
substance  which  may  be  obtained  from  ova,  blood-plasma,  and 
many  tissues  of  animals  and  plants. 

Albuminoids.  A  class  of  nitrogenous  substances  allied  to  the  albu- 
mins in  composition,  but  differing  from  them  in  many  important 
respects. 

Allantois.  A  vascular  out-growth  from  the  embryo  which  in  mam- 
mals helps  to  form  the  placenta,  and  in  birds  becomes  the  respi- 
ratory organ  of  the  embryo. 

Alveoli.  The  term  used  to  denote  the  small  cavities  found  in  some 
parts,  such  as  the  air-spaces  of  the  lungs. 

Amnion.  The  membranous  sac  which  grows  around  the  embryo 
and  envelops  the  foetus  while  it  is  being  developed  in  utero. 


724  GI>OSSARY. 

Amoeba.  A  unicellular  organism  consisting  of  a  nucleated  mass  of 
protoplasm. 

Amorphous.  Without  definite  or  regular  form  ;  the  opposite  of 
crystalline. 

Amylolytic.  Kelatiug  to  the  conversion  of  starch  into  dextrin  and 
grape-sugar. 

Amylopsin.  A  ferment  in  the  pancreatic  juice,  which  converts 
starch  into  sugar. 

Analgesia.  A  condition  of  the  nervous  centres  in  which  pain  can- 
not he  felt,  hut  ordinary  tactile  and  other  sensations  remain  un- 
impaired. 

Analysis.  A  separation  into  component  parts  ;  the  splitting  up  of 
a  chemical  compound  into  its  constituents. 

Anastomoses.  The  direct  union  of  bloodvessels  without  the  inter- 
vention of  a  capillary  network. 

Anelectrotonus.  A  peculiar  electric  condition  of  a  nerve,  resulting 
from  the  passage  of  a  current  through  the  nerve,  but  confined 
to  the  region  where  the  current  enters,  i.e.,  the  neighborhood 
of  the  positive  pole. 

Anode.  The  positive  pole  or  electrode — i.e.,  the  pole  by  which  the 
electric  current  enters  a  substance. 

Apnoea.  A  state  of  cessation  of  the  breathing  movements  from 
non-excitation  of  the  respiratory  nerve  centre  on  account  of  an 
unusually  arterial  state  of  the  blood. 

Area  opaca.  The  outer  zone  of  the  blastoderm  from  which  the 
foetal  membranes  are  developed. 

Area  pellucida.  The  central  spot  of  the  blastoderm  from  which 
the  embryo  chick  is  developed. 

Arteriole.  A  small  artery ;  usually  applied  to  those  vessels  the 
walls  of  which  are  largely  composed  of  muscle  tissue. 

Arthroses.     Movable  joints  which  have  a  synovial  membrane. 

Asphyxia.  A  term  meaning,  literally,  cessation  of  the  pulse,  such 
as  occurs  from  interruption  of  respiration,  now  commonly  used 
as  synonymous  with  suffocation. 

Assimilation.  The  chemical  combination  of  new  material  (nutri- 
ment) with  living  tissues.  This  forms  the  most  characteristic 
property  of  living  matter. 

Astigmatism.  TJnevenness  of  the  refracting  surfaces  of  the  eye  ; 
when  engaging  the  entire  cornea,  it  is  called  "  regular,"  and 
affecting  a  local  part,  "irregular  "  astigmatism. 

Atoms.     The  ultimate  indivisible  particles  of  matter. 


QLOSSAKY.  725 

Atrophy.     A  wasting  from  insufficient  nutrition. 

Automatic.     Self-moving— ie.,  acting  without  extrinsic  aid  ;  a  term 

applied  to  the  independent  activity  of  certain  tissues  (such  as 

the  nerve  centres),  the  exciting  energies  of  which  are  not  readily 

determined. 
Axis  cylinder.     The  essential  conducting  part  of  a  nerve  fibre, 

which  is  composed  of  fine  strands  of  protoplasm. 

Bacteria.  A  class  of  minute  fungi,  which  occur  in  all  decomposing 
animal  or  vegetable  substances. 

Bilirubin.  The  red  coloring-matter  of  the  bile  of  man  and  carni- 
vora. 

Biliverdin.  The  greenish  coloring-matter  of  the  bile  of  herbivorous 
animals. 

Binocular.  Pertaining  to  vision  with  two  eyes.  A  combination  of 
the  effect  of  two  retinal  impressions  by  means  of  which  the 
appearances  of  distance  and  solidity  are  arrived  at. 

Biology.    The  science  of  life. 

Blastoderm.  The  primitive  cellular  membrane  formed  by  the  seg- 
mentation of  the  ovum,  in  a  part  of  which  the  embryo  is  devel- 
oped. 

Blood-pressure.  The  force  exercised  by  the  blood  against  the  walls 
of  the  vessels.  It  is  very  great  in  the  arteries,  and  therefore 
causes  a  constant  stream  through  the  capillaries  to  the  veins. 

Canaliculi.  Minute  channels  which  connect  the  small  cell-spaces 
or  lacunae  of  bone  with  each  other,  and  contain  protoplasmic 
filaments  uniting  the  neighboring  cells. 

Carbohydrates.  Compounds  of  carbon,  hydrogen,  and  oxygen,  in 
which  the  oxygen  and  hydrogen  exist  in  the  proportions  requi- 
site to  form  water. 

Cardiograph.  An  instrument  by  means  of  which  the  heart's  im- 
pulse is  transmitted,  through  an  air  tube,  from  a  tambour  on 
the  chest-wall  to  another  which  makes  a  record  on  a  moving 
surface  by  means  of  a  lever. 

Catelectrotonus.  A  peculiar  electric  state  of  a  nerve  in  the  region 
where  a  current  passing  through  it  leaves  the  nerve,  i.e.,  near 
the  negative  pole. 

Cathode.  The  negative  pole  or  electrode — i.e.,  the  one  by  which  the 
electric  current  leaves  a  substance  through  which  it  is  passing. 

Centrifugal.    Efferent. 


726  GLOSSARY. 

Centripetal.    Afferent. 

Cerebral  vesicles.  Primitive  swellings  on  the  primary  neural  tube 
of  the  early  cm])ryo  which  develop  into  the  brain. 

Chemical  elements.  Substances  which  cannot  be  split  up  into  com- 
ponents, and  therefore  arc  regarded  as  simple. 

Chlorophyll.  The  green  coloring-matter  of  the  cells  of  plants.  It 
is  supposed  to  be  the  agent  which,  under  the  influence  of  light, 
decomposes  carbon  dioxide  and  water  to  form  the  cellulose  and 
starch  of  the  plant. 

Cholestenn.  A  substance  occurring  in  the  bile,  white  matter  of 
the  brain  and  spinal  cord,  and  in  small  quantities  in  many 
other  tissues.     Chemically  it  is  a  triatomic  alcohol. 

Chorda  dorsalis.  The  precursor  of  the  vertebral  column  of  the 
embr3'0. 

Chorion.  The  outer  layer  of  the  membranes  of  the  ovum,  part  of 
which  becomes  vascular,  and  helps  to  form  the  placenta. 

Choroid.     The  vascular  coat  of  the  eyeball. 

Chromatic  aberration.  The  alteration  of  white  light  into  prismatic 
colors  during  its  passage  through  an  oi-dinary  lens. 

Chyle.     The  fluid  absorbed  from  the  small  intestines  by  thelacteals. 

Cilia.  Minute  vibratile  processes  which  occur  on  the  surface  cells 
of  the  respiratory  and  many  other  epithelial  membranes. 

Circumvallate.  Large  papillae  situated  at  the  back  of  the  tongue. 
They  are  surrounded  by  a  fossa  in  the  walls  of  which  lie  taste 
buds. 

Cloaca,  The  common  opening  of  the  genito-urinary  organs  into 
the  primitive  hind-gut  of  the  embryo.  The  cloaca  persists  in 
birds. 

Colloid.  That  condition  of  quasi-dissolved  matter  in  which  it  will 
not  diffuse  through  a  membrane  such  as  parchment.  The  op- 
posite of  crystalloid. 

Co-ordination,  The  adjustment  of  separate  actions  for  a  definite 
result,  as  when  the  nerve-centres  cause  various  distinct  muscles 
to  act  together  and  produce  complex  movements, 

Cytod.  A  term  suggested  to  denote  a  living  protoplasmic  unit 
which  has  no  nucleus. 

Decidua  reflexa.     The  outgrowth  of  the  uterine  mucous  membrane 

which  surrounds  the  ovum. 
Decidua  serotina.    The  part  of  the  modified  mucous  membrane  of 

the  uterus  in  which  the  fecundated  ovum  is  lodged. 


GLOSSARY.  727 

Decidua  vera.  The  altered  mucous  membrane  of  the  uterus,  which 
lines  that  organ  during  gestation. 

Deglutition.     The  act  of  swallowing. 

Desquamation.  The  term  used  to  denote  the  casting  off  of  the 
outer  laj^er  of  the  skin. 

Dialysis.  The  diffusion  of  soluble  crystalloid  substances  through 
membranes  such  as  parchment. 

Diastole.     The  period  of  relaxation  and  rest  of  the  heart's  muscle. 

Dicrotism.  The  double  wave  of  the  arterial  pulse.  The  dicrotic 
wave  is  seen  on  the  descending  part  of  the  pulse  curve. 

Dioptric  media.  Transparent  bodies,  such  as  those  parts  of  the 
eye  which  so  refract  the  light  that  images  come  to  a  focus  on 
the  retina. 

Distal.     A  term  used  to  denote  a  part  relatively  far  from  the  centre. 

Ductus  arteriosus.  A  short  bond  of  union  between  the  pulmonary 
artery  and  the  aorta,  which,  in  the  foetus,  carries  blood  from  the 
right  side  of  the  heart  into  the  aorta. 

Ductus  venosus.  A  vessel  which,  in  the  fcetus,  carries  blood  from 
the  umbilical  vein  to  the  vena  cava.  After  birth  it  becomes  a 
fibrous  cord. 

Ductus  vitello-intestinalis.  The  union  between  the  yolk  sac  and 
the  intestine  of  the  embryo. 

Dyspnoea.  Difficulty  of  breathing ;  it  is  a  condition  in  which  inor- 
dinate respiratory  movements  are  excited  by  an  unusually  ven- 
ous state  of  the  blood  in  the  respiratory  nerve  centre. 

Ectoderm.     The  outer  layer  of  simple  organisms. 

Ectosarc.  A  term  applied  to  certain  unicellular  organisms,  mean- 
ing the  outer  layer  or  covering. 

Electrodes.  The  two  terminals  which  are  applied  to  a  substance 
in  order  to  complete  the  circuit  when  it  is  required  to  pass  a 
current  through  the  substance. 

Electrotonus.  A  peculiar  electric  state  of  nerves  resulting  from  the 
passage  of  a  continuous  current  through  them. 

Embryo.  The  name  given  to  the  animal  at  the  earliest  period  of  its 
development. 

Emmetropic.  A  term  applied  to  the  normal  eye,  in  which  parallel 
rays  of  light  are  brought  to  a  focus  at  the  retina  without  accom- 
modation. 

Emnlsification.  The  suspension  of  very  fine  particles  of  liquid  or 
solid  in  a  liquid  which  is  not  able  to  dissolve  them. 


728  GLOSSARY. 

Endoderm.    The  inner  layer  of  simple  organisms. 

Endogenous  reproduction.  The  formation  of  new  cells  or  organ- 
isms within  the  hody  of  the  parent  individual. 

Endolymph.  The  liquid  contained  within  the  membranous  laby- 
rinth of  the  ear. 

Endosmosis.  The  diffusion  of  a  fluid  into  a  vessel  through  its  walls 
from  the  exterior. 

Endothelium.  The  single  layer  of  thin  cells  which  lines  the  serous 
cavities,  the  lymphatic  and  bloodvessels,  and  all  spaces  in  the 
connective  tissues  fmesoblastic  lining  cells). 

Epiblast.  The  uppermost  of  the  three  layers  of  the  blastoderm, 
from  which  the  epidermis  and  the  nerve-centres  are  developed. 

Epithelium.  The  non-vascular  cellular  tissue  developed  from  the 
epi-  and  hypoblast  of  the  blastoderm. 

Eupncea.  A  term  used  to  denote  the  normal  rhythm  of  respiratory 
movements  in  contradistinction  from  Oyi^pKna  and  apnoea. 

Excito-motor.     Impulses  which,  reflexly,  call  forth  motion. 

Excito-secretory.  Impulses  calling  forth  the  activity  of  gland  cells, 
commonly  applied  to  aflerent  influences  which  act  reflexly. 

Fibrinogen.    A  form  of  globulin  obtained  from  serous  fluids,  which, 

on  being  added  to  a  liquid  containing  serum-globulin,  gives  rise 

to  the  formation  of  fibrin. 
Fibrinoplastin.     A  term  sometimes  applied  to  paraglobulin  or 

serum-globulin. 
Filiform.    A  name  given  to  a  certain  class  of  the  papillae  of  the 

tongue,  the  points  of  which  taper  off  to  a  thread. 
Foetus.     The  fully-formed  embryo  while  in  the  uterus  or  egg. 
Fungiform.     A  name  given  to  a  certain  class  of  papillae  of  the 

tongue,  which  are  shaped  like  a  toadstool. 

Galvanometer.  An  instrument  for  measuring  the  direction  and 
strength  of  electric  currents  by  means  of  the  deflection  of  a  mag- 
netic needle. 

Ganglion.  A  swelling.  Chiefly  used  to  denote  swellings  on  nerves 
which  contain  nerve  corpuscles.  Hence  any  group  or  mass  of 
nerve  cells. 

Gastrula.  A  stage  in  the  development  of  animals  in  which  they 
consist  of  a  small  sac  composed  of  two  layers  of  cells. 


GLOSSARY.  729 

Gemmation.  Budding — a  process  of  reproduction  in  which  a  bud 
forms  on  the  parent  organism,  and  finally  separates  as  a  distinct 
being. 

Globulin.  A  form  of  albumin  insoluble  in  pure  water  but  soluble  in 
weak  solutions  of  common  salt. 

Glomerulus.  A  bundle  of  capillary  loops  which  form  part  of  the 
Malpighian  body  of  the  kidney. 

Glycocholic  acid.  An  acid  existing  in  large  quantities  combined 
with  soda  in  the  bile  of  herbivorous  animals,  and  in  lesser  quanti- 
ties in  man. 

Glycogen.  Animal  starch  ;  a  substance  belonging  to  the  carbohy- 
drates, which  is  made  in  the  liver.  It  may  be  readily  converted 
into  grape  sugar — from  which  fact  its  name  is  derived. 

Gustatory.    Pertaining  to  the  sense  of  taste. 

Haematin.  A  dark-red  amorphous  body  containing  iron  ;  obtained 
from  the  decomposition  of  the  coloring  matter  of  the  blood 
( haemoglobin). 

Haemin.  Hydrochlorate  of  heematin  ;  easily  obtained,  as  small  dark 
crystals,  by  boiling  blood  to  which  some  common  salt  and  gla- 
cial acetic  acid  have  been  added. 

Holoblastic.  The  form  of  ova  the  entire  yolk  of  which  enters  into 
the  process  of  segmentation. 

Homceothermic.  Even  temperatured — a  term  applied  to  those  ani- 
mals that  keep  up  a  regular  temperature,  independent  of  their 
surroundings — warm-blooded  animals. 

Hyaloid.  Glass-like  ;  a  name  given  to  the  delicate  membrane  in- 
closing the  vitreous  humor. 

Hydrocarbons.  Compounds  of  carbon  and  hydrogen.  Fats,  though 
containing  oxygen  in  addition,  have  been  considered  as  hydro- 
carbons. 

Hypermetropic.  A  term  applied  to  eyes  in  which  the  focus  of  par- 
allel rays  of  light  lies  beyond  the  retina ;  also  called  long  sight. 

Hypertrophy.    Increased  growth  from  excessive  nutrition. 

Hypoblast.  The  undermost  of  the  layers  of  the  blastoderm,  from 
which  the  pulmonary  and  alimentary  tracts  and  their  glands 
are  formed. 

Inhibition.  A  checking  or  preventive  action  exercised  by  some 
nervous  mechanisms  over  nerve  corpuscles  and  other  active 
tissues. 

61 


730  GLOSSARY. 

Infusoria.    A  name  given  to  a  large  class  of  simple  organisms  which 

are  found  in  dirty  water. 
Irradiation.     The  phenomenon  that  bright  objects  appear  larger 

than  they  really  are.     It  is  due  to  the  extension  of  the  effect  of 

light  on  the  retina  to  those  parts  immediately  adjacent  to  where 

the  light  rays  impinge. 

Kymograph.  An  instrument  used  for  recording  graphically  the  un- 
dulations of  blood-pressure,  measured  directly  from  a  bloodves- 
sel by  means  of  a  manometer. 

Lachrymal.    Pertaining  to  the  secretion  of  tears. 

Lacunae.  Small  spaces  in  the  substance  of  bone  tissue,  occupied 
during  life  by  the  bone-cells.  They  appear  black  in  sections  of 
dry  bone  owing  to  their  containing  air,  which  replaces  the 
shrivelled  cells. 

Latency,  or  Latent  period.  The  time  that  elapses  between  the 
moment  of  stimulation  and  the  response  given  by  an  active  tissue. 

Leucin.  This  is  a  common  product  of  the  decomposition  of  proteids. 
It  is  formed  in  the  later  stages  of  pancreatic  digestion. 

Leucocytes.  A  term  applied  to  the  white  blood-corpuscles  and  lymph 
cells. 

Lumen.  The  open  space  seen  on  section  of  a  tube,  vessel  or  glandu- 
lar saccule  ;  the  cavity  surrounded  by  the  gland-cells,  in  which 
the  secretion  collects. 

Lymph.  The  liquid  collected  by  the  absorbent  vessels  from  the  tis- 
sues ;  the  return  flow  of  the  irrigation  stream  escaping  from  the 
bloodvessels  to  nourish  the  tissues. 

Manometer.  An  instrument  for  measuring  pressure  ;  made  of  a  U- 
shaped  tube  containing  liquid,  commonly  mercury. 

Medullary  sheath.  A  soft  clear  sheath  around  the  axis  cylinder  of 
medullated  nerves,  which,  owing  to  its  refracting  power,  gives 
them  the  white  appearance. 

Menstruation.  The  monthly  change  in  the  mucous  membrane  of 
the  uterus,  which  accompanies  the  discharge  of  the  ovum. 

Meroblastic.  The  form  of  ova  in  which  the  yolk  does  not  undergo 
complete  segmentation,  as  that  of  birds. 

Mesoblast.  The  middle  of  the  three  layers  of  the  blastoderm  from 
which  the  connective  tissues  and  vascular  apparatus  of  the  em- 
bryo are  formed. 


GLOSSABY.  731 

Metabolism.  The  intimate  chemical  changes  occurring  in  the 
various  organs  and  tissues  upon  which  their  nutrition  and  func- 
tions depend. 

Metanephros.  The  hinder  portion  of  the  Wolffian  duct  which  de- 
velops into  the  kidney  and  ureter. 

Metazoa.  A  term  used  to  denote  all  those  animals  whose  ova  undergo 
division,  in  contradistinction  to  Protozoa. 

Micrococcus.  An  extremely  minute  fungus  of  a  round  shape.  Mi- 
crococci occur  in  many  solutions  of  decomposing  organic  matter. 

Micturition.    The  act  of  voiding  urine. 

Molecules.  The  smallest  physical  particles  of  matter  that  can  exist 
in  a  separate  state.  They  are  probably  always  constituted  of 
two  or  more  atoms. 

Morphology.  The  science  which  treats  of  the  forms  and  structure 
of  living  beings. 

Morula.  The  stage  of  development  of  the  ovum  after  segmentation 
in  which  all  the  young  cells  are  alike,  before  the  blastoderm  is 
formed. 

Mullerian  duct.  An  embryonic  structure  from  which  are  formed 
the  genital  passages  in  the  female,  viz.,  Fallopian  tube,  uterus, 
and  vagina. 

Myograph.  An  instrument  for  graphically  recording  muscle  con- 
traction. 

Myosin.  The  substance  formed  by  the  coagulation  of  muscle  plasma . 
It  is  one  of  the  globulins. 

Natural  nerve  currents.  The  electrical  currents  passing  through 
an  exposed  muscle  or  nerve  while  in  the  state  of  rest. 

Neuroglia.  The  reticular  connective  tissue  which  binds  together 
the  elements  of  the  nerve-centres. 

Non-polarizable  electrodes.  Specially  constructed  electric  termi- 
nals which  do  not  set  up  secondary  currents  on  application  to 
the  moist  living  tissues. 

Notochord.     The  primitive  vertebral  axis  of  the  embryo. 

Nucleolus,    A  small  spot  observable  in  some  nuclei. 

Nucleus.  A  central  part  of  a  cell  differentiated  from  the  main 
protoplasm,  commonly  round,  but  sometimes  elongated  as  in 
muscle. 

Odontoblasts.  Living  cells  lining  the  pulp-cavity  of  the  interior 
of  a  tooth,  and  presiding  over  the  growth  and  nutrition  of  the 
dentine. 


782  GLOSSARY. 

Olfactory.    Pertaining  to  the  special  sense  of  smell. 

Omphalo-mesenteric.  The  vessels  connecting  the  embryonic  circu- 
lation with  the  yolk  sac,  which  are  early  obliterated  in  the  mam- 
malian fcetus. 

Ophthalmoscope.  An  instrument  consisting  of  a  small  mirror  by 
which  the  interior  of  the  eye  can  be  illuminated  so  that  the  fun- 
dus may  be  viewed. 

Optic  cup.  The  involuted  optic  vesicle  which  is  developed  into  the 
retina,  etc. 

Oxyhaemoglobin.    The  coloring  matter  of  the  red  blood-corpuscles. 

Paramaecium.  A  unicellular  organism  composed  of  a  soft  mass 
of  protoplasm  inclosed  in  a  firmer  case  and  covered  with  motile 
cilia. 

Parapeptone.  A  stage  in  the  formation  of  peptone  produced  in  gas- 
tric digestion. 

Pepsin.  A  ferment  existing  in  the  gastric  juice  which  converts  pro- 
teids  into  peptones. 

Peptone.  A  form  of  albumin  which  is  produced  during  the  diges- 
tion of  proteids ;  it  is  very  soluble  and  diffuses  readily  through 
membrane. 

Perilymph.  The  liquid  surrounding  the  membranous  labyrinth  of 
the  ear. 

Peristalsis.  The  mode  of  contraction  of  the  muscular  walls  of  cer- 
tain tubes,  as  the  oesophagus  and  intestine,  the  effect  of  which 
is  to  cause  a  progressive  constriction,  and  so  force  the  contents 
of  the  tube  onwards. 

Phakoscope.  An  instrument  for  estimating  the  changes  in  the  shape 
of  the  lens  during  accommodation  by  doubling  the  reflected 
images  with  a  prism. 

Placenta.  The  intra-uterine  organ  by  means  of  which  the  foetal 
blood  is  brought  into  close  relationship  to  that  of  the  mother, 
so  as  to  gain  nutriment  and  oxygen  and  get  rid  of  effete  matters. 

Plasma.  A  term  meaning  anything  formed  or  moulded ;  it  is  ap- 
plied in  physiology  to  indicate  chemically  complex  kinds  of 
matter  which  subserve  to  the  formation  of  the  living  tissues. 

Poikilothermic.  Varying  in  temperature.  A  term  applied  to  those 
animals  whose  temperature  varies  with  that  of  the  surrounding 
medium — "cold-blooded  animals." 

Presbyopia.  A  term  denoting  the  loss  of  power  of  accommodation 
for  near  vision,  which  accompanies  old  age. 


GLOSSARY.  733 

Protista.    A  term  used  to  denote  the  large  group  of  organisms 

which  remain  in  the  primitive  state  of  a  single  cell  during  all 

their  lifetime. 
Protococcus.     A  unicellular  vegetable  organism,  the  protoplasm  of 

which  contains  chlorophyll. 
Protoplasm.     The  substance  which  gives  rise  to  the  primitive  vital 

phenomena,  seen  in  unicellular  organisms,  and  which  is  the 

chief  agent  in  executing  the  functions  of  all  the  active  tissues. 
Protovertebrae.     The  primitive  segments  of  the  mesoblast  in  the 

site  of  the  future  vertebral  column. 
Protozoa.     The  division  of  the  protista  which  has  been  assigned  to 

the  animal  kingdom. 
Proximal.    A  term  used  to  denote  a  part  relatively  nearer  to  the 

centre. 
Pseudopodia.     A  term  applied  to  the  projections  thrown  out  by 

moving  protoplasm,  by  means  of  which  cells,  such  as  amoebae, 

move. 
Ptyalin.     The  ferment  of  the  saliva.    In  a  weak  alkaline  solution 

it  converts  starch  into  dextrine  and  sugar. 

Reflex  action.  The  activity  caused  by  a  ganglion  cell  reflecting  an 
afferent  impulse  along  an  efferent  nerve  to  the  neighborhood  of 
original  stimulation. 

Refraction.  The  bending  which  rays  of  light  undergo  when  passin«y 
obliquely  from  one  medium  to  another  of  diflerent  density. 

Reticulum.  A  network ;  a  term  applied  to  the  interlacement  of 
fibres,  such  as  is  seen  in  reticulated  connective  tissue,  etc. 

Rheoscopic  frog-  An  arrangement  by  which  the  change  in  the 
electric  current  of  one  muscle  of  a  frog  is  made  to  act  as  a  stim- 
ulus to  the  nerve  of  another. 

Saponification.  The  formation  of  soap  ;  the  decomposition  of  oils 
or  fats  by  means  of  alkalies  into  salts  of  the  fatty  acid  and  glyce- 
rin. 

Sarcolactic  acid.  The  principal  acid  in  dead  muscle.  It  has  a 
dextro-rotatory  power  on  polarized  light,  which  ordinary  lactic 
acid  does  not  possess. 

Sarcolemma.  The  deUcate  sheath  surrounding  the  fibres  of  skeletal 
muscles. 

Sclerotic.    The  fibrous  coat  of  the  eye-ball. 


734  GLOSSARY. 

Sensorium.    That  part  of  the  nerve  centres  which  is  supposed  to 

perceive  sensory  impressions. 
Somatopleure.    The  subdivision  of  the  mesoblast  which,  with  the 

attached  epiblast,  forms  the  bodj'-walls  of  the  embryo. 
Specific  gravity.     The  relation  of  the  weight  of  a  given  volume  of 

any  substance  to  the  weight  of  an  equal  volume  of  distilled  water 

at  r-  c. 

Spherical  aberration.  The  different  degrees  of  refraction  at  differ- 
ent parts  of  a  lens  giving  rise  to  different  focal  lengths,  and 
causing  an  indistinctness  of  the  image. 

Sphygmograph.  An  instrument  for  obtaining  a  graphic  representa- 
tion of  the  pulse-wave  by  means  of  a  lever  applied  to  the  radial 
artery  at  the  wrist. 

Splanchnopleure.  The  subdivision  of  the  mesoblast  which  with 
the  attached  hypoblast  forms  the  chief  visceral  cavities  of  the 
embryo. 

Sporadic  ganglia.  Swellings  occurring  in  the  course  of  the  peri- 
pheral nerves  caused  by  a  group  of  nerve  corpuscles. 

Steapsin.  A  ferment  existing  in  the  pancreatic  juice  which  causes 
or  aids  the  saponification  of  the  fats. 

Sudoriferous  glands.  The  small  tubular  glands  of  the  skin  which 
secrete  the  perspiration. 

Summation.  The  adding  together  of  several  single  contractions  of 
muscle  to  form  a  tetanic  contraction  ;  the  accumulation  of 
stimuli. 

Sutures.  Unions  formed  by  the  direct  apposition  of  bones  without 
intervening  cartilage.     They  do  not  permit  of  motion. 

Sympathetic  nerve.  The  ganglionic  nervous  cord  on  either  side 
of  the  vertebral  column.  It  transmits  most  of  the  vaso-motor 
impulses  coming  from  the  cerebro-spinal  centres. 

Symphysis.  A  form  of  joint  without  synovial  membrane  in  which 
the  bones  are  fixed  together  by  fibro-cartilage. 

Synthesis.  The  artificial  building  up  or  construction  of  a  chemical 
compound  from  simpler  materials.  Natural  processes  are  not 
termed  syntheses. 

Systole.     The  period  of  contraction  of  the  heart's  muscle. 

Taurocholic  acid.  An  acid  existing  in  combination  with  soda  in 
the  bile  of  man  and  of  carnivorous  animals. 

Tetanus.  In  physiology  is  used  to  denote  the  prolonged  contrac- 
tion of  the  skeletal  muscles  which  follows  rapidly  repeated  stimu- 
lations or  nervous  impulse. 


GLOSSARY.  735 

Thalamencephalon.  The  part  of  the  anterior  cerebral  vesicle  which 
is  left  after  the  differentiation  of  the  optic  lobes  and  cerebral 
hemispheres. 

Thrombosis.  The  occlusion  of  a  vessel  by  a  local  coagulation  of  the 
blood. 

Trabeculae .  A  term  used  to  denote  the  supporting  bars  of  tissue 
that  pass  through  some  organs,  such  as  those  proceeding  from 
the  capsule  to  the  interior  of  the  spleen  or  lymphatic  glands. 

Trophic.     Relating  to  nutrition. 

Tr3rpsin.  A  ferment  in  the  pancreatic  juice  which  in  alkaline  solu- 
tions converts  proteids  into  peptones. 

Tyrosin.  A  substance  formed  together  with  leucin  during  pancre- 
atic digestion  ;  it  is  also  produced  by  putrefaction  of  proteids. 

Urachus.  The  bond  of  union  which  at  an  early  period  connects  the 
urinary  bladder  with  the  allantois  in  the  embryo  ;  it  is  subse- 
quently obliterated  in  the  foetus. 

Vacuoles.  Small  cavities  such  as  occur  in  cells.  They  are  supposed 
to  have  important  functions  in  many  unicellular  organisms. 

Vagus.  The  part  of  the  eighth  pair  of  nerves  distributed  to  the  vis- 
cera of  the  thorax  and  abdomen ;  it  is  the  great  regulating  nerve 
of  the  vegetative  functions. 

Vaso-motor.  The  name  given  to  the  nervous  mechanisms  control- 
ling the  movements  of  the  muscle-wall  of  the  bloodvessels. 

Villus.  A  hair-like  process.  A  term  applied  to  the  small  projec- 
tions characteristic  of  the  small  intestine.  They  contain  blood- 
vessels and  lacteals,  and  are  important  in  absorption. 

Vitellus.  The  yolk  of  the  ovum,  which  in  mammals  divides  com- 
pletely to  form  the  embryo.  In  birds  only  a  part  divides,  and 
the  rest  serves  to  nourish  the  chick. 

Vorticella.  Bell  animalcule,  a  bell-shaped  unicellular  organism 
with  a  rudimentary  ciliated  mouth  cavity  and  rapidly  contractile 
stalk. 

Wolffian  body.  An  embryonic  structure  the  forerunner  of  certain 
parts  of  the  genito-urinary  apparatus. 

Zymogen.  A  peculiar  substance  existing  in  the  secretion  of  the 
pancreas  supposed  to  give  rise  to  the  pancreatic  ferments. 


INDEX. 


Abdominal  respiration,  327 
Abscissa,  459 
Absorption,  186 

methods  of,  198 
Accelerator  nerves  of  the  heart,  277 
Accommodation,  defects  of,  571 

mechanism  of,  569 
Acinous  glands,  125 
Adenoid  tissue,  364 
Afferent  cardiac  nerves,  278 

nerve-fibres,  494 
Air  passages,  322 
Air,  pressure  differences  in  the,  336 

volume  of,  337 
Ampullae  of  the  semicircular  canals, 

605 
Amylopsin,  162 
Albumin,  acid,  61 

alkali,  62 

coagulated,  62 
Albuminates,  61 
Albuminoids,  63 
Albuminous  bodies,  59 
Albumins,  classification  of,  60 

derived,  61 

tests  for,  60 
Alimentary  canal,   development  of, 

683 
Alimentary  tract,  103 
Allantoin,  68 
Allantois,  660,  664 
Alveoli,  335 
Amnion,  660 
Amoeba,  32 

assimilation  of,  83 

discrimination  of,  85 

movements  of,  75,  84 
Analgesia,  616 
Anastomoses,  280 
Anelectrotonus,  506 
Animal  heat,  425 


Animal  heat,  expenditure  of,  429 

gain  of,  430 

mode  of  production  of,  428 

nervous  control  of,  430,  435 

radiation     and     conduction    of, 
431 
Animals,  food  of,  89 
Anode,  501 
Anus,  120 

Anvil  bone  of  ear,  incus,  601 
Aorta,  256 
Aortic  arches,  704 
Apnoea,  341 
Aqueous  humor,  558 
Area  opaca,  658 

pellucida,  658 
Arterial  blood,  351 

pulse,  the,  302 

system  in  the  foetus,  704 

tone,  312 
Arteries,  280 

development  of  the,  705 
Arthrosis,  476 
Asexual  reproduction,  650 
Asphyxia,  351,  358,  621 
Assimilation,  23 
Astigmatism,  573 
Atmosphere,  composition  of  the,  319, 

347 
Atoms,  21 
Auditory  nerve  endings,  605 

nerve,  cochlear  division  of  the, 
606 
Auerbach's  plexus,  122 
Augmentation  of  nerve  cells,  518 
Auricles  of  the  heart,  254 
Automatic  centres  in  spinal  cord,  613 
Automatism,  626 

of  nerve  cells,  516 
Axis  cylinder,  41,  495 

of  spinal  cord,  611 


62 


738 


INDEX. 


Bacteria,  81 

wound  infection,  82 
Basal  ganglia,  fi41 
Basement  membrane,  37 
Belladonna,  action  on  the  eye,  571 
Bell  animalcule,  86 
Bile,  163 

composition  of,  170 
ducts,  167 

functions  of  the,  175 
method  of  obtaining,  169 
salts,  65 

secretion  of,  173 

tests,  172 
Bilirubin,  172 
Biliverdin,  172 
Binocular  vision,  592 
Blastoderm,  30 

of  egg,  655 
Blind  spot,  581 
Blood,  amount  of,  211 

arterial,  351 

carbon  dioxide  in  the,  240 

change  from  venous  to  arterial, 
351 

changes  in  the  tissues,  354 

changes  in  the  spleen,  368 

chemical  interchanges  in  respi- 
ration, 340 

circulation  of  the,  253 

circumstances  influencing  coagu- 
lation, 244 

coagulation  of,  215,  241 

coloring  matter  of,  231 

current,  velocity  of  the,  309 

destiny  of  the  red  disks,  237 

fibrin  of,  214 

fibrin  ferment,  218 

fibrin  formation,  248 

gases  in  the,  239,  354 

general    characteristics   of    the, 
210 

globin,  237 

haematin  crystals,  235 

hsemin  crystals,  236 

haemoglobin  crystals,  232 

liquor  sanguinis,  213 

nitrogen  in  the,  240 

origin  of  white  corpuscles,  223 

oxygen  in  the,  239 

plasma,  58,  213 

plasma,  composition  of,  216 

red  corpuscles,  95 


Blood,  red  corpuscles,  development. of 
the,  237 
serum,  215,  219 
spectra  of,  234 
tension  of  gases  in  the,  354 
Valentine's  method,  212 
vascular  system,  development  of, 

696 
venous,  351 
Weber's  method   of  estimating 

amount  of,  211 
Welcker's  method,  212 
white  corpuscles,  221 
Blood  corpuscles,  213 

corpuscles,  action  of  reagents  on, 

227 
corpuscles,  method  of  counting, 

230 
corpuscles,  size  and  shape  of,  226 
Blood -pressure,  287 

changes  with  respiratory  move- 
ment, 299 
curve,  298 

measurement  of  the,  291 
relative  height  of,  294 
respiratory  wave  of,  299 
tracing,  314 
variations  in  the,  295 
Bloodvessels,  279 

nervous  control  of  the,  316 
relative  capacity  of,  284 
Bone,  50 

Bones  of  middle  ear,  601 
Brain,  629,  636 

development  of,  679 
effect  of  stimulation  of,  646 
fibres  and  cells  in  the,  636 
function  of  the,  637 
recovery  after  injury  of,  648 
result  of  removal  of  parts  of,  638, 

647 
ventricles  of,  629 
Breaking  shock,  501 
Bronchial  tubes,  321 
Brvinner's  glands,  179 
Buccal  cavity,  development  of  the, 

720 
Butter,  96 


Calabar  bean,  action  on  the  eye,  571 
Calamus  scriptorius,  340 
Camera,  563 


INDEX. 


739 


Canaliculi,  51 
Capillaries,  252,  281 
Carbohydrates,  69 

Carbonic    acid   in   the    atmosphere, 
347 

in  expired  air,  348 

gas,  72 
Cardiac  centre  in  medulla  oblongata, 
634 

nerve  centres,  274 
Cardiograph,  268 
Cartilage,  elastic  tibro-,  50 

hyaline,  50 

ossifying,  52 

white  fibro-,  50 
Casein,  62,  380 
Catelectrotonus,  506 
Cathode,  501 
C«ll  contents,  29 

modification  of  original,  32 

reproduction,  77 

wall,  26,  28 
Cells,  25 

animal,  26 

budding  of,  78 

development  of,  33 

difierentiated,  31 

endogenous  reproduction  of,  78 

indifferent,  30 

in  spinal  cord,  automatic  action 
of,  626 

life  history  of,  78 

nerve,  494 

varieties  of,  30 
Cellulose,  83 
Centrifugal  nerves,  494 
Centripetal  nerves,  494 
Cephalic  or  head  fold,  659 
Cerebellum,  636 

Cerebral  functions,  localization  of  the, 
646 

hemispheres,  637,  644 
Cerebrin,  66 

Cerebrum,  histology  of  the,  643 
Cervix  uteri,  669 
Chalaza  of  eggs,  655 
Changes  in  pancreatic  cells,  158 
Cheese,  96 
Chemical  elements,  21 

basis  of  body,  54 

stimulation  of  muscle,  449 

stimulation  of  nerve,  499 
Chloride  of  sodium,  73 


Chlorophyll,  83 

Cholesterin,  66,  172 

Cholin,  66 

Chondrin,  64 

Chorda  dorsalis,  or  notochord,  672 

Choroid,  555 

Chorion,  666 

Choroidal  fissure,  713 

Chromatic  aberration,  572 

Chyme,  154 

Ciliary  ganglion,  526 

muscle,  556 

processes,  555 
Ciliated  epithelium  of  bronchi,  323 
Circulation  of  the  blood  in  the  foetus, 

699,  706 
Circulation,  physical  forces   of  the, 

285 
Circumvallate  papillae,  125,  548 
Cochlea,  604 

basilar  membrane  of,  607 

development  of  the,  717 

nerve  endings  in  the,  604 

organ  of  Corti,  606 

rods  of  Corti,  606 

reticulated  membrane  of,  606 
Cold-blooded  animals,  426 
Colon,  119 
Colostrum,  380 
Color  perception,  586 
Common  salt,  73 
Complemental  air,  337 
Complementary  colors,  587 
Connective  tissue  corpuscles,  49 
Contractile  tissues,  42,  44,  438 

vesicle  of  paramaecium,  85 
Convex  lenses,  563 
Convergence  of  rays  of  light,  566 
Convulsions,  620 
Coordination,  639 

of  muscular  movements,  619 

of  nerve  cells,  515 
Cornea,  555 
Corpora  quadrigemina,  621,  636 

striata,  641 
Corpus  callosnm,  629 
Corpuscles,  blood,  213 

of  Hassall,  364 

Malpighian,  365 

salivary,  129 
Coughing,  345 

centre,  631 
Cranial  nerves,  520 


740 


INDEX. 


Crura  cerebri,  640 
Crying,  345 
Crystalline  lens,  559 

development  of  the,  559 

structure  of,  560 
Curara,  449 

Cutaneous  desquamation,  385 
Cvtode,  27 


Daniell's  battery,  500 
Decidua  reflexa,  668 

serotina,  665 

vera,  668 
Defecation,  mechanism  of,  119 
Deglutition,  106 

nervous  mechanism  of,  111 
Depressor  nerve,  314 
Development,  655 

of  alimentary  canal,  682 

of  arterial  system,  704 

of  blood  vascular  system,  696 

of  brain,  679 

of  ear,  715 

of  eye,  710 

of  genito-urinary  apparatus,  689 

of  heart,  696 

of  kidneys,  692 

of  liver,  688 

of  lungs,  688 

of  nose  and  mouth,  720 

of  oesophagus,  688 

of  pancreas,  688 

of  sexual  organs,  694 

of  skull  and  face,  719 

of  spinal  cord,  677 

of  spleen,  688 

of  venous  system,  707 
Dextrose,  70 
Diaphragm,  329 
Diastole,  263     • 
Diet  table,  423 

Digestion,  mechanism  of,  102 
Dioptric  apparatus,  defects  of,  572 

media  of  eyeball,  559 
Direct  vision,  583 
Discus  proligerus,  653 
Drum  of  the  ear,  599 
Du    Bois    Reymond's    spring    myo- 
graph, 459 
Ductless  glands,  361 
Ductus  arteriosus,  254,  705 

venosus,  703 


Ductus  vitello-iDteetinalis,  664 
Dyspnoea,  341 


Ear,  auditory  canal,  599 

conduction  of  sound  through  the, 
598 

cochlea,  603 

development  of  the,  715 

Eustachian  tube,  603 

labyrinth  of  the,  599,  603 

nerve  endings  in  the,  604 

organ  of  Corti,  606 

ossicles  of,  600 

semicircular  canals,  604 

tympanum  of,  599 

o"f  birds,  599 

of  fishes,  598 
Ectoderm,  33 

Ectosarc  of  paramsecium,  86 
Efferent  nerve  fibres,  494 
Efl'ete  products,  67 
Egg  albumin,  60 
Egg,  development  of  the,  654 
Eggs,  98 
Elastin,  65 
Electric  shock,  500 

stimulation  of  muscle,  449 

stimulation  of  nerve,  500 
Electrodes,  non-polarizable,  446 
Electrotonus,  504 
Embryonic  chick,  558 
Emmetropic  eye,  569 
Emulsification,  161 
End  bulbs  (Krause's),  536 
Endoderm,  33 

Endogenous  division  of  cells,  78 
Endolymph  of  inner  ear,  602 
Endosarc  of  paramsecium,  86 
Endothelium,  194 
Epiblast,  30,  35,  658 
Epithelial  tissue,  35 
Epithelium,  ciliated,  38 

columnar,  38 

glandular,  38 

scaly,  38 

stratified,  37 
Equilibration,  639 
Eupnoea,  341 
Eustachian  tube,  602 

development  of  the,  718 
Excretions,  383 
Expiration,  327 


INDEX. 


741 


Expiration,  muscles  of,  333 

Eye,  554 

development  of  the,  710 
dioptrics  of  the,  562 
motor  nerve  of  the,  520 
range  of  distinct  vision,  568 
tunics  of  the,  555 

Eyeball,  dioptric  media  of,  558 

Eyeballs,  movements  of  the,  590 
muscles  of  the,  591 


Face,  development  of  the,  718 

Fallopian  tubes,  653 

Fats,  71 

Feh ling's  solution,  141 

Fever,  variations  of  temperature  in, 

434 
Fibrin,  62,  214 
Fibrinogen,  fil,  217 
Fibrinoplastin,  218 
Fibrous  tissue,  47 
Pick's  pendulum  myograph,  458 

spring  manometer,  247 
Filiform  papillae,  125,  548 
Pieces,  184 

Fa?tal  circulation,  699,  706 
Food,  changes  in  the  mouth,  139 

chemical  composition  of,  92 

inorganic,  92 

organic,  91 

requirements,  90,  416 

special  forms  of,  93 

suitable   proportion  for  healthy 
nourishment,  423 

stuffs,  ultimate  use  of,  423 
Foramen  ovale  of  fcetal  heart,  698 
Fovea  centralis,  564 
Fungiform  papillae,  548 


Gall-bladder,  175 
Galvanometer,  446 
Ganglion  ceils,  40,  513 

in  the  spinal  cord,  611 

heart,  43 

sympathetic,  43 
Gastric  ghinds,  144 
Gastric  juice,  action  of,  149 

characters  of,  145 

metluKl  of  obtaining,  146 

secretion  of,  147 
Gastrula,  34 


Gelatin,  64,  421 
Gemmation,  77 

Genitourinary   apparatus,    develop- 
ment of,  689 
Germ  epithelium,  651 
Germinal  spot,  654 

vesicle.  654 
Giddiness,  547 
Gills,  320 
Glands,  agminate,  364 

blood-elaborating,  360 

lachrymal,  374 

mammary,  378 

meibomian,  378 

mucous,  375 

sebaceous,  377 

sudoriferous,  383 
Globulin,  61 

Glossopharyngeal  nerve,  527 
Glottis,  322,"  483 
Glycin,  68 

Glyco-cholic  acid,  66,  171 
Glvcocoll,  68 
Glycogen,  70,  370 

preparation  of,  371 
Glycogenic  function  of  liver,  369 
Gmelin's  test  for  bile,  173 
Goblet  cells,  39,  196 
Graafian  follicle,  652 
Grape  sugar,  70 
Gravid  uterus,  669 
Gustatory  nerves,  548 


Hammer  bone  of  ear,  malleus,  601 

Haversian  system,  51 

Hearing,  594 

Heart,  251 

action  of  drugs  on  the,  277 
development  of,  696 
innervation  of  the,  271 
movements  of  the,  262 
muscle,  257,  440 
muscular  fibres  of,  256 
rhythm  of  the,  263 
soimds,  269 
valvesofthe,  254,  259 
work  done  by  the,  311 

Heart  beat,  cycle  of  the,  265 

Heart's  impulse,  266 

Heat  regulation,  437 

Hepatic  vein,  165 

Hiccough,  346 


742 


INDEX. 


Hippuric  acid,  69 
in  urine,  400 
Holoblastic  ovum,  657 
Homoeotliermic  temperature,  425 
Hunger  and  thirst,  546 
Hvaloid  membrane  of  eve,  558 
Hydrocele  fluid,  216 
Hydrochloric  acid,  72 
Hvpermetropia,  571 
Hyjwblast,  30,  34,  658 
Hypoglossal  nerve,  531 


Ideas,  545 

Ileo-caecal  valve,  118 

Incus,  601 

Indican,  69 

Indifferent  gases,  356 

Indirect  vision,  583 

Indol,  69 

Induced  current,  502 

Induction  coil,  Du  Bois  Reymond's, 

451 
Infusorium,  85 
Inhibition  of  nerve  cells,  516 
Inhibitory  action,  621 
Inorganic  bodies,  71 
Inosit,  70 
Insalivati(m,  139 
Inspiration,  327 

forced,  332 
Inspiratory  muscles,  329 
Intercellular  substance,  28 
Intercostal  muscles,  331 
Interlobular  vein,  166 
Interstitial  absorption,  188 
Intestinal  absorption,  195 

juice,  functions  of,  181 

motion,  nervous  mechanism  of, 
121 

secretion,  178 

seci-etion,  method  of  obtaining, 
180 

movements,  117 
Intestine,  development  of,  685 

large,  183 
•    lymph  follicles  of,  197,  198 

putrefactive  fermentation  in  the, 
184 

structure  of  small,  178 
Intralobular  vein,  165 
Inversion  of  the  image,  565 
Irradiation,  584 


Irrespirable  gases,  356 
Iris,  556,  574 


Jejunum,  119,  183 
Joints,  475 


Keratin,  65,  385 

Kidney,  bloodvessels  of,  389 
convoluted  tubes  of,  388 
development  of  the,  592 
glomerulus  of,  390 
pyramids  of,  389 
structure  of  the,  388 

Kreatin,  67,  406 

Kreatinin,  68 
in  urine,  400 

Kymograph,  Ludwig's,  293 
Pick's  spring,  297 

Kymographic  tracing,  314 


Labyrinth  of  ear,  594,  595,  603 
Lachrymal  glands,  374 
Lacteals,  180,  187,  196 
Lactic  fermentation,  70 
Lactose,  70 
Lacunae,  51 
Larynx,  322 

anatomy  of,  483 
Latent  period,  460 
Lateral  plates  of  embryo,  673 
Laughing,  345 
Law  of  contraction,  510 
Lecithin,  66 
Lens,  crystalline,  559 
Leucin,  68,  161 
Leucocytes,  221 
Levatores  costarum,  331 
Levers,  orders  of,  475 
Lieberkiihn's  follicles,  179 
Light  impressions,  578 
Light,  stimulation  of  retina  by,  585 
Listing's  measurements,  566 
Liver  cells,  166 
Liver,  development  of,  687 

glycogenic  function  of  the,  369 

structure  of,  165 
Long  sight,  571 
Lumen  of  cell,  158 
Lungs,  321 
Lung  sounds,  334,  339 


INDEX. 


743 


Lung  tissue,  323 

Lungs,  development  of,  687 

Lymph  and  ohyle,  204,  360 
corpuscles,  205 
follicles,  197,  198 
movement  of  the,  207 
spaces  in  tendon,  181 

Lymphatic  glands,  190 

Lymphatics,  187 


Making  shock,  499 

Malassez'    apparatus     for     counting 

blood  corpuscles,  229 
Male  and  female  generative  elements, 

origin  of,  650 
Malleus,  601 
Malpighian  bodies  of  spleen,  368 

corpuscles  of  kidney,  389 
Mammary  glands,  378 
Mastication,  104 
Meat,  97 

Mechanical   stimulation   of   muscle, 
450 

of  nerve,  499 
Mercurial  manometer,  290 
Medulla-oblongata  as  central  organ, 
630 

as  conductor,  628 

decussation  of  fibres  in,  629 

respiratory  centre  in,  630 

vaso-motor  centre  in,  632 
Medullary  canal,  672 

folds,  658,  671 

groove,  658,  672 

sheath,  43,  496 
Meibomian  glands,  378 
Meissner's  plexus,  123,  147 
Membrane  of  Keisner,  606 
Membranes  of  the  chick,  660 
Memory,  545 
Menstruation,  668 

and  ovulation,  653 
Meroblastic  ovum,  657 
Mesencephalon,  tlie,  637 
Mesoblast,  30,  34,  658 
Metabolism,  360 
Metazoa,  33 
Micrococci,  80 
Micturition,  409 

nervous  mechanism  of,  411 
Milk,  94 

action  of  gastric  juice  on,  153 


Milk,  composition  of,  380, 

influence  of  nervous  system  on 
secretion  of,  382 

mode  of  secretion  of,  381 
Milk  sugar,  70,  380 

tests,  95 
Mitral  valve,  255 
Moist  chamber,  458 
Molecules,  22 
Morphology,  17 
Morula,  33 

stage  of  the  ovum,  657 
Motor  nerve  roots,  517 
Mouth,  development  of  the,  720 
Mouth  digestion,  125 
Movements  of  the  body,  477 
Mucin,  64,  375 
Mucous  glands,  375 

of  tongue,  128 

tissue,  46 
Mullerian  duct,  691 
Muscles,  antagonistic,  475 

of  the  eyeball,  590 

of  mastication,  104 

origin  and  insertion  of,  474 

synergetic,  475 
Muscle,  42 

active  state  of,  448 

change  in  form  during  contrac- 
tion, 457 

changes  in  structure  during  con- 
traction of,  452 

chemical    changes   during    con- 
traction of,  452 

chemical  change  in,  443 

chemical  composition  of,  442 

consistence  of,  442 

contraction  of,  457 

elasticity  of,  443 

electrical  changes  in,  454 

electric  phenomena,  445 

fatigue,  461,  468 

heart,  257 

histology  of,  439 

irritability  of,  448 

latent  period,  460 

maximum  contraction,  464 

natural  currents  in,  445 

negative    variation    of    current, 
453 

non-striated,  44 

passive  state  of,  442 

plasma,  58,  443 


744 


INDEX. 


Muscle,  recording  contraction  of,  458 

serum,  442 

single  contraction,  459 

stimuli,  449 

striated,  44,  440 

summation,  464 

temperature  change  during  con- 
traction, 455 

tetanus,  465 

tone,  467 

unstriated,  473 

variations  in  the  single  contrac- 
tion, 462 

wave  of  contraction,  463 
Muscle  plates  of  embryo,  675 
Muscular  coordination,  619 

stimuli,  diflerent  forms  of,  449, 
450 
Muscularis  mucosae,  118,  143 
Musical  notes  or  tones,  596 
Myographs,  458 
Myopia,  571 
Myosin,  61,  442 


Nasal  ganglion,  526 

Nausea,  547 

Negative  after  image,  584 

variation  of  muscle  current,  455 
Nerve,  active  state  of,  499 

ascending  and  descending   cur- 
rents, 509 

electric  properties  of,  498 

electric  change  in,  504 

electrotonus,  504 

force,  velocity  of,  502 

natural  currents  in,  49S 

optic,  554 

tissue,  39 

roots,  517 

stimulation,  negative  variation, 
504 

stimuli,  499 

terminals,  510 
Nerves,  afferent,  40,  496 

cranial.  520 

efferent,  40,  496 

excito-motor,  496 

gray.  496 

inhibitory,  496 

intercentral,  497 

mixed,  517 

motor,  496 


Nerves,  reflex,  496 

secretory,  496 

sensory,  496 

special  physiology  of,  517 

spinal,  517 

vaso-motor,  497 

white,  496 

third  pair  of,  520 

fourth  ])air  of,  521 

fifth  pair  of,  521 

sixth  pair  of,  522 

seventh  pair  of,  522 

eightli  pair  of,  527 

ninth  pair  of,  531 
Nerve  cells,  40 

functions  of,  513 

in  retina,  519 

bipolar,  513 

multipolar,  43,  513 

unipolar,  518 

corpuscles,  494,  511 

endings,  536 

fibres,  40,  494 

chemistry  of,  498 

fatigue  of,  506 

irritability  of,  506 
Nerve  muscle  preparation,  455,  499 
Nervous  system,  494 

medulla  oblongata,  628 

reflex  action,  617 

spinal  cord,  494 
Neurin,  66 
Neuroglia,  494 
Neuro- muscular  cells,  39 
Nitrogen,  73 

in  expired  air,  348 

in  the  atmosphere,  347 
Nose,  551 

development  of  the,  719 
Notochord,  658 
Nucleolus,  25 
Nucleus,  25.  27 
Nutrition.  413 

and  food  stuffs,  88 

ti.ssue  changes,  414 

Odontoblasts,  106 
Qilsophagus,  111 

development  of,  688 
Olfactory  bulb,  552 

mucous  membrane,  552 
Omphalo-mesenteric  vessels,  702 


INDEX. 


745 


Ophthalmoscope,  577 
Optic  axis  of  the  eye,  564 

disk,  577 

lobes,  639 

nerve,  5^0 

nerve,  terminals  of  the,  581 

thalarai,  641,  643 

vesicle,  711 
Ora  serrata,  583 
Organ  of  Corti,  606 
Organisms,  characters  of,  20 

vital  characters  of,  74 
Ossicles  of  the  middle  ear,  600 
Ossifying  cartilage,  52 
Osteoblasts,  51 
Otic  ganglion,  527 

vesicle,  605 
Otoliths,  605 
Ovarv,  652 
Ovoid  cells,  133 
Ovum,  650 

Ovum,  changes  in  the,  656 
Oxalic  acid  in  urine,  401 
Oxygen,  73 

in  expired  air,  348 

in  the  atmosphere,  347 
Oxyhsemoglobin,  58,  352 


Paccinian  corpuscles,  538 

Pain,  505 

Pancreas,  development  of,  687 

structure  of,  155 
Pancreatic  digestion,  160 
Pancreatic  juice,  composition  of,  156 

mode  of  secretion  of,  156 
Papillae  of  tongue,  125,  549 
Paraglobulin  (fihrinoplastin),  61 
Paramaecium,  35,  85 
Parapeptone,  150 
Parietal  cells,  145 
Parotid  gland,  129 
Pavy's  solution,  141 
Peduncles  of  cereljrum,  629 
Pepsin,  150 
Peptic  cells,  145 
Peptone,  63 

conversion  of  proteid  into,  151 

testa  for,  63 
Perception,  534 
Perilymph  of  inner  ear,  602 
Peristaltic   contraction   of  intestine, 
114,  122 


Perspiration,   effect  of   nervous   in- 
fluence on,  385 

insensible,  3S3 

quantity  given  off.  384 

sensible,  384 
Pettenkofer's  test  for  bile,  65,  172 
Peyer's  patch,  199 
Phakoscope,  570 
Pharvnx,  muscles  of,  108 
Placenta,  666,  701 

functions  of  the,  670 
Plants,  food  of,  88 
Plasmata,  56 

Pleura,  function  of  the,  334 
Pneumogastric  nerves,  343,  528 
Pneumothorax,  335 
Poikilothermic  animals,  426 
Poisonous  gases,  356 
Polarizing  current,  505 
Pons  Varolii,  636 
Portal  blood.  367 

vein,  165 
Portio  dura  of  seventh  nerve,  522 

mollis  of  seventh  nerve,  594 
Porus  opticus,  581 
Positive  after  image,  584 
Potassium  chloride,  73 
Potatoes,  100 
Presbyopia,  572 
Primitive  groove,  34 

nerve  sheath,  42,  495 

streak,  671 
Products  of  tissue  change,  65 
Protamceba,  27 
Protagon,  66 
Protista,  32 
Protococcus,  83 
Protoplasm,  21,  27,  57 

effect  of  chemical  stimulation,  76 

effect  of  electric  stimulation,  76 

effect  of  mechanical  irritation  of, 
76 

effect  of  temperature  on,  75 

sensiliveness  of,  76 

movements  of,  75 
Protoplasmic    movements,    differen- 
tiation of,  438 
Protovertebra,  674 
Protozoa,  33 
Pseudopodia,  75 
Ptyalin,  140 
Puerile  breathing,  339 
Pulmonary  capillaries,  350 


746 


INDEX. 


Pulse,  the,  302 

tracings,  305 

variations  in  the,  307 
Pnpil  of  eye,  556 

Pupil,   circumstances   affecting    the, 
676 

contraction  of,  576 

dilatation  of,  576 
Pylorus,  114 


Quadratus  lumborum,  330 


Ranvier's  nodes,  40,  495 
Receptacnlum  chyli,  190 
Recording  apparatus,  292 
Recti  muscles  of  the  eye,  591 
Reflex  action,  515 

experiment  on   human  subject, 
620 

experiment  on  frogs,  619 

in  tlie  spinal  cord,  616 

theory  of,  022 
Reflex  centres,  special,  625 
Reflexion,  626 

of  nerve  cells,  515 
Refraction,  562 

Refracting  media  of  the  eye,  564 
Reproduction,  651 
Reserve  air,  3'i7 
Residual  air,  337 

Respiration,    afferent    and     efferent 
nerves,  343 

automatic  nerve  centre,  342 

chemistry  of,  347 

differences  in  male  and  female, 
328,  349 

external,  319 

internal  or  tissue,  319,  355 

mechanism  of,  318 

nervous  mechanism  of,  339 

of  abnormal  air,  355 

variations  of  pressure  in,  336 
Respirations,  rate  of,  327 
Respiratory  centre  in  medulla  oblon- 
gata, 630 

gas  interchange,  349 

sounds,  339 
Restiform  bodies,  629 
Reticulum,  365 
Retina,  558,  567,  577 

stimulation  of  the,  583 


Retina,  structure  of,  580 
Revolving  cylinder,  292 
Rheoscopic  frog,  455 
Ril)s,  324 
Rigor  mortis,  471 
Ritter's  tetanus,  500 
Rods  and  cones,  581 

of  Corti,  607 
Rotation  of  the  eyeball,  590 


Saccules  of  ear,  717 
Sacculated  glands,  125 
Saliva,  composition  of,  129 
Salivary  corpuscles,  129 

glands,  120 
Salivary  secretion,  nerve  mechanism 
of,  132 

method  of,  130 
Saponification,  161 
Sarcolactic  acid,  453 
Sarcous  elements,  441 
Sarcolemma,  43,  440 
Scaleni  muscles,  331 
Scheiner's  experiment,  567 
Sclerotic  coat  of  eye,  555 
Sebaceous  glands,  377 
Secretions,  374 

Secreting  gland  cells,  changes  in,  137 
Segmentation  in  the  ovum,  657 
Semicircular  canals,  604 

development  of  the,  717 
Semilunar  valves,  255 
Sensations,  general,  545 
Sense  of  touch,  535 
Sensorium,  541 
Sensory  nerve  roots,  517 
Serratus  posticus  inferior,  333 
Serum  albumin,  60 
Seventh  nerve,  494 
Sexual  distinction,  693 

organs,  development  of  the,  694 
Shivering,  547 
Sighing,  346 
Sight,  long,  571 

short,  671 
Skeletal  muscles,  474 
Skin  sensations,  535 
Skull  and  face,  development  of  the, 

718 
Smell,  sense  of,  551 
Sneezing,  345 

centre,  631 


INDEX. 


747 


Sobbing,  345 
Solar  spectrum,  587 
Somatopleure,  660 
Sound,  51)4 

amplitude  of  vibration,  596 

conduction  of,  598 

conduction  of,  through  the  ear, 

601 
conveyance  of,  in  cochlea,  607 
over-tones,  597,  609 
pitch  of  note,  595 
quality  of  notes,  596,  609 
rate  or  period  of  vibration,  595 
tone,  noise,  608 
tones  or  musical  notes,  596 
transmission  of,  596 
transmission  of,  to  the  brain,  609 
Sounds,  classification  of,  492 
Special    reflex   centres,    micturition, 

etc.,  625 
Special  senses,  532 
smell,  551 
taste,  548 
touch,  539 
vision,  554 
Spectrum,  solar,  587 
Speech,  491 
Spermatozoa,  650 
Sphenopalatine  ganglion,  526 
Spherical  aberration,  573 
Sphygmograph,  Marey's,  304 
Spinal  accessory  nerve,  528 
Spinal  cord,  611 

automatic  centres  in  the,  611 
centres  presiding  over  tonic  mus- 
cular contraction,  626 
coordination  of  movements,  618 
decussation  of  fibres  in  the,  616 
development  of,  677 
direction  of  nerve  fibres  in  the, 

614 
effect  of  section  of,  616 
ganglion  cells  in  the,  611 
nerve  cells  in  the,  612 
reflex  action  in  the,  617 
sweating  centres  in  the,  626 
vafio-motor  centres,  626 
white  and  gray  substance  of,  611 
Spinal  ganglion,  519 
Spinal  nerves,  517 

anterior  and  posterior  roots  of, 
615 
Spiral  lamina  of  cochlea,  605 


Splanchnopleure,  661 
Spleen,  365 

changes  of  blood  in,  368 
development  of,  688 
functions  of  the,  367 
Splenic  pulp,  365 
Sputum,  376 
Standing,  478 

Stapedius  muscle  of  ear,  602 
Stapes,  601 
Starch,  tests  for,  141 

into  grape-sugar,  conversion  of, 
140 
Starvation,  414 
Stationary  air,  338 
Steno's  duct.  128 
Steapsin,  161 

Stirrup  bone  of  ear,  stapes,  601 
Stomach,  digestion,  143 
epithelium  of,  144 
development  of,  685 
structure  of,  113 
motion  of,  113 
nerve  influence  on,  114 
Striated  muscle,  440 
Sublingual  gland,  129 
Submaxillarv  ganglion,  527 

gland, 129 
Sudoriferous  glands,  383 
Summation,  464,  620 
Supra-renal  capsule,  362 
Suspensory  ligament  of  lens,  560 
Sutures,  475 
Swallowing,  107 

Sweat,  chemical  composition  of,  384 
Sweat  glands,  383 

Sweating  centres  in  spinal  cord,  626 
Symphysis,  476 
Svntonin,  61 
Systole.  263 


Tactile  nerve  endings,  511 

Tambour,  Marey's,  268 

Taste  buds,  548 
sense  of,  548 

Taurin,  68 

Tauro-cholic  acid,  65,  171 

Tegmentum,  640 

Temperature,  external  variations  of, 
435 
internal  variations  of,  434 
maintenance  of  iniiform,  432 


748 


INDEX. 


Temperature  of  mammals,  425 

of  man,  426 

measurement  of,  426 

variations  of,  426 
Tendon  cells,  47 
Tensor  tynipani  muscle,  601 
Tetanus,"  465 
Thermic  stimulation  of  muscle,  450 

of  nerve,  500 
Thermometer,  clinical,  426 
Thoracic  duct,  190 

movements,  326 

respiration,  327 
Thorax,  333 

construction  of,  321 
Thrombosis,  247 
Thymus  gland,  364 
ThVroid  bodv,  363 
Tidal  air,  337 
Tissue  changes,  414 
Tissues,  classification  of,  35 

contractile,  438 
Titillation,  547 
Tone,  608 
Tongue,  549 

papilla  of,  125 
Tonic  contraction,  312 
Tooth,  crusta  petrosa,  105 

dentine,  105 

enamel,  104 

pulp  cavity,  105 
Tornla  cerevisia  (yeast  plant),  82 
Touch  corpuscles  (Meissner's),  536 

sense  of  locality,  539 

sense  of  pressure,  542 

temperature  sense,  543 
Trabeculffi.  190 

of  spleen,  365 
Triangularis  sterni,  333 
Tricuspid  valve,  254 
Trigeminus  nerve,  523 
Trochlear  nerve,  521 
Trommer's  test,  141 
Trypsin,  159 
Tubuli  seminiferi,  651 
Tunica  adventitia,  280 

fibrosa,  653 

granulosa,  653 

intima,  281 

media,  280 

propria,  653 

vasculosa,  653 


Tuning  fork,  demonstration  of  vibra- 
tions, 595 
uses  of,  in  measuring  time,  458 
Tympanic  membrane,  599 
Tyrosin,  68,  182 


Umbilical  vessels,  701 
Umbilicus,  662 
Unicellular  organisms,  32 
Unstriated  muscle,  439,  473 
Uracil  us,  666 
Urea,  67,  397 

source  of,  405 

preparation  of,  398 

volumetric  estimation  of,  399 
Uric  acid,  68,  399 
Ureters,  408 
Urinary  calculi,  404 

excretion,  387 

secretion,  nervous  mechanism  of, 
407 
Urine,  391 

abnormal  constituents  of,  403 

chemical  composition  of,  397 

coloring  matters  of  the,  401 

gases  in,  403 

inorganic  salts  in,  402 

passage  into  bladder,  408 

specific  gravity  of,  393 

secretion  of,  393 
Uterus,  667 
Utricle  of  ear,  716 


Vacuoles,  25 

of  paramsecium,  86 
Vagus  nerve,  277,  529 

effect  on  respiration,  344 

effect  on  heart,  276 
Valsalva's  experiment,  602 
Valvule  conniventes,  178 
Vaso-motor  centre  in  medulla  oblon- 
gata, 632 

centres  in  spinal  cord,  626 

nerves,  312 
Vascular  system,  development  of,  696 
Vas-deferens,  development  of,  691 
Vegetable  cells,  25 

action  of  light  on,  88 
Vegetable  food,  98 
Veins,  283 


INDEX, 


749 


Veins,  development  of  the,  706 
Velocity  of  blood  current,  308 
Venae  advehentes,  703 
Vena  porta,  369 
Venous  blood,  351 

system,  development  of  the,  707 
Ventilation,  357 
Ventricles  of  brain,  629 

of  heart,  254 
Vesicular  breathing,  339 
Vestibule  of  ear,  601 
Vertebral  plates  of  embryo,  673 
Villi,  vessels  of,  180 
Vision,  554 

accommodation,  568 

binocular,  592 

inversion  of  image,  565 

light  impressions,  578 

refraction,  563 
Visual  perceptions,  588 

purple,  585 
Vital  capacity,  338 

phenomena,  23 

point  (noeud  vital),  340 
Vitellin,  61 
Vitelline  membrane,  654 

veins,  698 
Vitreous  humor,  559 
Vocal  cords,  483 
Vocalization,  mechanism  of,  486 
Voice,  483 


Voice,  nervous  mechanism  of  491 
properties  of  the  human,  488 

Volition,  626 

Vomiting,  115 

Vorticella,  35,  87 

ciliary  motion  of,  87 
contractile  stalk  of,  87 


Walking  and  running,  481 

Warm-blooded  animals,  425 

Water,  71 

Wharton's  duct,  118 

White  substance  of  Schwann,  496 

Wolffian  bodies,  690 


Xanthin  in  urine,  400 


Yawning,  346 
Yeast  plant,  82 
Yellow  elastic  tissue,  50 
Yellow  spot,  578 

structure  of,  583 
Yolk  sac,  the,  653 


Zona  pellucida,  654 
Zymogen,  159 


N 


"v 


